I'm just an electronics hobbyist and your videos have taught me a lot. If you had been doing these videos 30 years ago I'm sure I would have changed my major to electrical engineering rather than finance. When you retire please consider teaching engineering or math at the college level. You would have a great impact on young minds.
I think he's having a much larger impact here, on youtube, teaching at the "hobbyist level", rather than teaching at the college level. Inspirational. Technical details (unfortunately?) are needed at some point to do advanced things, and those require a formal (university level) training, which can be more boring.
I wish I could get all the other TH-cam "instructors" to watch your videos. You are so polished. You could make a living instructing the instructors and I wish you would.
I remember watching this video years ago, and I was really in the very early stages of my electronics hobby. I vaguely understood it, but I thought to myself that hey, nice video, but let's be honest, I'll never need to know how to do this... Well, I was wrong. Today I need to know how to measure output impedance, and I also could use a primer on transmission line termination. w2aew to the rescue! Your drawings and practical examples have taught me more than anything else. Thank you, genuinely!
Still valuable and excellent, and it will be even in two decades from now on. A really outstanding quality in terms of clarity, accuracy and style. Can't thank you enough for your work!
For the beginers What Alan does here is : He devide's 2.5V bij 1000 ohms (1K) of R load. That give the current flows in R load which result 0.0025 Amps (2.5 milli amp's) that will also be current flows in Ro (since current is equal everywhere in a close circuit). We also know that when R load is connected and current start passing it creates 1.5 Voltage drop on Ro 4Volts - 2.5 (V drop on R load that is because the sommof all voltage drops must be 4Volts ). So 1.5V voltage drop on Ro devided withthe current flow's in it will give the value of R out which is 600 Ω (ohms).
I am glad you still have this video posted. The equations used are easy to derive once you think about the relations, but when electronics is a hobby and not a field of study it sure speeds up the process to see it laid out so nicely as in this video.
Hi Alan, great video and tutorial to measure output impedance. Most of us also wants to know how to measure an unknown network impedance from 100kHz to 1GHz or higher but this one can only be achieved using a network analyzer. In my experience in Electronics, EMC and RF, and before I started my ECErcuit TH-cam channel, I been working in understanding how to measure unknown network impedance using spectrum analyser and signal generator. In somehow, I have managed and successfully measured unknown network impedance accurately from 100kHz to at least 100MHz. Measurement was achieved by deriving a formula through a series of derivation and calculations. I automate the system by writing and application to control the spectrum analyser and signal generator to process the measurement against the calibration reference using the formula which gives you the result of impedance plot and data. I'm thinking of sharing this work on my TH-cam channel but I'm not sure if someone is interested to know. Cheers, ECErcuit.
Alan, when I watch your vids, it's like learning all over again. I must admit, I feel that same giggly sensation that I felt when I first learned these principles many years ago. I've found that a huge number of beginning electronics students, hobbyist and on occasion, even some tech's I've spoken to, have had a hard time understanding this. You sir, have accomplished the perfect explanation of the very concept of output impedance and how to measure it in a simple straight forward way.... I'm always amazed by your teaching skills which seem so easy but in fact, are difficult in practice. God indeed blessed you with an outstanding teaching ability and a keen mind as well. I'm so glad that you share this talent with others. It will unlock a new world of understanding for some they never knew existed. Again, Kudos!!! Job Well Done!
Just dropped in to see what was happening. LOL! Had just used the resistance substitution method (5:06) to get output impedance of an amplifier. Thanks again, Alan.
Another EXCELLENT review!! If you are still considering suggestions for tutorials, Have you considered FET's and in particular their utility in audio. I have been amazed how well FET's sound in preamps and low power amps. Being a voltage device apparently is a plus at audio freq's. My experience has shown them to rival many tubes I have played with such as 45, 2A3, 6L6 tubes. I have played with MANY transistors and opamps at audio freqs but they sound flat and dull compared to FETs...Many of the people I know are OK with transistor theory but don't understand FET's....Another OUTSTANDING audio device is the nuvistor for low level work like preamps...Microphonic as heck but once tamed, they are VERY nice not to mention fast and reasonably cheap.. And a great big OSCILLATOR to you too...dit dit
Very nice presentation once again. A couple of observations: I did this on a breadboard with the output impedance of my awg set to 50 ohm. I used a small 100 ohm trim pot and adjusted the pot until Vpp on the oscilloscope measured the set voltage on the awg. I got 47 ohm. Not bad. It just seems to me that using a pot makes it much easier to fine tune the load resistance compared to a decade box. Secondly this method assumes that the circuit under test consists of linear circuit elements which can be modeled as a thevenin voltage and resistance. The failure of the first method (2:47) to work all the time may be an indication of this. Using a second load resistance measurement (6:54) may or may not satisfactorily adjust for this. Best way is to measure open circuit voltage (Vth), calculate voltage at several load resistance levels, calculate current V/R at each point and plot voltage vs current. (You can do a least squares approximation on the data if the line is not perfectly straight) Getting a straight line insures that you are actually dealing with linear circuit elements. As V (measured) = - Rth*I + Vth. The slope of this line is the negative of the output impedance (Rth).
Please note that the 50 ohm setting on your AWG is likely *NOT* setting the output impedance. The output impedance is *always* 50 ohms. This setting is telling the AWG what the *load impedance* is that you're connecting to. It uses this to properly set and display the voltage at the load.
@@w2aew Yes I know. When I say I set the output impedance I should have said the input impedance of the load. My Siglent SDX1032X has two options for load impedance 50 ohm and HiZ. When set at 50 ohm the open circuit voltage is twice the displayed voltage and when set at HiZ the open circuit voltage is equal to the displayed (set) voltage. The question I had is what is considered a high load impedance? If we desire less than a 1% discrepancy between the displayed and measured voltages (across the load) we have (at the Hiz setting) Vm (measured) = R*Vset/( R + 50 ). Where Vs is the set (displayed) voltage. If we require Vm to be >= 99% of Vset then R (load) must be greater than 4950 ohms. If, instead, can live with a 5% discrepancy then R must be greater than 950 ohms. In practice I actually tested this out and found that at a 700 ohm load the output voltage was within 5% of the displayed voltage and that at greater than 1750 ohms the output voltage was identical to the displayed voltage. So the question arises what if our actual load impedance is below 1750 ohm (for 1%) or less than 700 ohm (for 5%)? Well with the the awg set to HiZ we have Vm = Vset*R/( R + 50) we can turn this around (setting Vm to the desired voltage) and Vset = (1 + 50/R)*Vdesired. Eg lets say we have a 100 ohm load and desire 4 volts. We would then set the awg to Vset = (1 + 50/100)*4 = 6 volts. OTOH if the awg were set to expect a 50 ohm load and we had a load resistance R other than 50 ohm (but less than 700) then the formula would be 1/2 of this Vset = (0.5 + 25/R)*Vdesired.
Wow, this is great! Have you considered publishing all of your notepad explanations with links to videos (or a companion DVD...)? I think this would make a fantastic workbook for those of us without that elusive EE degree!
