[time-nuts] Fast risetime pulse generator

Rob Sherwood. rob at nc0b.com
Tue Apr 12 22:51:45 EDT 2016


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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