On 2/4/20 4:23 PM, AC0XU (Jim) wrote:
Time Nuts-
I recently purchased (Ebay) a GPSDO from an overseas manufacturer. This is a
new item and the Ebay description seemed quite encouraging. Having had one in
the lab for several weeks what I observe is that:
1) ADEV for short to medium durations (Tau <1,000 sec) is comparable to or
better than other GPSDOs in my lab. Not as good as the best undisciplined
oscillators in my collection, as expected, but no problem here.
2) ADEV for longer durations (Tau>10,000 sec) is considerably worse than other
GPSDOs and Cesiums in my lab. I measure well over 1E-11 at tau=100,000 sec for
the new unit. This is my problem with it. As far as I can see, it behaves like it
is not actually locked to GPS.
I queried the vendor about the apparent lack of synchronization with GPS and
this was his response.
"I understand everything. When constructing this device, I was guided by the
need to get a good 100 MHz and 10 MHz signal, which will be maintained with an
accuracy better than 0.1 ppb throughout its life. I used the GPS signal to correct
the frequency of the main OCXO as the elements ages. I tried to ensure that the
frequent loss of GPS signal and its quality did not affect the parameters of OCXO
itself. For this reason, the adaptive algorithm is very delicate in the final phase.
The frequency correction program causes it to stop regulating OCXO in some
cases.
This happens when:
1. OCXO will achieve accuracy of 0.01 ppb.
2. No GPS signal.
The control system is temporarily frozen when the frequency accuracy of 0.01 ppb is
reached. Therefore, ADEV for T> 1000 seconds resembles the typical behavior of OCXO
during free operation. These parameters do not change when the GPS signal appears
suddenly after a long absence. When constructing the device, it was assumed that all
stability will be dominated by OCXO, and a value better than 0.01 ppb is not needed in
telecommunications applications. I would add that the value of 1E-13 has never been a
goal to achieve, especially due to the additional costs. Maybe if I gave up the 100 MHz
section, I could do it for the price."
It surprised and dismayed me that large-tau behavior is so poor in a GPSDO. The
several other GPSDOs in my lab all have ADEV around 1E-13 at 100,000 secs
(based on a mass assessment of the GPSDOs and cesium oscillators). It makes me
wonder what the point was in adding the GPS receiver to this device. Is there
something that I am not understanding?
They were interested in compensating for aging to 1E-11, and good ADEV
in short time, but not large tau ADEV. So they have a solution that met
their requirements. You'd need the GPS receiver to be able to measure
the aging and compensate it.
For instance, a Wenzel Streamline 10MHz OCXO has a aging spec of
5E-10/day - that wouldn't meet his 0.01ppb requirement, but with a GPS,
they can meet that.
This is a typical kind of spec, for what it's worth. There are a lot of
applications where you need good long term accuracy (so you can acquire
the signal without a-priori knowledge), but you're not making long term
measurements.
It's a lower accuracy application, but in satellite downlinks, there's
often a requirement that the downlink frequency be known to much better
than 1ppm, so a TCXO won't get you there. But you don't want to spend
the power on an OCXO with good aging properties. So you use a VCTCXO and
some "other reference" (like GPS or a CSAC) to periodically measure your
TCXO and adjust it.
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