# [volt-nuts] Yet another sub-nanosecond pulse generator thread

BIll Ezell wje at quackers.net
Tue Apr 12 16:27:14 EDT 2016

```(cross-posting to time-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. 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.

```