I connect one signal to the LO input of the demodulator, and the other signal to the RF input. I should draw up a schematic of this, and will do so.
Dana On Mon, Aug 23, 2021 at 4:59 PM Robert DiRosario <[email protected]> wrote: > 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 > >>> ********************** > >>> > >>> _______________________________________________ > >>> time-nuts mailing list -- [email protected] -- To unsubscribe > >> send an > >>> email to [email protected] To unsubscribe, go to and > follow > >>> the instructions there. > >> > >> > >> _______________________________________________ > >> time-nuts mailing list -- [email protected] -- To unsubscribe > send > >> an email to [email protected] > >> To unsubscribe, go to and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- [email protected] -- To unsubscribe > send an email to [email protected] > > To unsubscribe, go to and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send > an email to [email protected] > To unsubscribe, go to and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
