#847 

I think possibly you are mostly being bandwidth limited by the scope itself---1ns rise time is in general approximately 350MHz of bandwidth. Recently I looked at the output of an HP comb generator (step recovery diode with pulse output approximately 100-200ps) on my Lecroy 7200A with 500MHz 7242B plugin. I was only able to measure the rise time at about 900ps, and that was running directly into the scope with no probe. I had hoped to be able to measure it a bit more accurately but obviously my scope bandwidth was the limiting factor. So the solution is easy---go buy a Tek 11000 series sampling scope with a 20GHz plugin and redo the video...but don't forget to clear off 45 cubic feet of space on your bench first. :-D

FYI, a really interesting article related to this is "Secret world of oscilloscope probes" by Doug Ford. This was published by "Silicon Chip" in October 2009. One of the interesting features I was not aware of, is that the probe cables are lossy by design to suppress signal reflections. Another great video - thanks !

One confounding factor could be that it appears to be a *very* narrow pulse, so the maximum observed amplitude is itself a function of the probe/scope’s bandwidth. The 10/90% points thus end up being relative to a level that’s in part determined by the probe’s bandwidth. I think this would tend to make slower probes seem faster than they actually are. I can’t properly think through the math in my head, but suspect you’d get significantly different results if you were looking at a step function vs an impulse. With a step, the ultimate voltage would always be the same regardless of bandwidth, which I think would tend to make the slower probes appear more proportionately slower than they seemed to be in the tests here. (Related to this, I wonder if the larger signal from the cheap Chinese probe that you remarked upon might simply have been a result of its apparently higher bandwidth, vs improper calibration: I’d be interested to hear how it’s calibration compared to the others when measuring continuous, low-frequency waveforms…) Thanks as always for a super-interesting video!

If you are going to use or look at high speed, repetitive signals, you might try a sampling probe. It is a diode bridge made from 4 Schottky diodes. I used one such during the mid 90s and I could then use my 2.5 maxed out o-scope (like the ones that were once used to fix TVs with) to look at stuff right up to about 3 gig or so. The trigger that was used to open the bridge in the probe heat had 4 small coaxed cables running to it from the control box and used an avalanche set up like the one in the clip. The control box was all transistors, no ICs. ICs, I guess, might be pretty noisy at the higher frequencies.

That reminds me of a student project. For a pulse laser diode I had to build a square wave generator capable of 1A with a rise time of 1ns back in 1980. I used a 4 stage avalanche setup with open coax load line. Each stage produced a rise time of 250ps at 250mA triggered all at the same time to get 1A. The triggering also used an avalanche pulser. The open coax gives a nice square pulse compared to just using a cap

Interesting. I have nothing certain to offer but a few things to consider. 1) The lead length of the 3904 is a bit long. 2) There exist avalanche specific transistors in SMD, I don't recall number. 3) When building the pulse circuit 50ohm output is not magic, try lower. 4) Build also a pulse circuit that connects directly to the scope thru a BNC w/o cable. cheers dan

You're right up against the bandwidth limitation of the scope. Rise time is given by .35/freq, so for your 350 MHz scope, you can expect a rise time of 1 nanosecond. Add in the rise time of the probes to the rise time of the scope, and you are at the limit of your ability to measure. Time to spend more money!

I recall using a simple pulse generator in a college Physics class. It consisted of a mercury wetted relay with a form C contact. A small, ceramic disk capacitor was connected between the common pole of the relay and ground. When the relay coil was de-energized, it charged the capacitor through a resistor from a DC power source. When the relay was energized, it discharged the capacitor directly into the load. This produced short pulses with a very fast rise time. Sorry, I don't have any actual rise time values to post here.

Read the app note. Start by driving the signal directly into the scope with 50ohm termination direct coupled and do the BW calculation. BTW, your dead bug construction is marginal for sub nanosecond rise time signals.

Nice Vid I have a Leo Bodnar pulse gen , BNC version 32ps rise , 30ps fall Measured with CSA803 w/SD-30 head

It would be interesting to see the effect of using the probes' ground leads - the additional series inductance could have quite an effect and would show the care needed when trying to measure a circuit so that you measure what you want to measure.

You did not mention that the measured risetime is the root of the sum of the 3 risetimes squared. These 3 risetimes are the risetimes of the generator, the probe and the scope. When the generator is fast enough (or assumed to be) and the risetime of the scope is given or measured via bandwidth the risetime of the probes can be measured. (It might not be a perfect measurement with the given set up.)

12:19 jab at the "Cheap Chinese probes" which actually kicked ass with one of the fastest rise times.

