PSN-L Email List Message

Subject: Re: Not so simple photoelectrics, or are they?
From: "David A. Latsch" blottobear@..........
Date: Sat, 02 Jun 2001 23:36:28 -0600


Greetings all:

I am used to dealing with teraohms, femtowatts, and
picoseconds(much less nanoseconds) in my long and
illustrious dealings with those magical electrons-but
how did an analog signal with<10hz or so of bandwidth
get involved with those fleeting nanoseconds? I must
have been taking one of my catnaps or something.
The antique seismos  used a moving mirror and an
INCOHERENT light source and wrote on a rotating drum
located in a small vault-not hundreds of feet away! The trace
on the film was 1-2 inches or so for a typical (M7-9) large
quake. I think Sean-Thomas may have one of these puppies
stashed in the basement at SLU.
Yea, I sing the praises of the CCD, for it will set you free!!

Vaya con Dios,
DL

Thomas Leiper wrote:

> Yeah, I must have missed an order of magnitude in my mental
> guestimation as well, because 2.6 ns is out of the question.
> So it's back to the old coil. I wouldn't have wasted my time
> anyway. I suppose you could just slow down your sample rate, but
> then you would have the flutter factor to deal with. There must
> be SOME extremely complicated way we can replace the magnet and
> coil that work so well. How about a force-balance scheme where
> we just use some silver (or gold) contacts and keep them barely
> touching by PW modulating the coil with a high frequency square wave...
>
> Tom
>
> ------Original Message------
> From: "Charles R. Patton" 
> To: psn-l@..............
> Sent: June 1, 2001 8:44:42 PM GMT
> Subject: Re: Not so simple photoelectrics, or are they?
>
> Thomas Leiper wrote:
> >
> > Not sure you get it, Charlie. You don't have to
> > resolve the pulse width, only the timing of the
> > relationship between the reference pulse and the
> > detector pulse.
>
> Tom,
>
> I did make a mistake in my math by thinking in magnitudes rather than
> powers of two.  But the point I was trying to make has just been made
> very well by Chris Chapman who has just posted, "... 1 arc second is
> 1/60*60*360 of a revolution - 1/1,296,000, so at 30 RPS, 0.1 arc sec
> represents ~2.6 nano sec"  His observation about diffraction errors is
> probably quite accurate.
>
> Although parts are available at 300 MHz, they generally cannot be used
> with simple perf board techniques.  They require ground plane techniques
> and careful attention to lead lengths and so forth - not a construction
> area kind to the beginner.
>
> And as Chris's post and other posts have mentioned, it still doesn't
> solve the need for a small dot in order to get that 2.6 ns edge.  Other
> factors are the speed and position variations one will find in the
> scanner mechanism.  One can expect at best perhaps random 5 uin position
> errors at the motor mirror if the best bearings are used (not likely),
> and more likely much worse due to imperfections in the ball bearings.
> Those errors can be overcome with hydrostatic bearings, but you just
> left the few dollars territory and headed into kilobuck land.  Some of
> these position errors will translate as angle errors and thereby timing
> that is somewhat random with respect to the timing photodiode.
> Additionally there will be speed variations greater than and perhaps
> much greater than 10 us per revolution.  Some of it will be cancelled by
> the timing pulse, but unless the timing pulse and the measurement pulse
> are co-located, there will be that inaccuracy to add in.
>
> Regards,
> Charles R. Patton
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Larry Cochrane <cochrane@..............>