OH GOD, finally got that equation. For a very long time, I am just tryin Rload backandforth to get 2Vloaded=Vunloaded, like dumb. Thank you so much w2aew!
Lowering load Z (or R) also shifts the HPF created by output DC blocking capacitor (if any) upwards, so, f must be high enough compared to the cut-off freq of that.
Some active devices instead of running in a resistive mode, run in a constant current mode. For example in a high negative feedback amplifier, the output voltage changes very little with a changing load, so the impedance appears to be very low and the inverse of the impedance called damping factor is used. A common high performance amp with a damping factor of 30 would have an apparent impedance of 1/30th of an ohm or 33 miliohms. However protection first limits current to about 10A(varies) and then the output protection relay or other shutdown trips. Now at 60 volts at 10 Amps, the limit is about 600Watts peak into 6 ohms. Protection kicks in before the output can be dropped to 1/2 voltage.
Perfect, thank you! This helps a diy guy a lot :) When measure a heaphone output that can have something between 1 and 30 Ohms, will it fit when using 50 and 100 Ohm resistors and do the third type of measurement explained in the video?
Thanks Alan for that video, it will definitely be usefull. Correct me if I'm wrong, but with some manipulations there would be a way to determine an input impedance as well, right ? I was thinking to perform the steps you described to evaluate the output impedance of an opamp circuit with 2 resistors, R1 and R2 as you did. Therefore once the opamp output impedance is known, measurements could be performed using the input of the circuit we want to know the input impedance in place of R2. Having access to the opamp output impedance value, along with the V1, R1 and the newly measured V2, we could deduct the value of R2, which would normally be equivalent to the input impedance of the circuit being tested. I understand that if either the opamp output or the tested circuit input input are significally inductive or capacitive, a close attention will need to be paid to the frequency, but generally speaking, I would be lead to believe that what I described could be achieved too. Let me know if that make sense. Keep up with the good work !
When you have a chance, could you do a video that addresses the case where the signal is high-frequency and not sinusoidal -- for example, a high-speed clock (say a few hundred MHz)? Also (or separately), how might one make this measurement on a differential signal (such as LVDS)? Many thanks for your many instructive videos... I've learned a ton both about theory and practice and I'm always excited to see new posts from you. Cheers, Alex
Hi, Alan I guess my question is about the differences (if any) between… 'Output Impedance’ vs. 'Internal Resistance’, ... or ( 'Source Impedance' *). Are each of these terms completely synonymous? or is there enough of a difference between them where they are all needed in order to properly describe slightly different things at work. --- In the class I’m taking we did an experiment to determine the INTERNAL RESISTANCE (R_int) of a voltage supply. The 'SPECIAL CASE’ section of your video here basically covers the same process and formula we used… with the minor difference that… V-open / 2 = R_int (in our experiment). In the experiment we did I used a potentiometer as the R_load,… and adjusted it until V_load = V_open/2. After which, the POT was measured to determine it’s resistor value… and we approximated that to being (R_int). [ ... we ultimately did this for a variety of R_load values… and calculated the Power Differences as R_load values shifted away from matching what R_int is; … and more calculation were done to determine dB gains and losses-relative to the values of the matched pair ( R_int = R_load ). ] I guess In hindsight… we were doing a little more than just finding out what R_int was… it was also about maximizing power transfer, …and maybe starting to flirt with impedance matching too? though, I’m still not 100% solid on understanding everything that impedance is. I have been reading about it (a lot)… but still need more real experience with it, and intuitive understanding… in terms of circuit building, and more hands on testing. Thanks for the Great Channel, Alan… I really LOVE IT. and it’s helping me a lot in my attempts to learn about electronics. Take Care, Jim * My mention of the 'Source Impedance' comes from watching your impedance matching videos, which I'm just now starting to look at. .
Hey, great video. I was wondering if you'd be willing to post the derivation for the second formula. I can't seem to set up that equation from scratch. Much appreciated if you find the time.
Rather than memorizing a formula you can also see that the open circuit voltage equals the internal, Thevenin equivalent voltage since no current flows through the output (Thevenin) resistance. When you add a load resistance, you can divide the new output voltage by the load resistance to find the current flowing through the loop. Also, the change in output voltage MUST be due to Ohm's law applied to the output resistance. Thus, V_loaded = V_open - IR_output or R_output = (V_open - V_loaded)/I. Since I = V_loaded / R_load this yields the formula. ALWAYS the output resistance is R_out = -dV/dI by definition. You need only apply two different loads (resistances), measure the two output voltages, deduce the two output currents, and compute this slope. Be aware that taking the difference between two nearly equal numbers always loses precision so you need to make the two loads as different as practical.
U have a great understanding of how circuits work and how to compare two or more method's of measuring things. Could u show us the sinad and signal to noise methods of measuring receiver sensitivity and any trick or tips u know of how and what to use to make these measurements with equipment on a budget. For example I can remember reading of a method of using an analogue multi tester to monitor carrier level. Thanks Alan
Well your right. Just because I have a three prong could be misleading. I will have to investigate by removing the panel and looking for myself. Thank you Alan.......
The first formula is simply re-arranging the typical voltage divider equation: V1 = Vopen * (R1 / (R1+R2)), solving for R2. The second is starting with the ratio of two of these equations and solving for the source resistance.
Good video, of course the next question that comes to mind is: How do I measure input impedance of device. I have one of those eBay 2.4GHz frequency meter modules without much documentation of course ;) Knowing my signal generators output impedance of 50 ohms, would I place say a 10 turn 10K pot in parallel with the two devices and adjust for half the signal voltage and calculate from there? This may be an easy answer but sleep has been a precious commodity the past few days lol. As always thanks.
A wonderful collection of videos. I could listen to your voice all day... but can you help me with a problem on home made swr meters? I have constructed many types using single or double pcb's.... The forward measurement is spot on but the reflected is always higher. I'm thinking, if the impedance of the stripline is out, that's my basic error to be added to the real error on the test load.... SO. how do I measure the impedance of any swr meter?
hello I just did this test on BK Precision 3011 function generator and I got 480.625 of its output impedance. I checked it on a new Siglent SDX 1102X oscilloscope. What I am trying to do is measure the ESR on 2 caps that are in an old Interstate F-44 High Voltage Sweep Generator that I am trying to repair. And the video I watched said you need a function generator with 50 ohm output impedance. I guess my BK 3011 isn't 50 ohm. The resister I used was a wire wound 1000 ohm 10 watt that is covered in a hard porcelain or pottery substance. And I used cables just like you did in video. I think I will try it with different resistor values, I thought most function generators had 50 ohm output. thanks newbie
According to the specs in this manual, the output impedance is 50ohms. You should recheck your measurements and math. bkpmedia.s3.amazonaws.com/downloads/manuals/en-us/3011A_manual.pdf
Hi, Thank you for your great education videos! Would you please explain how to measure the input impedance of a receiver ( typically with input sensitivity of -110dBm). I tried to do the job using a nano VNA and attenuators, but it seems the measurement is not precise since the nano VNA cannot work with very low level signals. It would be much appreciated if you share your rich knowledge. Thank you!
