[time-nuts] Fast risetime pulse generator

[Alexander Pummer]

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
>