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- เผยแพร่เมื่อ 3 ก.ค. 2012
- Testing of the Jim Williams designed pulse generator from Linear App Note 47:
www.linear.com/docs/4138
On a 500MHz Agilent 3000X series oscilloscope.
The build is by Munaut.S
people.osmocom.org/tnt/hw/puls...
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For the EE students out there: this formula of BW * Tr = 0.35 is easy to deduce. Make an RC low pass filter. What is its bandwidth? BW = 1/(2πRC). If I apply a step function to the input, what is the rise time? Calculate the delay to 0.9 of the final value, then subtract the delay to 0.1 of the final value. This gives you the 10% - 90% rise time, which is 2.2*RC.
Now multiply the two formulas, and you get BW * Tr = 2.2RC/(2πRC) = 2.2/2π = 0.35. There you go!
And yes, RG-59 is a 75Ω cable being used in a 50Ω environment. Both sides of the cable need impedance converters which can be done with resistor networks. The cable mismatch is the cause of the ringing (transmission line reflections, actually) that Dave cites in his video.
What a shame Jim Is gone now..........a legend in electronics
I am in high school and want to major in electrical engineering. I have been researching various things and have always seem to come across your videos. They are very helpful and have helped me a lot. Subscribed. Thanks for everything you've taught me :)
I've built a few of these -- not all transistor types do it, supposedly diffused junction types are required. Surprisingly, I've always had good results with 2N3904s (various makes and dates), an epitaxial type. They're probably a little slower than the '2369s, but faster than my scope in any case.
Transistors that don't break down may need different bias current (if it doesn't snap on and generate a pulse, it simmers like a zener), or different B-E resistance.
As Jim Williams said: Never touch a running system with your dirty fingers!
add a 2-50m open coax in parallel to C2. It will give you much longer pulse with fast rise time. Jim did it as well in later revisions of that circuit.
I can observe avalanching in a Philips/NXP BFG424F (ordered 10 and tried just one so far... and it works). Breaks down at about Vce=30V. My pulse width is much wider (50ns), but that's almost certainly because I'm using a breadboard for the moment.
I saw this circuit in AN-72, which discusses the applications of the LT1016 voltage comparator, which was replaced by the AD8561
Somebody should write a book about Legendary circuits. This should be on first page. Nice apparatus for anything. Cable length and reflection measurement TDR etc. Thanks Dave for good explanation.
RIP Jim Williams
To make it even faster, one need to reverse bias the base to emitter junction, so base should be at negative voltage. -- from solid state physics the minority carriers in the base region will swept away when the PN junction is reverse biased.
To make it faster yet, reverse the transistor so the emitter is in place of collector. In this case because of doping profile of NPN transistor ( something called collector efficiency)
the speed is even higher.
To test this reverse the transistor and connect the base to negative voltage, and hope to have a scope that is fast enough to measure it.
that's a brilliant.to generate a fast pulse, I never would have thought of it!
Samtec didn't have one either. I did however build a BNC to BNC cable of 800mm length in the parts builder and it had the option to sample the part
( RFA6T-74SP3-74SP3-0800 ) . They are pretty cool for sampling cables I have found. I even sampled a 2 port usb through whole plug from them to replace a broken one on a motherboard.
He Explained it - the Human body can function as an antenna - he catches teh 50Hz signal that comes from the wireing in yoru room etc and overloads the circut with it.
I Once build a -1V/nA Current voltage amplifier - when i moved my hand over it i could measure teh 50hz signal as soon as i got within 20cm of the curcuit. That also explains why you need a good shielding.
Dave,
To characterise the pulse, instead of a home valued super scope, why not use a Spectrum Analyser and simply work back the rise and fall from an inverse Fourier transform from the spectra ?
Isn't RJ59 a 75 ohm cable? Also, rather than a really fast scope, can't you look at the output of that board on a spectrum analyzer and infer the risetime?
Hi Dave...you said that you were using RG-59? I think that is 75 ohm coax so it would probably be better with actual 50 ohm coax like RG-58.
i was very excited at 03:48 when you pressed down on the bnc and the signal goes crazy what caused that?
Is there anything particularly special about the 2N2369? Are there other good candidates? Googling around I see mention of Philips/NXP BFG541 avalanching at 50V (also at Farnell for about EUR 1.9 ea -- same as the 2N2369). Anything else?
Good video!
RG-59 ???
How are you matching that 75 ohm cable to the (I assume) 52 ohm source and 'scope impedance?
May be why he's getting that slight ringing. Load reflections due to the mismatch.
hmm... guessing a FET in series with the BJT (with the gate to the IO line) might do the trick provided it's switched off as quickly as possible. I love this idea of 'abusing' transistors :-)
Nice DSO!
greetings from Belgium :)
How difficult would it be to modify this circuit to be a one-shot pulse triggered by a rising/falling edge of a MCU IO line? I'm thinking short range UWB radar ...
Why the 10k resistor at the base of Q1? Intuitively it seems that low impedance connection would be better?
Hi for traveling a very fast rise time pulse signal from one board to another have i use coax???
If I use normal two wires can it decrease the rise time of pulse???
"And Bob's your uncle!"
How about running the test on the Rogol scope....
No, i'm french and it's phonetically "manoh"
Apart from that, great video ! :)
Newbie here...can you use this device for calibrating an older 25mhz CRT CRO?
is peak voltage out from that only 50mV??
I'd really like to see what the cheaper Rigol scopes manage.
Please try this on a hacked Rigol.. Kept hoping you'd do that before the video ended.
oh i must have missed that i often do a little skipping in his videos to get passed the slower and repetitive stuff no offense Dave i Thoroughly enjoy your videos! =] so basically he is ground and it was screwing with the way the scope was viewing the signal with respect to his body I'm sure it has something to do with the resistance of his skin too right?
What are you going to use you're new 1 GHz bandwidth for Dave?
@Peter Mortensen no u
what was that flash at 3:49?
Will Dave eventually discover that he's using Sylvain's surname all the time which sounds rather strange?
Fyi: Sylvain Munaut (I'd pronounce it like "manoh" :) is a team member of the Osmocon Open Source GSM project and can be seen here: watch?v=fH_fXSr-FhU
that in the scheme?
When is Australia going to upgrade their power to 60 Hz?
:3
Realised that after the fact. He shouldn't have put his surname first on the silkscreen!
8:48 630 ps?? u mean 530rite?
If I did I would have used it. I thought I had one, but it's MIA.
I found a book called Triggering of Avalanche Transistor Pulse Circuits, can anyone help me find the PDF version, thank you very much!!!!!!!!!!!!!!!!!!!!
1 Mega Ohm resistor ??
I have a question i need help plz someone help im just getting into the oscilloscope i have acouple car amplifiers that are not workin that i want to out and see if i can fix i have a tekronix 5110 can i use a pulse generator? Or would i nees a function generator? Plz help im new and im tryin to learn
Audio amplifiers use sinewaves to test them, not pulses.
0oououououói
that voice ....
Yes. Goof.
Ba dum bum tssh.
I had one, it's MIA. Plenty of the other sex.
DO EXPERIMENT TO MEASURE BODY CAPITANCE....HOW DO IT?