[time-nuts] Fast risetime pulse generator

Alan Melia alan.melia at btinternet.com
Tue Apr 12 17:28:26 EDT 2016


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" <wje at quackers.net>
To: <time-nuts at 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.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there. 



More information about the time-nuts mailing list