#380
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- เผยแพร่เมื่อ 28 ก.ย. 2024
- This circuit fun video presents the Twin-Tee op amp oscillator - a fun circuit that can make low distortion (low THD) sine waves with simple resistors and capacitors, no inductors. It is a circuit you don't see written up too much.
Here is a link to the Texas Instruments article shown in the video:
www.ti.com/lit/...
More appnotes on sine wave oscillators...
www.ti.com/lit...
www.ti.com/lit...
My show notes:
www.qsl.net/w2a...
Related FFT and THD videos on my channel:
• #65: Basics of using F...
• #305: Measuring Total ...
• #202: Basics of using ...
Here's an abstract for the transcript, focusing on the electrical engineering aspects of the video: (generated with gemini ultra)
*Abstract*
This video explores the design and implementation of a low-distortion sine wave oscillator using op-amps, resistors, and capacitors. The focus is on the twin-T oscillator, a design noted for its simplicity and lack of inductors. The video begins with a discussion of oscillator theory and then compares the twin-T oscillator to other common types of RC op-amp oscillators (relaxation, phase shift, Wien bridge). A twin-T notch filter is constructed and tested with a Bode plot to demonstrate its frequency response. Finally, the twin-T network is incorporated into an op-amp oscillator circuit. The oscillator output's low distortion is demonstrated through visual observation, FFT analysis, and a THD measurement.
*Keywords:* oscillator, op-amp, sine wave, twin-T, distortion, RC filter, feedback
Very eloquent! Pinned comment to the top!
I'm a retired electronic engineer, but I still learn something new from most of Alan's videos. He's a superb teacher. He prepares everything well in advance so that there's no fumbling about and he explains just the right amount of detail to make the videos useful. His pace is just right too, for me at least.
You make a great teacher! Around 2001 I was taking my first EE course in college. I wanted to learn digital electronics, but couldn't wrap my head around analog concepts. I soon switched majors to computer science and didn't look back for 20 years. Then about 3 years ago I was bored during lockdown and started teaching myself electronics, mostly by watching TH-cam videos and by experimenting with my new oscilloscope. At first I could barely explain the function of a resistor, but I've now made a few successful mixed signal PCBs (mostly in the digital domain, but there's some RC and LC filters and comparators in there). It's neat for me to now see this kind of video and immediately come away with a good understanding of the circuit. This is all to say, thank you and other TH-camrs for making this kind of content!
I got a degree in electrical engineering in 2017 with a big focus in RF.
I haven't used a lick of it since then as I ended up a draftsman in mechanical eng. Like learning another language it can fade quickly unused. But I want to get into it again and you have been a huge resource for me to refresh and learn a lot from among some other channels. Many thanks for everything.
Yep, this channel is a goldmine! I’ve learned a TON from watching his videos too.
That is basically my story. Now that the kids don’t need me that much anymore i got myself back into EE and got my amateur radio licence. And Alan is one of the best teachers around
It is always a pleasure to follow your videos and explanations. Many thanks for that
This channel, Old Hack EE, Joe Smith's channel, and the DC to Daylight series that Element14 is doing are the channels I'm leaning on to keep my brain refreshed on all the stuff I haven't really been using since graduating
Nice scope Alan
I too, developed scope envy watching this!
Seeing others saying they wish they had these kinds of videos back in their time makes me nothing but more grateful to have access to these practical demonstrations. Please keep making videos like these.
Circuit Fun is my favorite segment of your channel!!! Please have more!!!
That's a nice video about sine wave oscillators. Back in the eighties in my 2nd year at university we had to design, build, and test our own sine wave oscillator. It had to be able to work in the frequency range from about 10Hz up to 100kHz. I chose to work with the Wien bridge principle, and that went well. Like you mention in your video, I used a FET as the controlling element. Most of the work went into the design of characterizing the FET, and creating a stable feedback loop, especially at the lower end of the frequency range. My design resembles a bit the design that you can find in the paper you mentioned. Distortion was below 0.5% between 100Hz and 20kHz, and it went up to just above 1% at the bottom and top end.
Beautifully simple circuit with such a clean sinusoid, and that scope is pretty sick, great video. Loved seeing the bode plots and the harmonics
...a "sick" scope?!
