[time-nuts] Fast risetime pulse generator

[Rob Sherwood.]

Oops, sorry I didn't realize you needed 150 ps rise time. Rob

Sent from my iPad

> On Apr 12, 2016, at 6:02 PM, "jimlux" <jimlux at earthlink.net> wrote:
> 
> 
> 
>> On 4/12/16 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.
>> 
> 
> how about a fast digital buffer?
> I don't think Potatochip semi is quite in the 150ps range (although you could look at their website and see).
> 
> But we use microwave dividers and such all the time at 8 GHz.  Check places like Hittite.
> 
> 
> 
> 
>> 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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