[time-nuts] Re: Efratom LPRO-101 Rubidium question....

[Dana Whitlow]

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 <time-nuts at welwarsky.de>
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 mpb45 at clanbaker.org 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   mpb45 at clanbaker.org
> > Micanopy/Gainesville  North Central Florida
> > **********************
> >
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