EEVblog #311 - Jim Williams Pulser Followup

Some follow-up measurements on the Jim Williams pulse generator circuit.
With a special guest appearance by a rather expensive scope...

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[jeromekerngarcia]

Nice go with the 35 cm of coax Dave. That turns it into what I call a charged-line pulser. I learned that term in the mid-late 1970s, when I worked for an org that dealt in FAST pulse phenomena. I built many charged line pulsers and my co-ax of choice was always semi-rigid. The solid outer conductor keeps all of the E field inside the coax, whereas the braid outer conductor of flex cable is "leaky". This means semi-rigid is not susceptible to body capacitance altering the pulse characteristics.

[mikeselectricstuff]

Are those six decimal places of picoseconds in the measurement really meaningful... one attosecond?

[ivanv]

That $140, 000 dollar scope... Dave, since you used it, I demand a teardown, review and, more importantly a drop test.

[nlhans]

I have to say, throwing scopes around is very convenient. It gets them out of the way so fast and easily :-)

[MinnoW]

My jaw dropped when you said 16Ghz. For a split second i was expecting 6Ghz, and i was already in awe.
Be nice to see what a scope like that is required for, I guess there are many reasons people require them.

Keep up the good (GREAT) work!!

[EEVblog]

Help from who? I'm director, producer, cinematographer, sound engineer, editor, grip, gopher, and on-screen talent.

[mychevysparkevdidntcatchfi]

You can't use the waveform as-shown to calculate scope bandwidth. You have to extrapolate the rise part of the waveform to full vertical width of display. Think of it this way; if the voltage out of generator is 1/10th taking up only one vertical division, you'd get rise time of 0.2 ns, but that doesn't mean your scope has 1.75 GHz bandwidth.


If you extrapolate, the rise time is close to 2.5 nsec, which is about 140 MHz. Yes, not as accurate as testing with sweep frequency, but close.

[ahbushnell]

That ramp up after the leading edge is caused by high frequency loss in the small coax you added. Try some bigger cable like RG-8

[fullmetaljacket]

we need a damage report of that scope you threw away

[Skyfox]

That attenuator looks like a prop from the original Star Trek.  :D

[BEdmonson]

You could add a 50Ω resistor to the end of your coax to get rid of the "step" in the rise time. That step is the signal hitting the end of the coax and reflecting back down the line due to not seeing a 50Ω termination. If you want to really get it as flat as possible you could use a pot instead of a fixed 50Ω resistor to dial it in perfectly. On an aside, if you know the velocity factor of the able you're using, you can measure its length using that "step" in the waveform and a bit of math. Cheers.

[philpem]

I suspect that stepping on your waveform (after removing the capacitor) is caused by signal reflection on your length of RG174.

If I'm right, you've essentially built something along the lines of a time-domain reflectometer (TDR) rig, which is measuring the length of that piece of RG174 cable (or possibly the path from the pulse-gen through the attenuator and to the scope).

[toxanbi]

After watching 4 GHz Tektronix scope teardown I am just wondered what kind of high-freq design should be inside this thing. And yeah, if there are 100 GHz scopes, how do they (a designers of 100 GHz scopes) debug and test their scopes on development stage?

[EEVblog]

Yes, I actually threw it. I'm not that adept at video editing to fake that.

[listerdave]

With the Rigol scope if you enable equivalent time sampling it changes something in the front end that increases the bandwidth somewhat, at least on my DS1102E. But I think the modified 1052E will probably do the same.

[EEVblog]

You'd be surprised. Anyone who does any sort of leading edge high speed design, which is a ton of stuff these days, would consider this tool essential. You cannot characterise your design without a tool like this. $140K is petty cash for a lot of large tech companies.

[EE_fun]

HI Dave!

I've seen this little flat bit in the rising edge at 7:00 min before, in my Signal Integrity class. The additional coax forms a transmission line of course, which is not terminated! :(
So you have to wait the propagation delay of the t-line to have the full peak due to the reflexion at the end.
I've modeled your case in LTSpice with the tline component and got a similar result.

Greetings from Germany ;)

[aptsys]

We have a 100GHz scope at work for all of our radio and satellite work. Cost is no problem when it's an essential tool

[bloomtom]

This video gave me a double heart attack followed by an envy induced seizure.
I will be sending you my medical bill, apparently you can afford it lol.

[CoolMusicToMyEars]

Hi Dave, When I was testing Bandwidth I used a Tektronix Tunnel Diode Pulser on a TM 500 series, with a calibrated cable for the Tunnel Diode Pulse Generator 067-0681-01 Calibration Fixture in the UKAS Instrument Test Laboratory,
For myself I managed to get a HP 54750A 50GHz Scope & repaired that, very nice unit, for fast pulse Tektronix 284.