Hello sir, can you make a video about how to measure the output impedance of RF amplifier circuit ? The methods in this video generally apply to relatively low frequency use cases, I want to know that is threre any approach to measure output impedance at RF frequencies , looking forward to your video, thanks a lot
+w2aew Thanks, yet another masterful video I've found in your archive that makes interesting topics so easy to understand! You mentioned "designing the output impedance" along the way, which got my attention as it is something I'm trying to research. Sorry if I've missed it, but I don't think you've covered this explicitly before? If I could add my vote, it would be great to see you explain how to go about designing for a specific output impedance (say for audio amp or function generator) and what practical circuit changes you might make depending whether the expected next stage is e.g. a low-impedance speaker or higher impedance mixer or power amp.
+pratalife I'll put that on my list. Most often you'd choose the output impedance based on what the circuit needs to drive. A low impedance load (50 ohm coax/load, loudspeaker, etc.) require a low output impedance. For maximum power transfer (if that is the goal) the output impedance should match the load impedance (complex conjugate actually). If the load is a high impedance, then the output impedance of the circuit driving it is less important.
+w2aew cheers, that would be neat. "If the load is a high impedance, then the output impedance of the circuit driving it is less important." ... I think you've already helped;-) I've been casually thinking about (low impedance) opamp circuits possibly plugged into high-impedance inputs and I've been mentally hung-up on the impedance mismatch. But why artificially make my source high-impedance? Doesn't seem to make much sense .. perhaps I need to go and revise power transfer theory! In the meantime, always look forward to your videos no matter the topic .. more often than not, give me something to experiment with for the week;-)
+pratalife Sometimes other factors figure into why the output impedance is what it is - power dissipation requirements, amplifier design constraints, etc.
I've wondered what output impedance is for ... years. Knowing that it is a design choice explains why downstream elements like antennas must match it. I am still struggling to understand in a physical sense what it is. Alan, is there anything else you can say about it? Many thanks.
Dear Alan, I have RF magnitude and phase probe circuit with two toroids connected to 4 diode based peak detectors to give output voltage corresponding to magnitude and phase errors detected. I am somehow not sure how that voltage is mapped to magnitude and phase errors and in what ratio. This circuit is inside a 300W auto matching network which is used to do impedance matching. Load is a sputter source which is used to do sputtering. Generator is a 13.56 MHz, 300W unit. The circuit which I was talking about senses this magnitude and phase error and gives a feedback to a microcontroller which in turn rotates ganged series and shunt capacitor to match the impedance. I just want to understand the circuit better and make know how to make which motor move to get to 50 ohms. I am missing some bits and parts due to which my understanding about this thing is not complete. Please help me understand this thing. Let me know if you want any circuit diagram from or anything else. Thanks !
What is the difference between an impedance curve compared to a frequency response curve? For Speakers there is two curve graphs that are completely different from each other, there is a speaker impedance curve and a frequency response curve. What I'm confused on is that the you input an audio signal from the function generator and sweep a frequency range. The graphs will curve out the speakers frequency response curve but the Impedance curve is completely different. I'm confused because I would think the frequency response curve would show you what the speaker frequency response "profile" is but the speaker impedance curve is actually what the speaker will be not the frequency response curve? What I'm saying is that the speakers frequency response is not really the graph to look at but the impedance curve is, plus they are not related to each other it seems. You sweep 20hz to 20K hz and the speakers frequency response will graph a curve profile from 20hz to 20khz. This is what most people think would be the correct profile. But the speakers impedance curve from 20hz to 20khz is the actual graph profile the speaker will be it seems.
Frequency Response = the audible response of the speaker - in other words, how well it reproduces sound over the specified frequency range. It's what you hear. This is what people want to know when buying a speaker - how well it produces sound at various frequencies. Impedance Curve - the electrical impedance at the speaker terminals vs. frequency - which is the electrical load that the amplifier sees when driving the speaker. This is what the amplifier designer needs to know in order to properly design his amplifier circuit.
I am aircraft technician and I need clarification on the following impedance subject related to ARINC 429 circuits. The transmitting source output impedance should be 75 W ± 5 W divided equally between Line A and Line B (line A and B are twisted pairs i.e 4 wires two for transmission and two for receive). This balanced output should closely match the impedance of the cable. The receiving sink must have an effective input impedance of 8k W minimums. …How is this physically measure?
The output impedance of the collector of a Darlington transistor is higher or lower compared to a bjt's collector output impedance? But I have heard is that a darlington's output impedance on The collector is much lower compared to a bjt's output and complaints on The collector but what is the advantage of the output impedance of a darlington's collector compared to a bjt's output impedance collector?
Hi Alan. I'm building a CW transmitter. I have a buffer amplifier after the crystal oscillator. I tried measuring the output impedance of that stage using the method you described (with two load resistors). I calculated the output impedance . I then tried another pair of resistors (with different values) to see if I come up with the same output impedance. I did not. It was quite different. Could the output impedance change based on the load?
How do you measure the output impedance of high frequency devices, for example a crystal oscillator (if it makes sense) or an amplifier? As usual, thanks for your videos!
Great video, but you fail to mention this is a use of the fundamental idea of a "voltage divider". I only make this comment as it seems like a mystic rule to work out the R_o impedance, when in fact it's just a manipulation of the basic relationship between R_o, R_L, V_o and V_L in the humble voltage divider. A nice experiment. How sensitive would typical transistor amplifiers (common base, emitter, collector versions) be to this sort of probing to find their output impedances?
Yet another excellent video. Can you use the same method to measure the input impedance of lets say a common emitter amplifier or any other type of electronic circuit?
I have a TC Helicon Harmonizer pedal. MIc in, through the effect then back out to mic in amp etc. It works as expected on many different amps, but does not work well with my Fender Acoustic Jr GO. The amp 100W and with the TC pedal, out put volume is lower than an plain acoustic guitar with no amplification. It is not my amp it is an incompatibility with this model of Fender, tried 4 amps. I am suspecting an impedance mis match. TC is 400 ohm out. I have no input data for the Fender but both channels accept XLR for mic or 1/4 unbalanced for a guitar. Anybody's thoughts on what might being going on, and also ways to test/measure or add something between the pedal and amp mic input greatly appreciated. Cheers
Please understand that I really enjoyed your video and you are a good teacher. On top of that did your thumbnail clearly show that your video was about low frequency. But I am looking for high-frequency impedance measurements, do you know someone with your teaching skill level that is covering that subject? :-)
EXCELLENT videos! I love these videos and they mean so much to me that I put most of them into my list of Favorites, or in my "Watch Later" lists. THANK YOU to YOU, and to EVERYONE who puts so much thought, time, effort and energy into making instructional videos of this caliber and on SO many different topics, for us all to FREELY learn from! It is difficult to stay current with the changes, new information, new methods, new technology, new tools, new test equipment, etc, in our personal and in our professional fields of interest, but videos like this that we can watch, listen to, pause, think on and digest at our own individual rates or abilities is just wonderful! My sincere appreciation and thanks to everyone who makes instructional videos or who takes time to teach, mentor, guide, enlighten or help anyone who is interested and wants to know more about ANY worthy craft or skill. You are someone who is doing what you can to make the world a better place. I myself have often been helped, taught, mentored, encouraged and moved on down the path of life by good men and good women such as you. Who knows where or how much further along I might be had I always given you all the time, the respect, you deserved back when I was young? Young people, let this be 'the word to the wise!"
