[ntp:questions] site with lots of GPS's and accessories

Chris Albertson albertson.chris at gmail.com
Thu Mar 22 00:10:18 UTC 2012

On Wed, Mar 21, 2012 at 10:48 AM, unruh <unruh at invalid.ca> wrote:

> >>> The MAX232 datasheet gives maximum for output slew as 30V/us.
> >>> The MC1488 circuit assumes a capacitor at the output and has
> >>> a graph of slew rate vs capacitance. I've used values between
> >>> 100pF and 330pF.
> >>
> >> No, it gives that as the maximum over all samples. Unless your
> >> particular one was cherry picked, the typical is more like 5. 5V/us slew
> >> rate means a transition time of 2us for -5V to 5V (even 30V/us is a
> >> transition time of .3us) That then goes along a long (or short) wire,
> >> which introduces noise) into an rs232 chip on the computer motherboard.

This is easy to measure if you have a counter.  Put the TTL pulse on
channel A and the RS232 on "B" and let the machine measure average of A-->B
for a thousand seconds or so.   My older HP Universal counter has only
100nS resolution and is not quite good enough to measure such a small
delay but I'm setting up one that has a 250pS resolution.

Pico seconds are well outside of the realm of NTP but the over all goal is
to study radio propagation and oscillator stability.  So in addition to NTP
I have an interest in higher precision timing so I've been collecting some
esoteric equipment.

I've measured a few things related to PPS and RS232.   First off yes the
PPS to RS232 deal can be messed up so bad as to make it useless.  That is
what happened on my first try.   Itried to use a MAC232 chip to drive 100
feet of cat-5 cable.   The result was very poor.   Noise at the hand full
of uSec level, unusable.

Current system has a TTL level PPS going through about 4 feet of coax
cable.  This controls the base on an RF NPN transistor that switches an 8V
rail, then a short length of wire goes to the DCD pin on the motherboard.
I can not yet measure the performance of this setup.

Chris Albertson
Redondo Beach, California

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