Scenix has a new SX48/52 line (not in production but samples available) that
includes two 16 bit timers. My quick scan of the data sheet indicates that
it has at least one operating mode that can be used with an external clock
(which could possibly be faster than the 50Mhz system clock). An external
200Mhz oscillator that starts when the pulse is sent and stops when the echo
is received would be the only additional circuit necessary. Of course, the
counter may not work at 200Mhz (data sheet doesn't say) and a similar system
could be constructed for a standard PIC with the addition of an external
(large) counter. Maybe someone knows of a many bit, high speed counter than
can be read via serial?

James Newton, webmaster http://get.to/techref
(hint: you can add your own private info to the techref)
mailto:jamesnewton@geocities.com
1-619-652-0593 phone



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@mitvma.mit.edu]On Behalf Of Dave VanHorn
Sent: Sunday, September 19, 1999 6:51 PM
To: PICLIST@mitvma.mit.edu
Subject: Re: reflectometry.. measuring the length of a cable


> Anyone got any bright ideas on a device which can be plugged into the end
of a single wire (say one core in a power cable) and the device will return
a length from the end of where the cable ends.

A TDR


> anyone know how to build such a beastie? Anyone ever built one?

Tektronix :)

> I envisage something along the lines of a pulse with a delay which is then
translated to length, but the detection of the echo, and processing speed
may make it harder. Would a PIC be able to keep up?

Doubtful indeed, unless the cable is very long, and you don't need any
accuracy.
 Most 50 ohm coaxial cable has a V factor of .66, and the pulse goes out and
comes back, so figure .33C as a starting point.

At 300 MHz, a wavelength in free space is 1M, so to get accuracy to the
nearest meter, you'd have to count at 600*.33 or 200 MHz

Even scenix can't play that game.

Other cables are different, basically the higher the Z, the higher the C.

A pic (or any other micro) could control it, but couldn't directly measure
it.