Allen, is there a way to calculate the feed point impedance of an Antenna? Like an EFHW uses a 49:1 impedance transformation. How do one come to a conclusion that for HF bands the feed point impedance of an EFHW is around 2500 Ohms and required a 49:1 impedance transformer? I couldn't find a satisfying reply anywhere.
so ...how does a working circuit with many parts come out to be an of 50/6oo ohms? do they just place a 50/6oo ohms in parallel with entire device?.....taking into account the parallel total impedance...?.
im just a aermodellling hobbyist, right now i use the video transmitter with 5.8ghz freq , i need to know is this method can be used to measure the output impedance for my video trasmitter
Probably not, since most hobbyists don't have a way to measure the amplitude of a 5.8GHz signal. You can safely assume that is is likely 50-75 ohms as most RF outputs are.
Can you make a video of how to bias a one transistor mixer (say BJT - 2N3904) with modulating signal applied at the base and the LO signal applied to the emitter. How to get clean AM signal with no distortion. Say like 12 volts power supply to this simple circuit. How figure out the LO drive and what kind of drive the modulating signal requires. Anyways it is just an idea if you have time. Best regards, Pete
You use the term impedance but only consider that the output impedance is purely resistive. What about the condition where the output "impedance" has reactive [capcitive or inductive] elements along with the resistive element? Should consider taking measurements using more than one frequency.
I’ve got most of the principles of ohms law figured out but I’m working on mastering impedance which some people blow-off and as just being resistance. My interest, as an amp tech, is to develop a checklist of measurements across every amp I fix. I was watching this video on measuring output impedance and am curious to know why, when (@3:14) you switched your scope to 1 m-ohm of input impedance, you identified that as an open circuit (and I know an open circuit causes a voltage spike .. assuming because of the pent up potential). As resistance goes, doesn’t the fact that 1 m-ohm exceeds the 600 ohms of output impedance mean that it applies more resistance? Isn’t 1-m ohm a bigger load? Or, is it “open” because the BNC-T was left unterminated?
I said that 1Mohm was effectively open circuit because it is so much higher than the 600 ohm output impedance of the source that the difference in loading of the 1Mohm load vs. a true open circuit would be negligible. I higher resistance load is a "lighter" load - easier to drive, because it doesn't draw much current. Low resistance loads are "heavier" loads, much harder to drive because they draw a lot of current.
Can I use the second method (about 6:42 on the video) to check the output impedance on my HF transceiver, see if it is truly 50 ohms? Perhaps use a 50 ohm dummy load and measure with my scope then drop in a 100 ohm dummy load and do the same measurement?
When looking at the first stage/front end of an amplifier circuit, how can you tell if the first stage/front end is High impedance or low impedance? how can you tell how "sensitive' the front end will be to a low impedance input signal?
By examination of the circuit you should be able to approximate the input impedance. For the most part, if the amplifier isn't designed for RF use, the input impedance will most likely be relatively high.
@@w2aew If the audio amplifier doesn't have any resistor to ground its just only a decoupling cap connected to Q1 and transistor#2/Q2 feedback resistor is biasing the base of Q1 would this be considered an input that is low impedance or high impedance? Because mostly you put a 1meg resistor to ground on the input jack with the decoupling cap in series to the base of Q1 transistor. The 1meg resistor to ground is mostly what determines the High impedance. But if you don't have any resistor to ground the biasing resistors R1 and R2 set the bias voltage for the base of the Q1 transistor. I don't think R1 and R2 determine the input impedance they only set the biasing voltage of the base. But other amplifier configurations omit R1 and R2 and just use a feedback resistor from Q2/transistor#2 to set the biasing voltage for the base voltage of Q1. The input impedance gets really sensitive to low impedance.
W2AEW, Engineers say that the higher that input impedance the more sensitive the cables capacitance will get rolled off. Example if the input impedance is 1Meg the cables capacitance in picofarads pf won't affect the high frequency roll off as much as if the input impedance is 3meg or 5meg the cables capacitance will Rolled off the high frequencies more. I don't understand how or why higher input impedance will Roll off higher frequencies more depending on the cables capacitance. The input impedance is more sensitive to the cables capacitance the higher the input impedance which I don't understand why the input impedance is more sensitive to cables capacitance the higher the input impedance.
Couldn't you output calculation as " LOAD ohms X (v2/(v1-v2)) = device input impedance " simple instead of oscilloscopes can be used diod bridge made of 4 diodes and a cap of 10uf simply make a switch that passes original signal v1 through switch to volt meter and another position on switch to pass signal including load in known ohms as v2 then voltages us as in my formula above ..in that case formula is spot on and can be used without oscilloscope.. I made such PCB for myself
I'm just an electronics hobbyist and your videos have taught me a lot. If you had been doing these videos 30 years ago I'm sure I would have changed my major to electrical engineering rather than finance. When you retire please consider teaching engineering or math at the college level. You would have a great impact on young minds.
I totally agree
I think he's having a much larger impact here, on youtube, teaching at the "hobbyist level", rather than teaching at the college level. Inspirational. Technical details (unfortunately?) are needed at some point to do advanced things, and those require a formal (university level) training, which can be more boring.
Alan, I often find myself going back to your videos to brush up on calculations I don’t use every day. It’s a great resource.
I wish I could get all the other TH-cam "instructors" to watch your videos. You are so polished. You could make a living instructing the instructors and I wish you would.
I remember watching this video years ago, and I was really in the very early stages of my electronics hobby. I vaguely understood it, but I thought to myself that hey, nice video, but let's be honest, I'll never need to know how to do this...
Well, I was wrong. Today I need to know how to measure output impedance, and I also could use a primer on transmission line termination. w2aew to the rescue!
Your drawings and practical examples have taught me more than anything else. Thank you, genuinely!
Glad to hear it! I hope you found the videos on transmission lines and terminations!
Still valuable and excellent, and it will be even in two decades from now on. A really outstanding quality in terms of clarity, accuracy and style. Can't thank you enough for your work!
I like your videos as they teach the fundamentals with real world, hands-on, models. If you don't already, you would make a great teacher!
Maybe when I retire...
Love that step resistor box. I need to make or buy one. You explained this process much MUCH better than my "engineer" instructor. Thanks Allan
For the beginers What Alan does here is : He devide's 2.5V bij 1000 ohms (1K) of R load. That give the current flows in R load which result 0.0025 Amps (2.5 milli amp's) that will also be current flows in Ro (since current is equal everywhere in a close circuit). We also know that when R load is connected and current start passing it creates 1.5 Voltage drop on Ro 4Volts - 2.5 (V drop on R load that is because the sommof all voltage drops must be 4Volts ). So 1.5V voltage drop on Ro devided withthe current flow's in it will give the value of R out which is 600 Ω (ohms).
