Dana,
Just curious, can you share some details on what you are using for the
IQ demodulator? Is this a 'roll your own' solution Gilbert cell mixer, etc?
Thanks,
Dan
On 8/23/2021 3:30 AM, [email protected] wrote:
Message: 4
Date: Sun, 22 Aug 2021 10:16:56 -0500
From: Dana Whitlow<[email protected]>
Subject: [time-nuts] Re: Efratom LPRO-101 Rubidium question....
To: Discussion of precise time and frequency measurement
<[email protected]>
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<cadhrwpeyiqa1pwrvtb_htmzc8oxe-wr+fzyz4oz4vucnc4h...@mail.gmail.com>
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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:
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