Pretty long lead length there on that 2N3904. Scope rise time is probably limiting you also. Try direct 50 Ohm coax into scope terminated in 50 Ohms just to see what is what. Longer pulse duration also would help maybe? Just to make sure you get full swing before pulse cuts off.

As I mostly work on audio frequenccies I have nothing to say, but, interesting results. )

*Summary* *Goal:* Measure the rise time of different oscilloscope probes. *Challenges:* * Requires a signal source with a rise time significantly faster than the probes being tested. * Scope bandwidth limits measurement accuracy (350 MHz scope = ~1 ns rise time). *Procedure:* 1. *(**0:00**) Initial attempt:* A 74AC-based pulser, achieving ~2.4 ns rise time, wasn't fast enough to differentiate the probes. 2. *(**1:58**) Improved pulse generator:* Built an avalanche transistor-based pulser aiming for sub-nanosecond rise times (design from Jim Williams' App Note 47). 3. *(**6:06**) Probe testing:* * Connected probes to the pulser using a low-inductance method. * *(**6:08**)* Measured the rise time of the Rigol PVP2350 (350 MHz) probe. * *(**6:52**)* Measured the rise time of an HP probe (bandwidth not mentioned). * *(7:49) Measured the rise time of a Tektronix P6106 (250 MHz) probe. * *(8:44) Measured the rise time of a Tektronix P2200 (200 MHz) probe. * *(11:06) Measured the rise time of a 100 MHz Tektronix probe. * *(12:17) Measured the rise time of a generic, unbranded "cheap Chinese" probe. * *(14:11) Measured the rise time of a vintage HP 275 MHz probe with a small form factor. *Results:* * The Rigol probes performed surprisingly well, comparable to higher-end probes in this specific test. * Measurement limitations due to scope bandwidth and pulse generator speed make it difficult to accurately differentiate the probes. *Future Plans:* * Obtaining a faster pulse generator (

Were they all compensated? What effect would that have on this test

You may be running into the rise time limits of the scope and the acquisition setup. Specs for 350MHz I believe say "

Do you have access to any tunnel diodes ? You could build a simple TD pulser and generate very fast rise times.

With a bit of cleaning up of your layout (VERY short leads and surface mount components) the risetime of that avalanche pulser is better than half a nanosecond. (limit of my 500mhz HP scope) Try directly connecting the circuit to a BNC that will plug onto the scope. Use the impedance switch on the scope to switch the input to 50 ohms instead of hi Z. This will give you the rise time of the scope. When using a slow scope like that it is difficult to accurately measure nanosecond pulses. That scope uses multiple iterations of the input signal to get the waveform. Any jitter in the signal (relaxation osc, oh my!) will skew the risetime edge measurement when at that short time scale ruining the accuracy. If you really want to measure that kind of speed, save up your money and get a good HP scope from ebay with 8ghz or higher sample rate.

Should have gone back to the first probe to see if the weird fall off was caused by heat/degradation of the transistor.

Bandwidth x risetime = 0.45 in modern scopes -> 350MHz bandwidth equals roughly 1.28ns rise time. That is what you are measuring. The probes have barely an influence I guess

I believe you can get the frequency response using a spectrum analyser with tracking generator. Personally, I would try to fi nd the -3dB frequency by using a sine wave and changing its frequency. of course, the probe bw should be lower than that of the oscilloscope.... Nice vídeo ;)

Your measuring is limited buy your scope. What your rise time with you pulse generator connected directly to your scope? Before you tested did you adjust the probes with your scope for a nice square wave?

I would have liked to see you use a short piece of coax, fold a little shield back as your ground and expose the center conductor all to fit in your fixture, I know the probe tip ficture as a Curly Q. Use a BNC on the other end. Then terminate the coax at the scope with 50 ohms. Yes, it is low impedance, but I'd like to see the waveform.

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???

12:54 measured rise time is lower but... fall time has more divs.

Leo Bodnar is amused.

I didn't see you do probe compensation?

Inverse Fourier transform of frequency response == impulse response

With 70MHZ bandwidth you are getting the right result. You need a faster scope to see a sub-ns risetime

Use an HDMI source for fast edges.

Why not use a network analyzer?

Isn't the fact that you have an impedance mismatch a problem? Your generator is 50 ohm but a 10 x probe is nowhere near 50 ohms.

Looking up the specs of the scope you are using I see that it has an analog bandwidth of only 70 MHz so these measurements are severely tainted by using the inadequate scope for your probe comparisons.

I think C1(22pF) is too large. this causing pulse height too high, looks like about 5V. I have avalanche pulse generator too. it is without C1, parasitic capacitance only. in #838, you measure 3db point is more than 500MHz. 1.2ns risetime is not your scope limit I guess (but for 5V pulse... maybe).

oh I know what's wrong . says right on the label: MADE IN CHINA 😁