I am glad you still have this video posted. The equations used are easy to derive once you think about the relations, but when electronics is a hobby and not a field of study it sure speeds up the process to see it laid out so nicely as in this video.
Hi Alan, great video and tutorial to measure output impedance. Most of us also wants to know how to measure an unknown network impedance from 100kHz to 1GHz or higher but this one can only be achieved using a network analyzer. In my experience in Electronics, EMC and RF, and before I started my ECErcuit TH-cam channel, I been working in understanding how to measure unknown network impedance using spectrum analyser and signal generator. In somehow, I have managed and successfully measured unknown network impedance accurately from 100kHz to at least 100MHz. Measurement was achieved by deriving a formula through a series of derivation and calculations. I automate the system by writing and application to control the spectrum analyser and signal generator to process the measurement against the calibration reference using the formula which gives you the result of impedance plot and data. I'm thinking of sharing this work on my TH-cam channel but I'm not sure if someone is interested to know. Cheers, ECErcuit.
Alan, when I watch your vids, it's like learning all over again. I must admit, I feel that same giggly sensation that I felt when I first learned these principles many years ago. I've found that a huge number of beginning electronics students, hobbyist and on occasion, even some tech's I've spoken to, have had a hard time understanding this. You sir, have accomplished the perfect explanation of the very concept of output impedance and how to measure it in a simple straight forward way.... I'm always amazed by your teaching skills which seem so easy but in fact, are difficult in practice. God indeed blessed you with an outstanding teaching ability and a keen mind as well. I'm so glad that you share this talent with others. It will unlock a new world of understanding for some they never knew existed. Again, Kudos!!! Job Well Done!
Just dropped in to see what was happening. LOL! Had just used the resistance substitution method (5:06) to get output impedance of an amplifier. Thanks again, Alan.
I watch these videos many times. Each tim I do I find something more. Thank you.
Thank goodness for your videos! They help me understand my Measurements and Troubleshooting laboratories.
Another EXCELLENT review!!
If you are still considering suggestions for tutorials, Have you considered FET's and in particular their utility in audio. I have been amazed how well FET's sound in preamps and low power amps. Being a voltage device apparently is a plus at audio freq's. My experience has shown them to rival many tubes I have played with such as 45, 2A3, 6L6 tubes. I have played with MANY transistors and opamps at audio freqs but they sound flat and dull compared to FETs...Many of the people I know are OK with transistor theory but don't understand FET's....Another OUTSTANDING audio device is the nuvistor for low level work like preamps...Microphonic as heck but once tamed, they are VERY nice not to mention fast and reasonably cheap..
And a great big OSCILLATOR to you too...dit dit
Very nice presentation once again. A couple of observations: I did this on a breadboard with the output impedance of my awg set to 50 ohm. I used a small 100 ohm trim pot and adjusted the pot until Vpp on the oscilloscope measured the set voltage on the awg. I got 47 ohm. Not bad. It just seems to me that using a pot makes it much easier to fine tune the load resistance compared to a decade box. Secondly this method assumes that the circuit under test consists of linear circuit elements which can be modeled as a thevenin voltage and resistance. The failure of the first method (2:47) to work all the time may be an indication of this. Using a second load resistance measurement (6:54) may or may not satisfactorily adjust for this. Best way is to measure open circuit voltage (Vth), calculate voltage at several load resistance levels, calculate current V/R at each point and plot voltage vs current. (You can do a least squares approximation on the data if the line is not perfectly straight) Getting a straight line insures that you are actually dealing with linear circuit elements. As V (measured) = - Rth*I + Vth. The slope of this line is the negative of the output impedance (Rth).
Please note that the 50 ohm setting on your AWG is likely *NOT* setting the output impedance. The output impedance is *always* 50 ohms. This setting is telling the AWG what the *load impedance* is that you're connecting to. It uses this to properly set and display the voltage at the load.
@@w2aew Yes I know. When I say I set the output impedance I should have said the input impedance of the load. My Siglent SDX1032X has two options for load impedance 50 ohm and HiZ. When set at 50 ohm the open circuit voltage is twice the displayed voltage and when set at HiZ the open circuit voltage is equal to the displayed (set) voltage. The question I had is what is considered a high load impedance? If we desire less than a 1% discrepancy between the displayed and measured voltages (across the load) we have (at the Hiz setting) Vm (measured) = R*Vset/( R + 50 ). Where Vs is the set (displayed) voltage. If we require Vm to be >= 99% of Vset then R (load) must be greater than 4950 ohms. If, instead, can live with a 5% discrepancy then R must be greater than 950 ohms. In practice I actually tested this out and found that at a 700 ohm load the output voltage was within 5% of the displayed voltage and that at greater than 1750 ohms the output voltage was identical to the displayed voltage. So the question arises what if our actual load impedance is below 1750 ohm (for 1%) or less than 700 ohm (for 5%)? Well with the the awg set to HiZ we have Vm = Vset*R/( R + 50) we can turn this around (setting Vm to the desired voltage) and Vset = (1 + 50/R)*Vdesired. Eg lets say we have a 100 ohm load and desire 4 volts. We would then set the awg to
Vset = (1 + 50/100)*4 = 6 volts. OTOH if the awg were set to expect a 50 ohm load and we had a load resistance R other than 50 ohm (but less than 700) then the formula would be 1/2 of this Vset = (0.5 + 25/R)*Vdesired.
ok I have to say im hooked on your videos. You explain this stuff very clearly. Thank you for your time
Wow, this is great! Have you considered publishing all of your notepad explanations with links to videos (or a companion DVD...)? I think this would make a fantastic workbook for those of us without that elusive EE degree!
Exactly the tutorial I was looking for.
Explained very well.
OH GOD, finally got that equation. For a very long time, I am just tryin Rload backandforth to get 2Vloaded=Vunloaded, like dumb. Thank you so much w2aew!
Man you rock! Great at explaining things. Very good teacher. Thanks!
Nice tutorial. You're a good instructor. Thanks for showing us.
Thank you for making this video.I just used it to measure the impedance of my flea market function generator.
Gotta say it. You have brilliant Forest Mims style graf paper skillz.
"Oscilater"
That... that actually hertz my head.
what happened to amplifier? oscillator. I can think of some more, probably wouldn't want to say them.... jim
Great video! Very well put together, perfectly explained, and super useful information. Thanks for your work!!
Lowering load Z (or R) also shifts the HPF created by output DC blocking capacitor (if any) upwards, so, f must be high enough compared to the cut-off freq of that.
Fantastic tutorial! I just discovered your channel.... now I gotta watch all your videos. Thanks....
I always learn something from your videos. Great work. Thanks!
Excellent! Thanks for posting this !
Just found your channel. Very interesting and informative. Also, I REALLY like that scope :)
Damn...that osciloscope is a beaaaauty!
Digital osciloscope with a crt...beaaauty!
Thanks. This is helpful. Best wishes from New Zealand.
your videos are indispensable. thank you.