You're amazing for supplying these literature goldmines.
Ti holds a library of amazing documentation but its hard to find, awesome you put these out there.
More than greatly appreciated
Your circuit fun and basics videos are the best, very helpful to people learning electronics. Thank you.
I to am a retired EE and enjoy your videos. Keep up the good work.
I discovered this oscillator back in college while using twin-Tees to measure distortion in my homebrew amplifiers. I always needed a little positive feedback to ensure oscillation, which is different from what this fellow achieved. The good part about the positive feedback is that you can add AGC to keep the wave from hitting the rails and distorting.
Great circuit. Simple and elegant. Whish i had known about it before. Perfect for synchro resolver excitation applications.
BSEE 1975 retired.
*I like your videos. One of the key structures they demonstrate is there are single topics that can be tackled in both a scholastic and lab environment, and you don't need to spend weeks on it.* Once you have intellectually secured a structure in most the fundamentals and math for the science of engineering, circuit analysis coupled with a lab will adequately help us advance.
I was just studying Twin-T RC networks and associated the last couple of months for audio applications, so an oscillator relative is a great topic for my brain's configuration ATM. In music audio, T-notch filters are abundant in modern equipment. Even with them incorporated into common tone architecture both fixed and variable, *you still constantly see production engineers in recording studios mastering songs, and they're scooping the midrange out of the instruments to balance out the tonal structure and make it more pleasing to our ears.*
I think a lot of that need is running through so many active circuits, and the midrange one piece of gear notched out is brought back into the output bandwidth by another because such is the nature of amplification stages, *even when it's unity gain.*
Have not seen your videos pop up on my feed lately. Glad to see this one. It was very educational. Thank you.
I really enjoy these circuit descriptions, Alan. Thanks for sharing them!
The limitation of tuning to a desired frequency is quite severe. Built these in the early '60s because I wanted a low THD sine wave for audio testing @ 1 kHz. Took days finding all sorts of MIL-surplus precision capacitors and resistors. The common carbon composition resistors back then were 5% and paper capacitors at 10% in the sub-uF range. I finally found some 0.1% Rs and some 2% Cs, but still too far from the desired frequency. So I spent days adding small resistors and capacitors in series and parallel to get to 1 kHz.
Also in the '60s, Popular Electronics Magazine had an article to make "Electronic Bongos" using Twin T oscillators biased just bellow oscillation and triggered by a touch pad and a differentiator. This made a damped sine wave which with some imagination sounded like small bongos.
Even today, generally available parts precision ranking is: #1 = Rs (1% common, 0.1% quite available) #2 = Cs (5% std, 2% available) and #3 = Ls (10% std., 5% available). Yes, all are available better than 0.01%, but they are often un-obtanium and un-affordium.
Keep up the great work! I was often called an OFAE, "Old Fart Analog Engineer", even in the last century, when I wasn't that old.
Great clip again,
Here is more research work:
It would be good to compare the phase noise and jitter of different oscillators.
Also, It also would be good to develop a formula for Q factor twin T filter based on R & C. That is how to get the best Q, when the product of R and C is constant so we set the frequency with that. Which one gives us the best Q when Xc>R or Xc
Wow, that’s quite elegant. Never heard of this type. Good paper, too!
It's a nice experiment, and I've done similar experiments, my conclusion is that it's very hard to beat even cheap DAC based devices. Something like the Scarlett 2 channel audio interface will output a 0.002% distortion sinewave at any frequency from 20Hz to 20kHz. I have not been able to achieve anything like that with any oscillator based circuit.
In the 1980's I built a portable audio mixer to operate with a 16mm camera. It needed a bloop generator to put a synchronising burst of 1khz on the tape to coincide with a white frame on the film.
I used a Wien oscillator with a lamp for stabilising the frequency. Worked well.
Very interesting video, Alan! A pleasure to watch, with lots of information. Thanks!
Merci beaucoup, j'ai travaillé beaucoup sur les oscillateurs sinusoïdaux, je ne connaissais pas celui là
Excellent video Alan.
