[time-nuts] How can this be?

Tom Van Baak tvb at leapsecond.com
Mon Oct 18 20:26:05 EDT 2004


They should stay within 100 ns of UTC and each other
at all times. Sounds like they are doing that just fine.

Short-term stability is determined by the OCXO. After
that, the main influence is the sum of all GPS system
and receiver noise, which will not be identical in time
for each receiver (each receiver makes its own solution
with its own observations using its own TCXO with its
own particular sawtooth behavior). Add to that the
current drift, frequency, and phase history of the two
different HP OCXO's and there's plenty of room for
variations on the order of tens of ns between your two
10 MHz Z3801A outputs. See also:


I'd be interested in anyone else's observations from a
raw Oncore (no OCXO discipline). I'm wondering how
much of the typical unit-to-unit variation is due to the
GPS engine alone ("dumb clock") and how much is
due to the rest of the GPSDO ("smart clock").


----- Original Message -----
From: "Bill Hawkins" <bill at iaxs.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Monday, October 18, 2004 17:04
Subject: [time-nuts] How can this be?

> Have two Z3801A receivers by HP and a Racal-Dana model 1992
> counter timer with a Phase A-B function. Measured the phase
> between the two 10 MHz outputs of the receivers and got
> values between 250 and 350 degrees with 2-3 degrees noise.
> The receivers are from the same production run. Each has its
> own HP antenna. The antennas are four feet apart on a mast
> of six inch plastic pipe. Usually, six satellites are active
> with signal strengths above 150. The lead-in is 50 feet of
> RG-8 for each antenna.
> They have been running for two days now, locked in 40 hours
> ago. Why is there all of this phase shifting?
> Regards,
> Bill Hawkins

More information about the time-nuts mailing list