Some active devices instead of running in a resistive mode, run in a constant current mode. For example in a high negative feedback amplifier, the output voltage changes very little with a changing load, so the impedance appears to be very low and the inverse of the impedance called damping factor is used. A common high performance amp with a damping factor of 30 would have an apparent impedance of 1/30th of an ohm or 33 miliohms. However protection first limits current to about 10A(varies) and then the output protection relay or other shutdown trips. Now at 60 volts at 10 Amps, the limit is about 600Watts peak into 6 ohms. Protection kicks in before the output can be dropped to 1/2 voltage.
Great video. Love the HP-15C!
Do you have a PDF of all of your notes ? This would be great as either an e-book. Great videos.
Occ-il-Later, you be a funny man. It's the imaginary component that trips me up when trying to figure out Impedance for RF
Perfect, thank you! This helps a diy guy a lot :)
When measure a heaphone output that can have something between 1 and 30 Ohms, will it fit when using 50 and 100 Ohm resistors and do the third type of measurement explained in the video?
Great video. Thanks for sharing your knowledge.
Thanks Alan for that video, it will definitely be usefull.
Correct me if I'm wrong, but with some manipulations there would be a way to determine an input impedance as well, right ?
I was thinking to perform the steps you described to evaluate the output impedance of an opamp circuit with 2 resistors, R1 and R2 as you did. Therefore once the opamp output impedance is known, measurements could be performed using the input of the circuit we want to know the input impedance in place of R2.
Having access to the opamp output impedance value, along with the V1, R1 and the newly measured V2, we could deduct the value of R2, which would normally be equivalent to the input impedance of the circuit being tested.
I understand that if either the opamp output or the tested circuit input input are significally inductive or capacitive, a close attention will need to be paid to the frequency, but generally speaking, I would be lead to believe that what I described could be achieved too.
Let me know if that make sense.
Keep up with the good work !
Nice and straightforward.
really awesome tutorial video for me. I have been wondered this issue for long time. Thanks.
When you have a chance, could you do a video that addresses the case where the signal is high-frequency and not sinusoidal -- for example, a high-speed clock (say a few hundred MHz)?
Also (or separately), how might one make this measurement on a differential signal (such as LVDS)?
Many thanks for your many instructive videos... I've learned a ton both about theory and practice and I'm always excited to see new posts from you.
Cheers,
Alex
th-cam.com/video/g_jxh0Qe_FY/w-d-xo.html
Hi, Alan
I guess my question is about the differences (if any) between… 'Output Impedance’ vs. 'Internal Resistance’, ... or ( 'Source Impedance' *).
Are each of these terms completely synonymous? or is there enough of a difference between them where they are all needed in order to properly describe slightly different things at work.
---
In the class I’m taking we did an experiment to determine the INTERNAL RESISTANCE (R_int) of a voltage supply.
The 'SPECIAL CASE’ section of your video here basically covers the same process and formula we used… with the minor difference that… V-open / 2 = R_int (in our experiment).
In the experiment we did I used a potentiometer as the R_load,… and adjusted it until V_load = V_open/2.
After which, the POT was measured to determine it’s resistor value… and we approximated that to being (R_int).
[ ... we ultimately did this for a variety of R_load values… and calculated the Power Differences as R_load values shifted away from matching what R_int is; … and more calculation were done to determine dB gains and losses-relative to the values of the matched pair ( R_int = R_load ). ]
I guess In hindsight… we were doing a little more than just finding out what R_int was… it was also about maximizing power transfer, …and maybe starting to flirt with impedance matching too? though, I’m still not 100% solid on understanding everything that impedance is. I have been reading about it (a lot)… but still need more real experience with it, and intuitive understanding… in terms of circuit building, and more hands on testing.
Thanks for the Great Channel, Alan… I really LOVE IT. and it’s helping me a lot in my attempts to learn about electronics.
Take Care,
Jim
* My mention of the 'Source Impedance' comes from watching your impedance matching videos, which I'm just now starting to look at.
.
In general, the "internal resistance" and "output impedance" in this context are basically the same thing.
Very clear explained. thanks
Hey, great video. I was wondering if you'd be willing to post the derivation for the second formula. I can't seem to set up that equation from scratch. Much appreciated if you find the time.
Rather than memorizing a formula you can also see that the open circuit voltage equals the internal, Thevenin equivalent voltage since no current flows through the output (Thevenin) resistance. When you add a load resistance, you can divide the new output voltage by the load resistance to find the current flowing through the loop. Also, the change in output voltage MUST be due to Ohm's law applied to the output resistance.
Thus, V_loaded = V_open - IR_output or R_output = (V_open - V_loaded)/I. Since
I = V_loaded / R_load this yields the formula.
ALWAYS the output resistance is R_out = -dV/dI by definition. You need only apply two different loads (resistances), measure the two output voltages, deduce the two output currents, and compute this slope. Be aware that taking the difference between two nearly equal numbers always loses precision so you need to make the two loads as different as practical.
U have a great understanding of how circuits work and how to compare two or more method's of measuring things. Could u show us the sinad and signal to noise methods of measuring receiver sensitivity and any trick or tips u know of how and what to use to make these measurements with equipment on a budget. For example I can remember reading of a method of using an analogue multi tester to monitor carrier level. Thanks Alan
I use the same HP 15, thanks.
Well your right. Just because I have a three prong could be misleading. I will have to investigate by removing the panel and looking for myself. Thank you Alan.......
Very well explained. Is there a derivation for the first and last formula?
The first formula is simply re-arranging the typical voltage divider equation:
V1 = Vopen * (R1 / (R1+R2)), solving for R2. The second is starting with the ratio of two of these equations and solving for the source resistance.
***** That's perfect. Thank you!
Good video, of course the next question that comes to mind is: How do I measure input impedance of device. I have one of those eBay 2.4GHz frequency meter modules without much documentation of course ;) Knowing my signal generators output impedance of 50 ohms, would I place say a 10 turn 10K pot in parallel with the two devices and adjust for half the signal voltage and calculate from there? This may be an easy answer but sleep has been a precious commodity the past few days lol. As always thanks.
A wonderful collection of videos.
I could listen to your voice all day...
but can you help me with a problem on home made swr meters?
I have constructed many types using single or double pcb's....
The forward measurement is spot on but the reflected is always higher.
I'm thinking, if the impedance of the stripline is out, that's my basic error
to be added to the real error on the test load....
SO. how do I measure the impedance of any swr meter?
Another very good video! Thanks very much Alan!
hello
I just did this test on BK Precision 3011 function generator and I got 480.625 of its output impedance. I checked it on a new Siglent SDX 1102X oscilloscope. What I am trying to do is measure the ESR on 2 caps that are in an old Interstate F-44 High Voltage Sweep Generator that I am trying to repair. And the video I watched said you need a function generator with 50 ohm output impedance. I guess my BK 3011 isn't 50 ohm.
The resister I used was a wire wound 1000 ohm 10 watt that is covered in a hard porcelain or pottery substance. And I used cables just like you did in video.
I think I will try it with different resistor values, I thought most function generators had 50 ohm output.
thanks
newbie
According to the specs in this manual, the output impedance is 50ohms. You should recheck your measurements and math.
bkpmedia.s3.amazonaws.com/downloads/manuals/en-us/3011A_manual.pdf
Thank you. The length was for just bench work mostly.
