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

[Robert DiRosario]

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 <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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