Great video lesson. My customer wanted to see a slow sinewave so, I dug into my Heath electronics course and built a twin tee for 1/9 Hz or so. My old Tek 464 with variable persistence displayed a smooth sinewave. I had to use large capacitors.
When I worked as a product design engineer for an audiological manufacturer, they needed a variable frequency, audio oscillator whose frequency could be set by the processor. I "inherited" a design which used a switched-capacitor, bandpass filter as the feedback element with a "wide-open" op-amp. The BIG issue we had wasn't THD, but *phase* distortion that was totally hosing up audiological measurements.
As a little trivia background ... we would put two sinusoidal signals into the ear (via tiny speakers). These two signals would mix in the inner ear (a non-linear system) and a third, related signal would return to the probe (to a tiny microphone). We would measure the amplitude of the returning signal to determine the health of the entire audio chain from eardrum to inner ear and back. Pretty slick, eh?
Like what Nura does?
Snazzy scope, Alan. This was a great video and I can’t wait to build it up
Always nice to see that theory matches reality. Btw.: nice scope
This reminds me of using an RLC tuned circuit to obtain 50VPP for fiber optics modulators.
Cool little circuit design! Sometimes I see designs like this and wonder why didn’t I think of that??? 🤔 Lol
I don’t think this has been asked, but how about an in-depth discussion of stability for this and/or similar opamp circuits? Textbook stuff, but your explanations are so much better. Which is why I refer every Elec and Computer engineering student I can to your channel.
Cristal clear explained, thanks Mister
Excellent video as always. Much appreciated :)
Have you ever discussed quadrature oscillators? This form of oscillator has the potential for various applications, including music synthesizers, signal scrambling, frequency shifters, ham and others.
...not yet...
Page 36, same TI paper.
Love your circuit fun videos! Wonder how low the THD would be with an additional filter stage at the output to attenuate the harmonics. Pretty fun circuit to play with.
Thanks for another informative video. I have an old book by Ray Marston which shows a similar circuit using a 741 op-amp, so the type of op-amp doesn't appear to be critical.
Really cool! Thanks for showing this!
Cool scope, nice informational video and thnx for the document.
So interesting and great fun to build.
👍 Would be interesting to compare it in terms of THD with a Wien bridge oscillator that uses diodes (not a lamp).
Haven't understand much since i am just learning, BUT the scope is a top notch! :) Thank you.
fun video, clean and informative also feel comfortable watching them!
Started looking through some of my Op Amp specific books and was surprised that this was not mentioned in any of them except for "Operational Amplifiers: Theory and Servicing" by Edward Bannon. I do have probably hundreds of electronics related books and maybe this circuit was mentioned in a few of them, but was surprised at how little information there was in print. I'll have to pull out the breadboard and play around with this when I get some time.
Lovely piece of work. Thanks.
Thanks for the video. Impressive distortion figures.
You always do an excellent job. Thanks so much.
Excellent video. Thanks.
Another great tip, thanks Alan! 73 - Dino KLØS
In most cases, the oscillator part is easy-ish, the amplitude stabilization part dominates the distortion of those low-frequency oscillators. Any op-amp oscillator without explicit amplitude control is hitting op-amp nonlinearities to limit amplitude. If the resonator isn’t very high-Q, the distortion is pretty bad. A scope with less than 16 bits of ADC resolution isn’t the best tool to measure such distortion.
Yeah, a 50dB notch ain’t doing low distortion anything much without an amplitude control loop. A 100dB deep notch (good luck) will do the trick without an ALC. A crystal resonator will do that job admirably. But a twin-tee? Nope. It’s not like only the Wien bridge needs amplitude control. They all need it unless a very high-Q filter is used.
It is possible to have >110dB null with Twin-T, but the tempco's drift it too fast without complicated correction. Bridged-T is simpler and 75dB is easy across the audio band with cheap NE5532 (10:1 cap ratio with a dual linear pot).
SFDR of the scope input amplifiers is typically the limiting factor for distortion measurement. If you notch the fundamental (60+ dB) and only look at harmonics, 8bit scope FFT can see minus 100dB! I use a bridged-T Q=3 with a 12bit scope and have a thd floor about -130dB re 1vrms.
I have been trying to understand op amps on and off for the last 35 years. I am still not there. I appear to be dense to this material. Smart, but this intersects with some weakness of mine.