Hi, Thank you for your great education videos! Would you please explain how to measure the input impedance of a receiver ( typically with input sensitivity of -110dBm). I tried to do the job using a nano VNA and attenuators, but it seems the measurement is not precise since the nano VNA cannot work with very low level signals. It would be much appreciated if you share your rich knowledge. Thank you!
👍Thank you sir.
Great video, sir. But how to measure the output impedance of an RF signal generator with a Directional Coupler and a Spectrum Analyzer?
Increase the load until you see a 6 dB power drop. That's the output impedance.
Hello sir, can you make a video about how to measure the output impedance of RF amplifier circuit ? The methods in this video generally apply to relatively low frequency use cases, I want to know that is threre any approach to measure output impedance at RF frequencies , looking forward to your video, thanks a lot
Thanks for all the videos!
+w2aew Thanks, yet another masterful video I've found in your archive that makes interesting topics so easy to understand!
You mentioned "designing the output impedance" along the way, which got my attention as it is something I'm trying to research. Sorry if I've missed it, but I don't think you've covered this explicitly before?
If I could add my vote, it would be great to see you explain how to go about designing for a specific output impedance (say for audio amp or function generator) and what practical circuit changes you might make depending whether the expected next stage is e.g. a low-impedance speaker or higher impedance mixer or power amp.
+pratalife I'll put that on my list. Most often you'd choose the output impedance based on what the circuit needs to drive. A low impedance load (50 ohm coax/load, loudspeaker, etc.) require a low output impedance. For maximum power transfer (if that is the goal) the output impedance should match the load impedance (complex conjugate actually). If the load is a high impedance, then the output impedance of the circuit driving it is less important.
+w2aew cheers, that would be neat. "If the load is a high impedance, then the output impedance of the circuit driving it is less important." ... I think you've already helped;-) I've been casually thinking about (low impedance) opamp circuits possibly plugged into high-impedance inputs and I've been mentally hung-up on the impedance mismatch. But why artificially make my source high-impedance? Doesn't seem to make much sense .. perhaps I need to go and revise power transfer theory!
In the meantime, always look forward to your videos no matter the topic .. more often than not, give me something to experiment with for the week;-)
+pratalife Sometimes other factors figure into why the output impedance is what it is - power dissipation requirements, amplifier design constraints, etc.
I've wondered what output impedance is for ... years. Knowing that it is a design choice explains why downstream elements like antennas must match it. I am still struggling to understand in a physical sense what it is. Alan, is there anything else you can say about it? Many thanks.
Dear Alan,
I have RF magnitude and phase probe circuit with two toroids connected to 4 diode based peak detectors to give output voltage corresponding to magnitude and phase errors detected. I am somehow not sure how that voltage is mapped to magnitude and phase errors and in what ratio. This circuit is inside a 300W auto matching network which is used to do impedance matching. Load is a sputter source which is used to do sputtering. Generator is a 13.56 MHz, 300W unit. The circuit which I was talking about senses this magnitude and phase error and gives a feedback to a microcontroller which in turn rotates ganged series and shunt capacitor to match the impedance. I just want to understand the circuit better and make know how to make which motor move to get to 50 ohms.
I am missing some bits and parts due to which my understanding about this thing is not complete. Please help me understand this thing. Let me know if you want any circuit diagram from or anything else. Thanks !
Nicely done, as always.
What is the difference between an impedance curve compared to a frequency response curve? For Speakers there is two curve graphs that are completely different from each other, there is a speaker impedance curve and a frequency response curve. What I'm confused on is that the you input an audio signal from the function generator and sweep a frequency range. The graphs will curve out the speakers frequency response curve but the Impedance curve is completely different. I'm confused because I would think the frequency response curve would show you what the speaker frequency response "profile" is but the speaker impedance curve is actually what the speaker will be not the frequency response curve? What I'm saying is that the speakers frequency response is not really the graph to look at but the impedance curve is, plus they are not related to each other it seems. You sweep 20hz to 20K hz and the speakers frequency response will graph a curve profile from 20hz to 20khz. This is what most people think would be the correct profile. But the speakers impedance curve from 20hz to 20khz is the actual graph profile the speaker will be it seems.
Frequency Response = the audible response of the speaker - in other words, how well it reproduces sound over the specified frequency range. It's what you hear. This is what people want to know when buying a speaker - how well it produces sound at various frequencies.
Impedance Curve - the electrical impedance at the speaker terminals vs. frequency - which is the electrical load that the amplifier sees when driving the speaker. This is what the amplifier designer needs to know in order to properly design his amplifier circuit.
Nice handwriting! Seriously, sure not calligraphy, but I always struggle to make my diagrams look neat like that.
Excellent video. Thank you!
How does one measure the output impedance of an RF circuit (oscillator / amplifier) without fancy SWR meters?
Awesome video as always. Thanks a ton!!
It is not an easy thing to do, unfortunately. Most common techniques are with a vector network analyzer or with a load-pull test set.
@@w2aew Alrighty thanks!
I am aircraft technician and I need clarification on the following impedance subject related to ARINC 429 circuits. The transmitting source output impedance should be 75 W ± 5 W divided equally between Line A and Line B (line A and B are twisted pairs i.e 4 wires two for transmission and two for receive). This balanced output should closely match the impedance of the cable. The receiving sink must have an effective input impedance of 8k W minimums. …How is this physically measure?
None of those measurements are impedances.
The output impedance of the collector of a Darlington transistor is higher or lower compared to a bjt's collector output impedance? But I have heard is that a darlington's output impedance on The collector is much lower compared to a bjt's output and complaints on The collector but what is the advantage of the output impedance of a darlington's collector compared to a bjt's output impedance collector?
Hi Alan. I'm building a CW transmitter. I have a buffer amplifier after the crystal oscillator. I tried measuring the output impedance of that stage using the method you described (with two load resistors). I calculated the output impedance . I then tried another pair of resistors (with different values) to see if I come up with the same output impedance. I did not. It was quite different. Could the output impedance change based on the load?
you're just so amaizing to teach us thus way
How about testing the output impedance of a ham radio in the HF range from 160 meters to 10 meters?
How do you measure the output impedance of high frequency devices, for example a crystal oscillator (if it makes sense) or an amplifier?
As usual, thanks for your videos!
Great video, but you fail to mention this is a use of the fundamental idea of a "voltage divider". I only make this comment as it seems like a mystic rule to work out the R_o impedance, when in fact it's just a manipulation of the basic relationship between R_o, R_L, V_o and V_L in the humble voltage divider. A nice experiment. How sensitive would typical transistor amplifiers (common base, emitter, collector versions) be to this sort of probing to find their output impedances?
Another great video. Thanks!
Thanks Alan. Great video.
Yet another excellent video. Can you use the same method to measure the input impedance of lets say a common emitter amplifier or any other type of electronic circuit?
Mind doing a video on building V/I Curve Tracer?
Thank you so much. I certainly did get valuable information from this video. A++
I have a TC Helicon Harmonizer pedal. MIc in, through the effect then back out to mic in amp etc.
