Its interesting that the Miller/Jupiter design appears to outperform
everything except the Z3801, in the GPS locked modes.  Maybe the KISS
principle at work.

SAIDJACK@aol.com wrote:

To me, the interesting part is that both the Thunderbolt and the Miller
designs appear to degrade the performance of the OCXO in locked mode at
short Tau compared to unlocked. The Z3801 does very well < 300sec, but
degrades the ADEV at Tau farther out. The Fury does best, never worse locked
than unlocked. The practical difference between the Fury and Z3801 at short
Tau looks to be mostly in the OCXO, the Fury is the one that actually has
the least degradation of the OCXO performance overall.

Performance at short Tau cannot be better locked than unlocked, so it is
driven by the OCXO and software, but the fact that two designs (we will
forgive the Miller design, which looks to be mated to a very good OCXO, but
the Thunderbolt is less excusable) actually degrade the ADEV at short Tau is
surprising to me.

Didier KO4BB

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3:20 PM

The interesting thing about the Miller example is that if you click on the
link that goes to his page, the OCXO that he shows is just a simple, cheap
Temex(?) part.  I bought 5 or 6 of these, or something that looks very much
like them, for about $20 each not long ago from a UK seller.  Nothing
special about it.

Tom, was this graph taken with his Isotemp 10.0 MHz OCXO, or the cheaper
one?  What was your reference... one of the masers?

-- john, KE5FX

John Miles wrote:

Tom

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

In the case of the Miller design either the GPS receiver 10KHz output is
extraordinarily stable with no phase jerking every second or something
is confounding the measurements.
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.

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

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.

Bruce

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

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?

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.

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.

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

On Feb 13, 2008 1:03 AM, Tom Van Baak tvb@leapsecond.com wrote:

Tom,

I think you might be the only person in the world who would consider
1.7 parts per trillion to be "quite a bit off" :)

Matt

Tom Van Baak wrote:

Tom

The Thunderbolt GPDSO appears to have an asymptotic performance more
like a few ns/Tau not a few ns /SQRT(Tau).
The Miller GPSDO appears to have a similar performance.

Comparing the performance of the locked and unlocked OCXO suggests that
a loop response time of perhaps a few tens of seconds at most may be in
use, do you have any idea what the actual filter time constant is?
Are all GPS receivers really that good, even an old design like the 5V
(non timing - at least there is no 0D mode) Jupiter receivers?
Since the receiver determines its location every second the  timing
solution will inevitably be noisier than  if a 0D mode were available.

However these receivers do make the GPS carrier phase data available.
They also offer serial port baud rates to 115kBaud.
All one needs is to lock the 10.95MHz signal to one's local standard to
facilitate high resolution (5 psec) measurement of the local standard's
phase error with respect to the GPS carrier.

The 5V (and 3V) Jupiter receivers have 3 crystals a 10.95MHz crystal
used by the RF section, a 29MHz crystal used by the DSP chip, and a
32KHz crystal used by the RTC.
If the 10KHz and PPS signals are  derived from the 10.95MHz crystal
oscillator then the 1PPS sawtooth timing error amplitude (and the 10KHz
phase jerk amplitude) will be either 22.83ns or 45.66ns pp depending on
whether the leading edge of the PPS signal (or the leading edge of the
10KHz signal that coincides with the PPS transition) is coincident with
the nearest transition (positive or negative slope) of the 10.95MHz
signal or say just to the nearest positive slope transition of the
10.95MHz signal.
The datasheet timing spec is within [-1us, +1us] of UTC. This was
probably specified when SA was turned on but expecting the error to be
100x better now that SA is off is perhaps somewhat optimistic.

A much larger offset would be better (although difficult to easily achieve).

RG58C has stranded inner conductor plus 95% coverage braided outer
conductor with no foil shields.
With the single 95% coverage braided outer conductor it is a bit leaky.
Heliax has a solid corrugated copper outer conductor and consequently is
well shielded but somewhat inflexible.

Pads and isolators wont help much if coupling is through the air.
A 10dB pad probably wont have enough attenuation.
At least 120dB (preferably more) reverse isolation is desirable.

Measuring the PPS jitter would also be useful.

The 5V version of the Jupiter GPS receiver is somewhat scarce, however
the 3.3V versions are less so (I have 5 plus a marine version in a
radome somewhere).
Maybe they could be useful for carrier phase discipling of an OCXO, they
are certainly relatively inexpensive.

Bruce

I was wondering with so many 10 mhz sources running.or even a single cesium
and gps receiver running, as I have now..and the 10 mhz is also routed to a
clock driver system (leitch).I cannot receive 10mhz wwv broadcasts at all
anytime,I am using a old radio shack sw radio on batt power..I tried even
almost 3 blocks away and all I get is my 10 mhz carrier..so even a rooftop
ant will not work ..I must have a lot of leakage somewhere to travel for the
signal to peg the radios meter blocks away..

----- Original Message -----
From: "Matt Ettus" boyscout@gmail.com
To: "Tom Van Baak" tvb@leapsecond.com; "Discussion of precise time and
frequency measurement" time-nuts@febo.com
Sent: Wednesday, February 13, 2008 3:17 AM
Subject: Re: [time-nuts] GPS Locked and Unlocked Performance Comparison

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