[time-nuts] Fast risetime pulse generator

[Logan Cummings]

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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