It works as expected on many different amps, but does not work well with my Fender Acoustic Jr GO.
The amp 100W and with the TC pedal, out put volume is lower than an plain acoustic guitar with no amplification. It is not my amp it is an incompatibility with this model of Fender, tried 4 amps.
I am suspecting an impedance mis match. TC is 400 ohm out. I have no input data for the Fender but both channels accept XLR for mic or 1/4 unbalanced for a guitar.
Anybody's thoughts on what might being going on, and also ways to test/measure or add something between the pedal and amp mic input greatly appreciated. Cheers
Hello Alan, thank you for your great explanations. Is it possible to put a link with the schematics of your generator ?
I built that homemade generator over 25 years ago, and the schematic is lost unfortunately.
Please understand that I really enjoyed your video and you are a good teacher. On top of that did your thumbnail clearly show that your video was about low frequency. But I am looking for high-frequency impedance measurements, do you know someone with your teaching skill level that is covering that subject? :-)
High frequency impedance measurements are typically done with a VNA (vector network analyzer)
EXCELLENT videos! I love these videos and they mean so much to me that I put most of them into my list of Favorites, or in my "Watch Later" lists.
THANK YOU to YOU, and to EVERYONE who puts so much thought, time, effort and energy into making instructional videos of this caliber and on SO many different topics, for us all to FREELY learn from!
It is difficult to stay current with the changes, new information, new methods, new technology, new tools, new test equipment, etc, in our personal and in our professional fields of interest, but videos like this that we can watch, listen to, pause, think on and digest at our own individual rates or abilities is just wonderful!
My sincere appreciation and thanks to everyone who makes instructional videos or who takes time to teach, mentor, guide, enlighten or help anyone who is interested and wants to know more about ANY worthy craft or skill.
You are someone who is doing what you can to make the world a better place. I myself have often been helped, taught, mentored, encouraged and moved on down the path of life by good men and good women such as you. Who knows where or how much further along I might be had I always given you all the time, the respect, you deserved back when I was young? Young people, let this be 'the word to the wise!"
Very good video and I like the new camera. OCIL LATER.
Allen, is there a way to calculate the feed point impedance of an Antenna? Like an EFHW uses a 49:1 impedance transformation. How do one come to a conclusion that for HF bands the feed point impedance of an EFHW is around 2500 Ohms and required a 49:1 impedance transformer? I couldn't find a satisfying reply anywhere.
That's exactly what an antenna analyzer does.
so ...how does a working circuit with many parts come out to be an of 50/6oo ohms? do they just place a 50/6oo ohms in parallel with entire device?.....taking into account the parallel total impedance...?.
Usually the output driver is low impedance, and a resistor is added in series to get to 50 ohms.
Loved it!
Very useful. Thank you!
im just a aermodellling hobbyist, right now i use the video transmitter with 5.8ghz freq , i need to know is this method can be used to measure the output impedance for my video trasmitter
Probably not, since most hobbyists don't have a way to measure the amplitude of a 5.8GHz signal. You can safely assume that is is likely 50-75 ohms as most RF outputs are.
Can you make a video of how to bias a one transistor mixer (say BJT - 2N3904)
with modulating signal applied at the base and the LO signal applied to the emitter. How to get clean AM signal with no distortion. Say like 12 volts power supply to this simple circuit. How figure out the LO drive and what kind of drive the modulating signal requires. Anyways it is just an idea if you have time.
Best regards,
Pete
You use the term impedance but only consider that the output impedance is purely resistive. What about the condition where the output "impedance" has reactive [capcitive or inductive] elements along with the resistive element? Should consider taking measurements using more than one frequency.
I’ve got most of the principles of ohms law figured out but I’m working on mastering impedance which some people blow-off and as just being resistance. My interest, as an amp tech, is to develop a checklist of measurements across every amp I fix. I was watching this video on measuring output impedance and am curious to know why, when (@3:14) you switched your scope to 1 m-ohm of input impedance, you identified that as an open circuit (and I know an open circuit causes a voltage spike .. assuming because of the pent up potential). As resistance goes, doesn’t the fact that 1 m-ohm exceeds the 600 ohms of output impedance mean that it applies more resistance? Isn’t 1-m ohm a bigger load? Or, is it “open” because the BNC-T was left unterminated?
I said that 1Mohm was effectively open circuit because it is so much higher than the 600 ohm output impedance of the source that the difference in loading of the 1Mohm load vs. a true open circuit would be negligible. I higher resistance load is a "lighter" load - easier to drive, because it doesn't draw much current. Low resistance loads are "heavier" loads, much harder to drive because they draw a lot of current.
@@w2aew Oh, doh. Not even sure why I asked. So obvious now. Thank you.
Can I use the second method (about 6:42 on the video) to check the output impedance on my HF transceiver, see if it is truly 50 ohms? Perhaps use a 50 ohm dummy load and measure with my scope then drop in a 100 ohm dummy load and do the same measurement?
Yes, that would work, provided your scope doesn't alter the impedance.
Interesting way to sine off.
When looking at the first stage/front end of an amplifier circuit, how can you tell if the first stage/front end is High impedance or low impedance? how can you tell how "sensitive' the front end will be to a low impedance input signal?
By examination of the circuit you should be able to approximate the input impedance. For the most part, if the amplifier isn't designed for RF use, the input impedance will most likely be relatively high.
@@w2aew If the audio amplifier doesn't have any resistor to ground its just only a decoupling cap connected to Q1 and transistor#2/Q2 feedback resistor is biasing the base of Q1 would this be considered an input that is low impedance or high impedance? Because mostly you put a 1meg resistor to ground on the input jack with the decoupling cap in series to the base of Q1 transistor. The 1meg resistor to ground is mostly what determines the High impedance. But if you don't have any resistor to ground the biasing resistors R1 and R2 set the bias voltage for the base of the Q1 transistor. I don't think R1 and R2 determine the input impedance they only set the biasing voltage of the base. But other amplifier configurations omit R1 and R2 and just use a feedback resistor from Q2/transistor#2 to set the biasing voltage for the base voltage of Q1. The input impedance gets really sensitive to low impedance.
W2AEW, Engineers say that the higher that input impedance the more sensitive the cables capacitance will get rolled off. Example if the input impedance is 1Meg the cables capacitance in picofarads pf won't affect the high frequency roll off as much as if the input impedance is 3meg or 5meg the cables capacitance will Rolled off the high frequencies more. I don't understand how or why higher input impedance will Roll off higher frequencies more depending on the cables capacitance. The input impedance is more sensitive to the cables capacitance the higher the input impedance which I don't understand why the input impedance is more sensitive to cables capacitance the higher the input impedance.
Couldn't you output calculation as " LOAD ohms X (v2/(v1-v2)) = device input impedance " simple instead of oscilloscopes can be used diod bridge made of 4 diodes and a cap of 10uf simply make a switch that passes original signal v1 through switch to volt meter and another position on switch to pass signal including load in known ohms as v2 then voltages us as in my formula above ..in that case formula is spot on and can be used without oscilloscope.. I made such PCB for myself
awesome, very simple, Thanks.