Re: Triggering on variable-height pulse

On Tue, 08 Aug 2006 14:33:51 -0400, Phil Hobbs
<pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Okay, so I have this big nasty laser. It produces 20-ps pulses tunable
over a wide range (420 nm-10 um), which is very nice. Two not-so nice
features: the rep rate is 20 Hz, and the pulse-to-pulse amplitude
variation is +- 5% or worse, which means it's close to 2:1 p-p. Using
that with a sampling scope translates to a 25.6 second sweep time for
512 points, plus lots and lots of averaging to get the noise down to
something reasonable. Say an hour per trace--frustrating.

A streak camera would be a beautiful solution--I could get a whole trace
per pulse, and would need many fewer averages, but they're too expensive
and take up too much room.

I have a nice Tek TDS7704 7 GHz scope coming, which will help some,
although it's really still too slow--the budget didn't stretch as far as
those 12-GHz monsters from Agilent.

The worst problem is how to trigger properly from a pulse train, in the
face of the amplitude noise. Right now, I use a ~2 GHz
photodetector(*) feeding a digital delay generator and a mixer to pick
off the 8th pulse in the sequence, and stuff that into the trigger input
of my 11801B sampling scope. This produces about 10 ps jitter, which is
uncomfortably large given the pulse width. Small pulses get sampled
later, which makes the amplitude variation worse.

I can look backward in time by about 50 ns due to optical path delay, so
it might be possible to normalize the pulse height somehow before the
trigger.

Any bright ideas?

Thanks,

Phil Hobbs

(*) It's a Thor Labs InGaAs photodiode running straight into a
Mini-Circuits MMIC amp with no coupling cap.


Sounds like you need the equivalent of a constant-fraction
discriminator. The classic delay-line plus comparator CFD is just
possible at your speeds, using one of the newish ADI ecl comparators.

But I'm thinking that there's also a passive network that could go
between the mmic output and the scope trigger input that would help a
lot. I've done a pseudo-CFD using an R-L-C network to get pulse-peak
triggers over a wide range of amplitudes and some range of pulse
shapes, for a 2-d delay-line imager behind a microchannel plate,
imaging individual ion hits. It was just...


in-------C-------+---------comparator/zero cross
|
|
R
|
|
L
|
|
gnd


which is just a partial differentiator with a bit of cheating; the
comparator threshold is nominally zero, but may benefit from a bit of
offset tweaking, too. The scope trigger front-end is pretty good
already, so it may pay to use it. Some sort of hacked coax thing might
work at your speeds, or just teeny surface-mount parts.

That photodiode is pretty slow, though.


John


.

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