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=85"  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)