Hi Bill
check out the classical fast rise time pulse generator the mercury
whetted relay, with a good lay-out you could get easily 50psec rise
time, but rather use an SMA than BNC.
You could look for an old Tektronix pulse generator used that principle
73
KJ6UHN
Alex
On 4/12/2016 1:30 PM, BIll Ezell wrote:
(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
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