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 > __________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email PSN-L-REQUEST@.............. with > the body of the message (first line only): unsubscribe > See http://www.seismicnet.com/maillist.html for more information. > > __________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email PSN-L-REQUEST@.............. with > the body of the message (first line only): unsubscribe > See http://www.seismicnet.com/maillist.html for more information. __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>