I can tell all of the information I need is in this video and in #75 Basics of Op Amp Circuits. I am going to have to watch them again. Thank you very much. de w0it
Ron Mancini! Yeah, I know that article. :-)))
One time I've built a 500kHz oscilator on a single OpAmp (embeded in MC3361, the filter amp) by his design.
Simple, elegant, practical. Thanks. 73
This is a great video! Amazing
Thank you, excellent concise explanation as always.
For some more fun with this circuit, set it up so that it doesn't quite oscillate, and then when you kick it with a pulse it'll give a nice effect, like ringing a bell or similar.
I have done that and it is quite nice driving a speaker as a "bell" circuit. You can also set it up so that it clips when the signal is large and is sounds even more like a real bell. Bells have other modes that die away quickly after being struck leaving just the main tone. A bit of distortion mimics that.
Nice demonstration, Alan. Not seen that one before. At least not knowingly!
Nice circuit and presentation. Thank you.
I made regenerative receivers from Wien bridge, twin T network and phase shift network circuits. :)
That was fun, thanks!
Why is nobody commenting on your wonderfully bad sign off? "Oscillator" for "I'll see ya later"? You got a good groan out of me!
Excellent!!!
That's a sweet 'scope!
I'll try this soon.
Excellent video, thank you!
Very good video. In need a 22khz filter circuit. I am going to experiment with putting the twin-t network in the feedback and see how it works as a filter.
Putting the twin-tee notch in the feedback path "inverts" its response, turning it input a narrow bandpass filter.
@@w2aew I need a sharp bandpass filter. I was not very clear on my comment. It will be the first active stage in a 22khz detector circuit after a voltage follower.
Dear Alan please make the video. Aligning the trim pot with spectrum analyzer (fft) to see the behavior of harmonic content .Thank you sir.
Done - see here: th-cam.com/video/Dm1YjETg3rU/w-d-xo.htmlsi=4jlU4JoKYPL9LPCT
Thank you very much for the video sir.
I might try a few hundred kOhms between output and inverting input. A little negative feedback to stabilize the gain and then adjust the filter for maximum signal.
Oscillator 😂. Great video!
Very cool ... Thank You for sharing .. Cheers :)
Best ever channel! TNX!
Когда то много лет назад программа EWB зависала насмерть, при попытке симуляции этого двойного т-моста
Very nice...
Great video. Any electronic books - recommendation.
One of my favs is The Art Of Electronics by Horowitz & Hill.
Anyone have a General Radio 1650B impedance bridge (early 1970's)? In my "for parts only" model pretty much the same circuit shown at 8:30 is used as a tight bandpass at 1 kHz in the detector. The op-amp is replaced by a 3-transistor (very high gain) negative feedback amplifier. The service manual mentions the possibility of it oscillating, which it does very well. What Barkhausen changes would make this a bandpass?
Having loop gain
@@w2aew I was going fishing to see if anyone had analyzed this circuit. I spent a lot of time on it and still can't figure it out, pretty complicated for something designed when everything probably used tubes. Google let me down.
They use the DC part of the notch filter to bias the amp like a single transistor amplifier collector feedback (just at Vbe) and, as mentioned in the video, the rejected part for high gain.
I recently got the same scope. How you finding it? I also coughed up for the 100MHz bandwidth, source and serial, and logic analyser options. Added a bit to the cost but definitely worth it (for me)
Great video as always. Is the MSO Series 2 new?
Yes, introduced last year.
"Oscillator = I'll see later" - w2aew
This is good, thank you. How high frequency you can go to design a stable oscillator without using crystal..??
This will depend on the op amp used.
I'm a bit surprised, that one resistor can regulate an amplitude. In theory, due to positive feedback in generator, once a loop amplification is high enough, amplitude will raise to infinity. Of course there is supply voltage limit, but that means clipping. Hence the lamp in Wien oscillator.
How it works here?
The non-linearities of the op amp when it reaches the rails have lot to do with it.
Can't say I'm a twin-T fan...due to the tuning difficulties. Years ago, I had the joy of doing production calibration of a device that used a twin-T 60 Hz notch filter. The time & frustration in adjusting three multi turn trimmers to both hit the frequency and get enough rejection was ridiculous, and doubtless subject to drift in the field. Really bad design choice.
