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" <w...@quackers.net>
To: <time-nuts@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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