[time-nuts] Fast risetime pulse generator

Wed Apr 13 05:03:03 UTC 2016

The Tektronix 067-0681-01 was widely used in the 1970's and 1980's for
oscilloscope calibration. It produces a <125 ps risetime edge using a
tunnel diode. But it was designed for use with a large driving voltage
slow pulse (60 to 100 V p-p):
http://w140.com/tek_067-0681-01.pdf
http://w140.com/tekwiki/wiki/067-0681-01

The Tektronix SD-24 TDR sampling head (which was used in the 1180x and
CSA80x mainframes, such as the 11801B and CSA803A) generated a 27 ps
risetime rising or falling 250 mV step from one or two channels. But
you would need a working sampling scope mainframe to operate the
sampling head.

There are many other products (past and present) which can generate <150
ps risetime pulses. For example, the Tektronix AWG70002A arbitrary
waveform generator has <27 ps risetime (10 bit D/A analog output, 500
mVp-p single-ended or 1 Vp-p differential) and <35 ps marker (digital 1
V p-p) outputs. This product uses SiGe technology. But I'm guessing that
you are looking for something inexpensive that you can lash up rather
than a commercial generator.

If you are building up something, you probably want a SRD (step
recovery diode) or tunnel diode. But both may be hard to find and
expensive these days.

I agree with earlier comments that the Analog Devices SiGe voltage
comparators appear to be a good choice, with 37 ps typical rise/fall
(20%/80%):
http://www.analog.com/media/en/technical-documentation/data-sheets/ADCMP580_581_582.pdf
The evaluation board single unit price is about $299:
http://www.analog.com/media/en/technical-documentation/evaluation-documentation/310187823ADCMP572_3_80_1_2EB_0.pdf

--
Bill Byrom N5BB

On Tue, Apr 12, 2016, at 09:51 PM, Rob Sherwood. wrote:
> I have both HP and Tektronix pulse generators.  Neither are
> expensive and
> both have been very reliable.  Since I need some really slow rep rates
> for receiver testing, I use my Tek 115 triggered with an HP 8904A
> synthesizer.
> Rob, NC0B
>
> Sent from my iPad
>
>> On Apr 12, 2016, at 5:05 PM, "Logan Cummings"
>> <logan.cummings at gmail.com> wrote:
>>
>>
>>
>> Hi Bill,
>>
>> Lots of resources out there on EEVblog forums and elsewhere. One nice
>> alternative to the avalanche pulser is a very fast comparator
>> like the
>> ADCMP580 from Analog Devices. Generate a pulse of whatever width
>> you want
>> with slower gear, and have the comparator sharpen the edges.
>>
>> See here:
>> http://www.eevblog.com/forum/testgear/inexpensive-diy-50ps-ultra-fast-pulse-generator/
>> and
>> http://www.starlino.com/build-a-really-fast-pulse-generator-50ps-rise-time-using-an-ultra-fast-sige-comparator.html
>>
>> For a couple of examples. The EEVblog thread also has a link to
>> info on an
>> avalanche pulser using a transistor that avalanches at 30V rather
>> than 70+.
>>
>> -Logan
>>
>> On Tue, Apr 12, 2016 at 2:28 PM, Alan Melia
>> <alan.melia at btinternet.com>
>> wrote:
>>
>>> Bill, Avalanche pulse gens only require high voltage because of
>>> the high
>>> VBRcbo and the gain of normal NPN transistors. I cant find the
>>> reference
>>> now it might have been a 1970s Ham Radio but if you use the same
>>> circuit as
>>> Jim but put an NPN "upside down" that is emitter where the
>>> collector is in
>>> Jim's circuit you can fire off fast pulses from a 12v supply,
>>> instead of
>>> requiring 70 to 100v. I do wish I could locate the source as I
>>> have had
>>> several arguments about it :-))....in the nicest possible way of
>>> course.
>>>
>>> Alan
>>> G3NYK
>>>
>>> ----- Original Message ----- From: "BIll Ezell" <wje at quackers.net>
>>> To: <time-nuts at febo.com>
>>> Sent: Tuesday, April 12, 2016 9:30 PM
>>> Subject: [time-nuts] Fast risetime pulse generator
>>>
>>>
>>>
>>> (cross-posted to volt-nuts)
>>>> After paying only limited attention to this topic, I suddenly have
>>>> a need
>>>> for a pulse generator that has <150 ps risetime and a pulse width
>>>> of at
>>>> least 2 ns. 100mv amplitude or more is fine. I've looked at the
>>>> classic Jim
>>>> Williams avalanche generator, but I don't want to have to deal
>>>> with the
>>>> (relatively) high voltage source needed.
>>>>
>>>> I've done microwave design using Gunn diodes, so I'm drawn to
>>>> using a
>>>> step-recovery diode. The topology seems very straightforward, and
>>>> I can
>>>> build it right onto a BNC connector, no PCB.
>>>>
>>>> I'm thinking using an SMD835 diode, biased at ~1ma. The (sketchy)
>>>> datasheet claims a T of 20 nsecs and a Tr of 85 ps, Cj of 0.4 to
>>>> 0.8 pf.
>>>>
>>>> Questions:
>>>>
>>>> The obvious, is it reasonable?
>>>>
>>>> Is the bias current reasonable? I'm assuming the bias current is
>>>> actually
>>>> dependent on the repetition rate, you need enough current to
>>>> replenish the
>>>> charge within one pulse cycle. I suppose I could compute it from
>>>> the stated
>>>> junction capacitance, but I'm not sure that's the only factor.
>>>>
>>>> Will the stored charge actually give me the desired transition
>>>> rate into
>>>> 50 ohms? Hmm, again I should be able to compute this, but any other
>>>> factors
>>>> ignoring the non-diode ones like cap inductance?
>>>>
>>>> How should I compute the coupling cap from the diode to the load?
>>>> Use the
>>>> impedance at the pulse rep rate? Seems reasonable. BTW, I don't
>>>> care about
>>>> droop in the  pulse, just the risetime.  (measuring overshoot in
>>>> an HF
>>>> amp). Again, just want to verify that the obvious answer is the
>>>> correct
>>>> one. I clearly need to be very careful about the inductance.
>>>>
>>>> Thanks, Bill
>>>>
>>>> --
>>>> Bill Ezell
>>>> ----------
>>>> The day Microsoft makes something that doesn't suck
>>>> will be the day they make vacuum cleaners.
>>>> Or maybe Windows 10.
>>>>
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