How can we make a microphone audio feedback or howling prevention circuit (especially to avoid feedback from condenser mics)? Is there a way to sense the howling frequency first and auto-adjust instantly?
One technique that has been used is a a very slight pitch shift (upconverting to a higher pitch, then down-converting to *almost* the original frequency). Not so much to be noticeable, but enough to prevent the positive feedback from happening. Modern systems likely use DSP techniques. There may be other techniques that I am not aware of.
Can I have the same experience with normal power supply ?
Would've been fun to see it get trimmed while looking at the FFT. In particular also to see how badly it does when *not* trimmed properly.
Done - see here: th-cam.com/video/Dm1YjETg3rU/w-d-xo.htmlsi=4jlU4JoKYPL9LPCT
Great video, so much so I that I can forgive that closing awful (funny) pun.
Another great video. I love that scope but my first true lovebis your other one haha. Ive been looking for an older tek curve tracer. I have about 1400cad. Im wondering if there's a model you know off hand that goes for that price range. Im not picky about the age or if its super high end. Im building some guitar pedals and want to sort a bunch of parts i ordered. I plan to donate it later this year to a makerspace so others can use it.
You should be able to find a 577 for that price range.
Anybody know what the upper-frequency limit of this circuit would be?
How stable would it be in the kHz or MHz ranges? All depends on the op-amp I suppose.
What makes an oscillator into a Relaxation Oscillator? When measuring the THD on the THD meter lets say it it measure 0.500, using the Spectrum Analyzer to measure THD how do you MATCH the measurements so that the spectrum analyzer THD measurement is also 0.500THD?
I am curious what the MCP in the name MCP6xxxx stands for. I looked at the parts on the microchip site and couldn't put together a good answer? Is it just a 'microchip' designation?
Probably just an abbreviation for MicroChip. They couldn't use just MC, because that was used by motorola.
Hi, I like your calculator.
FYI, superior reproductions are available from Swiss Micros. I love mine.
Thanks 👍
I am trying to build a self-contained component tester with a built-in sine wave generator. This way you can avoid needing a 110V plug, or a signal generator. It would be nice to power it up from a bench top 12V power supply. This would be a good start. But any clever idea how to make the sinewave oscillate around 0V with a single rail power supply? (assuming power supply's ground is tied to the same ground as the scope's, so that you can't just hook up the mid point of the sinewave to the scope ground...)
Simply put a capacitor in series with the output, followed by a resistor to ground. The signal on the resistor will swing above/below ground.
@@w2aew I've been thinking that, just like a speaker output on an audio amp. Thanks for confirming.
Is the “trimmer” a potentiometer?
Yes
Illseeyoulater👍
Not complaining. Nope..
Interesting if the circuit was built and described just to show the new Series 2 scopes. Nice free advertising for his employer, wonder if Alan gets to keep it or if it is just a loaner. Can not hurt with meeting his employee review goals !
Just a happy coincidence!
If I tell my fiends about this oscillator, I also get negative feedback. ;-)
Hm with -45dB notch dip THD should (theoretically) be 0.00316% but 2 potential problems might be of limiting factor: OPamp is not capable of that low of distortion itself (in the datasheet manufacturer is not really bragging with low THD performance so….), or as i saw in the video, there is still some clipping occurring during adjustment process, which is making this oscillator as complex as Wien oscillator, which is similar in nature just uses 1 order lower H/L pass filter combined into rejection notch filter.
I suspect it is a combination of things... the "trimming" process is actually de-tuning the twin-tee in order to achieve the 180 degree phase shift and 1/openloopgain to just meet the Barkhausen criteria, the inherent distortion in the op amp, and other non-linearities of the amp near the rails.
That was interesting and informative. Thanks for the tutorial.
Oscillator! 😅
I made three with light bulbs. 1st is LM386! Then transisters only and last a split power rail.
Thank you for the inspiration!
th-cam.com/video/bnwVpbLQU-s/w-d-xo.html
th-cam.com/video/_bDrKjrJBl0/w-d-xo.html
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