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From: whit3rd on 29 Mar 2010 17:40
On Mar 28, 6:45 pm, George Herold <ggher...(a)gmail.com> wrote:
 
>  The existing front end elecronics gives a pulse with
> ~100ns rise time and >300ns fall time.  (Kinda crappy for a PMT,)  And
> I'll live with that.  

If it matters, you can use an AC-coupled monostable to capture the
fast rise and generate a 'well-behaved' timing pulse, independent of
the slow fall times. For critical work (proportional counters) a
delay-line amplifier can do even better. The input pulse and its
attenuated/delayed/inverted copy are summed, and if the sum is done
carefully, the exponential downward tail of the input exactly
cancels against the upward tail of its delay, so all your pulses
look like little lumps on a dead-flat background.

Four meters of cat-5 cable with a lower-than-110 ohm terminator
generates
a negative reflected pulse about 100 ns late (there's four pairs in
the cable,
and the return pulse goes four lengths, reflects, and comes back four
more
lengths).
From: George Herold on 29 Mar 2010 21:09
On Mar 29, 5:40 pm, whit3rd <whit...(a)gmail.com> wrote:
> On Mar 28, 6:45 pm, George Herold <ggher...(a)gmail.com> wrote:
>  
>
> >  The existing front end elecronics gives a pulse with
> > ~100ns rise time and >300ns fall time.  (Kinda crappy for a PMT,)  And
> > I'll live with that.  
>
> If it matters, you can use an AC-coupled monostable to capture the
> fast rise and generate a 'well-behaved' timing pulse, independent of
> the slow fall times.  For critical work (proportional counters) a
> delay-line amplifier can do even better.  The input pulse and its
> attenuated/delayed/inverted copy are summed, and if the sum is done
> carefully, the exponential downward tail of the input exactly
> cancels against the upward tail of its delay, so all your pulses
> look like little lumps on a dead-flat background.
>
> Four meters of cat-5 cable with a lower-than-110 ohm terminator
> generates
> a negative reflected pulse about 100 ns late (there's four pairs in
> the cable,
> and the return pulse goes four lengths, reflects, and comes back four
> more
> lengths).

Ok the AC coupling idea is an interesting question. (Jon Kirwan
suggested that too...private email.) I wanted to DC couple
everything, as I figured the AC level would move around with the count
rate and I'd like to 'know' where the trigger level is. But I wonder
if AC coupling would be better (at not missing closely spaced double
pulses) at lower count rates?



I never heard of the cat 5 cable idea before.. sounds fun. I remember
doing similar tricks with a roll of coax cable...had about a 1us delay
and you could change the phase by opening or shorting the end.

George H.
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