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An amusing diversion

Reading through AN47 by Jim Williams, I *knew* I had to build a pulse generator. I mean it's just too f****ng simple.

So I thought, what low voltage, fast BJTs do I have, and I realized I have some PH2369's pulled from a monitor video drive board (they were the bottom of a cascode gain stage driving a class B emitter follower, of all things). Coincidentially, the transistor Jim used is a *2N*2369...

So I grabbed a 47 ohm resistor, 2.2k, 10k and 100k (so a bit more rapid than Jim's ;-). Discovering my bench supply tops out at 36V, a bit too low for avalanche breakdown, I wired up a MOSFET, inductor and fast diode to my signal generator for a quick 100V booster.

The results:

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Simple layout, tight; note red wire going under 47 ohm resistor and signal lead as quasi-ground plane. Reponse:

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5V/div vertical, 500ns/div horizontal. Note uncertainty in repetition rate -- okay, so the supply has a bit of hum on it...

Maximum sweep rate, 10ns/div:

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X10 button (1ns/div):
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Now this is with my Tek 475, which is "only" 200MHz, so this pulse ought to be considerably taller and thinner than shown.

And not to forget the second most important element: the transmission line is a 3' 50 ohm BNC cable from pulse generator to scope input. A tee and 50 ohm (looks solid enough) terminator complete the scope end.

Pretty bouncy after the pulse, could be a number of things. I'm not complaining, since this is the fastest thing I've observed, or generated for that matter.

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams
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BTDT, to evaluate a range of different scopes. whats really interesting is the behaviour of digital scopes - if you see a sin(x)/x type response, with wiggly bits before the pulse, you just measured the impulse response of the scope :)

Cheers Terry

Reply to
Terry Given

awsome protyping pics at the end

martin

Reply to
martin griffith

It's mostly the older, diffused transistors that avalanche nicely. I've tried a bunch of more modern, mostly epitaxials, and they avalanche wimpily if at all. The Zetex avalanche transistors generate mighty pulses (for mighty dollars) but aren't as fast as some of the older parts.

Some of Tek's sampling scopes got over 1 GHz bandwidth using an avalanche transistor to drive a 4-diode sampling gate... below 300 ps effective sample/hold.

John

Reply to
John Larkin

Tim Williams wrote: (snip)

This circuit reminds me of Marconi.

Reply to
John Popelish

If you'd like, with a few more volts I can send a spark-generated pulse through the cable. ;-)

Tim

-- Deep Fryer: a very philosophical monk. Website:

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Reply to
Tim Williams

Surely you meant macaroni.

John

Reply to
John Larkin

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