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An amusing diversion

T

Tim Williams

Jan 1, 1970
0
Reading through AN47 by Jim Williams, I *knew* I had to build a pulse
generator. I mean it's just too fucking 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:
http://webpages.charter.net/dawill/Images/Pulse_1.jpg

Simple layout, tight; note red wire going under 47 ohm resistor and signal
lead as quasi-ground plane.
Reponse:
http://webpages.charter.net/dawill/Images/Pulse_2.jpg
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:
http://webpages.charter.net/dawill/Images/Pulse_3.jpg
X10 button (1ns/div):
http://webpages.charter.net/dawill/Images/Pulse_4.jpg

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
 
T

Terry Given

Jan 1, 1970
0
Tim said:
Reading through AN47 by Jim Williams, I *knew* I had to build a pulse
generator. I mean it's just too fucking 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:
http://webpages.charter.net/dawill/Images/Pulse_1.jpg

Simple layout, tight; note red wire going under 47 ohm resistor and signal
lead as quasi-ground plane.
Reponse:
http://webpages.charter.net/dawill/Images/Pulse_2.jpg
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:
http://webpages.charter.net/dawill/Images/Pulse_3.jpg
X10 button (1ns/div):
http://webpages.charter.net/dawill/Images/Pulse_4.jpg

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

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
 
M

martin griffith

Jan 1, 1970
0
Reading through AN47 by Jim Williams, I *knew* I had to build a pulse
generator. I mean it's just too fucking simple.
awsome protyping pics at the end


martin
 
J

John Larkin

Jan 1, 1970
0
Reading through AN47 by Jim Williams, I *knew* I had to build a pulse
generator. I mean it's just too fucking 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:
http://webpages.charter.net/dawill/Images/Pulse_1.jpg

Simple layout, tight; note red wire going under 47 ohm resistor and signal
lead as quasi-ground plane.
Reponse:
http://webpages.charter.net/dawill/Images/Pulse_2.jpg
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:
http://webpages.charter.net/dawill/Images/Pulse_3.jpg
X10 button (1ns/div):
http://webpages.charter.net/dawill/Images/Pulse_4.jpg

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


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
 
J

John Popelish

Jan 1, 1970
0
Tim Williams wrote:
(snip)
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.

This circuit reminds me of Marconi.
 
T

Tim Williams

Jan 1, 1970
0
John Popelish said:
This circuit reminds me of Marconi.

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

Tim
 

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