[time-nuts] GPS Locked and Unlocked Performance Comparison

Tom Van Baak tvb at LeapSecond.com
Wed Feb 13 04:03:40 EST 2008


Thanks to all of you so far that made comments on the
GPSDO plots; I'll handle this one first.

> Tom
> 
> Both the Thunderbolt and Miller designs appear to measure at least 10x
> better than the most optimistic expectations.

Bruce,

Interesting thought. What calculations did you use to set
your expectations? The free OCXO in both those GPSDO
are well within reason, no? And both GPSDO follow a fairly
standard couple of ns over root tau line, yes?

> One possibility is injection locking of the OCXO via its EFC due to
> inadequate shielding or via its output due to its buffer amplifier
> having inadequate isolation.

To prevent, or at least detect, this effect I allow my 10 MHz
house reference to drift off-frequency by quite a bit (last
month it was 1.7e-12 off). That way there are no on-time
or on-frequency sources near the test setup.

> What cable types did you use to connect the GPSDO's and the reference
> signal to the Allen deviation measurement instrument?

For these tests I used a TSC 5120A; a 3 meter mil-spec
RG58C cable to the UUT; the reference comes into the
lab from another room over Andrew heliax FSJ1-50A.

I get the same performance with and without 10 dB pads,
and with or without RF isolators. Between the deliberate
frequency offset, the cables, and the pads/isolators I'm
pretty confident of the test setup. If you have additional
ideas, let me know, and I'll give them a try.

> Is there any way to measure the Allen deviation of the 10kHz GPS
> receiver output, or at least verify that the 10KHz phase is jerked every
> second.
> The jerk may be done by adjusting the length of the 10kHz cycle just
> before the second marker.

Yeah, I will try to measure the 10 kHz directly for you. I was
curious also, having never examined a Jupiter GPS in detail
before.
p.s. It's Allan, not Allen.

/tvb




More information about the time-nuts mailing list