[time-nuts] Fast risetime pulse generator

[Bruce Griffiths]

http://www.its.caltech.edu/~hajimiri/pdf/non-linear.transmission.pdf
The above indicates how cascaded NLTLs using CMOS processes can be used to 
construct NLTLs that sharpen both edges of a pulse. 2.5ps fwhm impulses can 
also be produced.
Bruce

On Tuesday, April 12, 2016 04:42:06 PM jimlux 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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