Fast Pulse Generator

Feed a square wave into one input of a fast XOR gate and feed a delayed version of that square wave into the other input. The delay could be an RC network or a short (10 to 20cm) length of coax.

John

There is an old app note by Jim Williams about applying a voltage higher than rated VCE to a NPN which creates a very short pulse

I am offline, so cannot find the reference, sorry

Cheers

Klaus

Here's the schematic of Jim Williams 350ps version.

The write is online, I don't have the link.

Mikek

As I've mentioned before:

You can get about 250ps rise / fall time from LVC.

With a shorted transmission line, you could turn an edge into a pulse.

Here is a video of a 74LVC04AD that I dead-bugged on a board, in this case the chip was an old one from Philips, now NXP. Unfortunately eevblog tested it with a lower freqency than I had intended, and so the on-board decoupling did not have enough capacitance at the lower frequency and there is quite a bit of ringing on the supply. The low-portion of the waveform looks much nicer because the positive supply ringing has less effect when the nmos fets are pulling the output down to ground. I think he also removed my ac-coupling and back termination on the output, but you can see that the edge itself has about 250ps rise time, even using whatever BNC adapters he used.

I have to say I like your thinking....

The delay can be another gate.

NC7SV86P5X is a CMOS 1 ns XOR gate.

You can go a lot faster with ECL.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

When I said

I don't know what you're up to, but to make rectangular fast pulses, I'd use an avalanche pulser. Risetime can be in the

150ps ballpark and pulse length is set by the length of a open-ended piece of coax. You'd get about 20 to 30V into 50 Ohms.

The trouble is finding transistors that will avalanche. Not all do. I know that a 2N2369 or BFG541 will work.

Jeroen Belleman

Incidentally the 2D21 gas-thyratron produces ripping-fast edges in a relaxation-oscillator configuration during the de-ionization period.

Not ps-fast of course but divided-down appropriately even fast-jellybean op-amps with 10 or 20 MHz GBW as buffers can't keep up.

Am 10.05.20 um 20:04 schrieb Jeroen Belleman:

2N3809 is also said to work good. Zetex / Diodes, Inc make one that is specified for that operating mode, but the price is not so cheap.

cheers, Gerhard

Probably Leo Bodnar:

Nice little gadget for testing scopes and the like.

-- john, KE5FX

~True, but only 1.5Vpp output? That's the obvious drawback AFAIC.

Right, that's it.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

You didn't specify swing.

What is this for?

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

I bought several dozen partly-used coax cable reels with anything from 20 to 300ft left on 'em. I don't fancy unravelling them all to find out how long they are. Anyway, time I got some shut-eye.... g'night!

You could measure capacitance. Or weigh it. Or resonate it.

But you don't need a sub-ns step for TDR. Cable attenuation would mush it up anyhow.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard