Dana,
I make phase comparisons between the 10 MHz outputs of a GPSDO and the Rb
under test using a simple quadrature demodulator, with the I & Q outputs
displayed on a 2-chan DSO.
Can you explain what you do? Do you just connect the two 10 MHz signals
to the input of the demodulator?
Robert
On 08/22/2021 11:16 AM, Dana Whitlow wrote:
C'mon guys- when you speak of a frequency error in Hz, you should also
specify "at
what frequency" *in the same breath*. Or better yet, always specify
frequency errors in
relative terms.
Hundreds of Hz at 10 Mhz is unthinkable for a Rb. Even at 8.2 GHz, 100 Hz
error is about
12E-9, which is likely to be outside the EFC tuning range of some (if not
most) Rb standards.
Both of my Rbs have a tuning range of only roughly 2E-9 via the EFC input.
Outside that
range, much sterner measures must be taken, which I frankly dread.
The two Rbs that I own (an L-Pro and a PRS-10) both tend to drift upwards
in frequency
to the tune of about 1E-11 or 2E-11 per month. Superimposed on that are
random
variations of around 1E-11 on a time scale of a few hours.
I make phase comparisons between the 10 MHz outputs of a GPSDO and the Rb
under
test using a simple quadrature demodulator, with the I & Q outputs
displayed on a 2-chan
DSO. I use the "roll mode" display feature on the DSO at its slowest speed
(1000 sec/div)
and just leave things running continuously for up to several days. On my
DSO a full screen
width is 14000 sec (slightly under 4 hours), and I just take a glance from
time to time as I
happen to pass by.
Someone suggested a 100 sec measurement with a counter, but that is right
in the realm
where GPSDOs are typically the most noisy, so a single measurement is
likely to have
rather large errors. One would have to record a fairly large number of
such measurements
(several hundreds of them) and plot them out to get a good assessment of
what the Rb is
actually doing. WIth the IQ phase difference display, one can get a pretty
decent estimate of
the needed tuning correction, without doing any real work at all, in a day
or so.
When I'm doing something requiring the best frequency accuracy, I keep the
'scope display
running while I'm doing the serious work, and note the frequency error of
the Rb at the time
for use in correcting the final result. BTW, I don't see much "settling"
effect after making
tuning changes- the correction made seems to take effect essentially
immediately (as best
as one can tell in the presence of GPS noise). By comparing two Rbs, I can
investigate
settling effects quite well without the noise having anything to do with it.
Dana
On Sun, Aug 22, 2021 at 8:41 AM Matthias Welwarsky <[email protected]>
wrote:
Hi,
a Rb will not be long-term accurate. I'd trust a GPSDO to be long-term
accurate. A hundreds-of-Hz offset between the LPRO and the GPSDO certainly
points into the direction of the LPRO being off the mark.
When you adjust the LPRO, be sure to give it ample time to stabilize.
BR,
Matthias
On Samstag, 21. August 2021 14:50:24 CEST [email protected] wrote:
Hello Time-Nutters--
I am working on building a downlinking AZ/EL tracking dish set up to
receive the 8.2 GHz telemetry from the Suomi JPSS polar orbit
meteorological/environmental satellite. The receiver seems to be
working OK but it periodically appears to slowly drift frequency by
300 Hz to 400 Hz. I need some way to accurately determine what
the actual RX receive frequency is.
I have a vintage Efratom LPRO-101 Rubidium 10 MHz oscillator.
I originally got it thinking that it would provide a decent
phase-lock reference for my freq-counters, o-scopes, spectrum
analyzers and Agilent/HP vector network analyzer.
The only other item I have that has (or should have) a reasonably
accurate 10 MHz reference output is a Trimble Thunderbolt.
However, there is a several hundred Hz freq difference between the
Efratom and the Trimble T-bolt.
My question is how do I go confirming the frequency of the T-Bolt
or the Efratom Rubidium? The Efratom has an adjustment pot
to fine-tune its output frequency. How do Time-Nutters go about
confirming the accuracy of frequency references such as my
T-Bolt or Efratom rubidium oscillator?
Thanks for any feedback / suggestions on this !!
Off-list communications on this via my email is OK!!
Mike Baker [email protected]
Micanopy/Gainesville North Central Florida
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