[time-nuts] Symmetricom TimeSource 2700
bell.peter at gmail.com
Wed Nov 30 02:54:07 UTC 2011
Assuming it's just tracking the CDMA pilots, the 1PPS output is likely
not aligned with UTC. The problem is that the pilot channel is just a
PN sequence with no modulating data - so when you lock to it you can
know that your local clock is 19200Hz * 64 chips/bit (1.228MHz) - but
that's all you know. Even the code phase doesn't tell you anything,
since there are two unknowns - the first is the distance to the cell
and the second is the code phase offset on this specific pilot (each
BTS has it's modulating sequence offset by an integer multiple of 64
chips to reduce mutual interference) - the second piece of information
you can obtain by reading one of the overhead channels, but the first
is basically not available just using a receiver (your phone can do
it, since it can ask transmit back to the BTS and measure the round
trip timing offset).
If you had access to the AGPS dats from the BTS, you could in
principle get a position fix from it and calculate the offset that way
- but when I played with this a few years ago, I couldn't find any way
to get that data from the system unless you were registered - and,
obviously, that is not something you can do just using a receiver.
On Wed, Nov 30, 2011 at 3:06 AM, Kevin Rosenberg <kevin at rosenberg.net> wrote:
> Hi Charles,
> Thanks very much for the information on the pinout of the TOD connector
> on the TS2700.
> I verified the PPS signal between pins 6 and 1. It's duration and voltage
> matches your report. I compared the timing of the PPS signal with the PPS
> of two synchronized thunderbolts.
> Interesting, the PPS signal from the TS2700 is delayed a rather constant
> 280ms compared to the two TBolts over about a minute of observation.
> One reason why I'm interested in the PPS output is to provide a varied
> NTP source for my Soekris 4501s. I have TBolts, M12M, Netclock/2 WWVB,
> and Garmin 18x. I thought it's be nice to have a CDMA clock as well.
> If of the PPS offset remains a constant, then I could use the TS2700 as
> an NTP reference clock. (I realize I'd have to write an NTP driver for
> the device). But, the fact that the TS2700 is a 1/4 sec off of GPS
> PPS makes me suspicious that the device was never interested to provide
> a precise PPS -- just a reference frequency source as advertised.
> Anyway, many thanks for all your help in understanding the device.
> I'll report more if I learn anything interesting.
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