How to determine that an external reference is active?

FrankO
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How to determine that an external reference is active?

Postby FrankO » Sun Jun 06, 2021 1:55 am

I'm having a hard time determining if there's any way to see in PowerSDR mRX v3.4.9 -- or on my Anan 100D itself -- whether it is correctly receiving input from my connected GPSDO. (Very possibly my eyes aren't sharp enough while looking at the display, or I'm using the wrong search terms in going through the docs.)

Can anyone refresh my memory on this? Or is it the case that there isn't any such display, and I need to note signal frequencies when I connect and disconnect the external reference? Thanks for any possible help.
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Tony EI7BMB
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Re: How to determine that an external reference is active?

Postby Tony EI7BMB » Sun Jun 06, 2021 8:42 am

If memory serves you could uses the Freq Cal on the calibration tab and the correction factor should be 1.00000000 if the GPSDO is connected.
FrankO
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Re: How to determine that an external reference is active?

Postby FrankO » Sun Jun 06, 2021 3:31 pm

Thanks! Found that (under Setup > General), and the correction factor is showing as 1.00000000. I'll assume that's good.
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w-u-2-o
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Re: How to determine that an external reference is active?

Postby w-u-2-o » Sun Jun 06, 2021 5:10 pm

Frank,

You can't just look at what's in the cal. factor box and assume all it good because it says "1.000..."

If I understand Tony's suggestion properly, it is that you actually have to go through the freq. cal. process and see if the result is 1.0. If you did not go through the freq. cal. process you can't know for certain.

However, there is an easier way:

1. Make certain your cal. factor is 1.0.
2. Tune to WWV.
3. Set mode to DSB.
4. Make sure ALL receive signal processing is OFF. No NR, NB, EQ,, none of that stuff can be on.
5. Set the spectral display mode to Phase.

With the GPSDO disconnected it is very unlikely you will be precisely on frequency. The Phase display will show a burst of squiggly lines circling or orbiting the center of the display. The speed with which they orbit is how many Hertz you are off frequency. Without messing with the cal. factor you can adjust your frequency in single Hertz steps to get it close to zero rev's per second.

Now go back to the correct frequency for WWV. Attach your GPSDO. The mess of squiggly lines should stop orbiting the center of the phase display. It may drift very slowly. This means you have achieved well below 1Hz accuracy.

With the GPSDO the very slow drift is indicative of ionospheric Doppler effects and not absolute accuracy. With a good GPSDO your error will be in the milliHertz range.

Finally, the resolution and precision of the panadapter display mode is not sufficient to measure below 1 or 2Hz, depending on your settings. You have to use the Phase mode as described above.

See also this topic: viewtopic.php?f=17&t=2499

73,

Scott
FrankO
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Re: How to determine that an external reference is active?

Postby FrankO » Mon Jun 07, 2021 3:43 pm

Thanks for the information, Scott, I'll check out phase mode.

For what it's worth, when I bought the 100D last year, the seller said that in addition to a clock option that supports proper function of the 10 MHz reference input, the radio also has a high-accuracy internal 10MHz TCXO used when the external ref isn't attached. Among my GPSDOs, my Trimble Thunderbolt has been running continuously for a couple of years, and with other radio it supports accuracy on most bands to the milliHertz level. Depending on the frequency and time of day, Doppler can cause the frequency of WWV signals seen here to vary by a much greater amount. I don't yet know the typical error of the 100D's TCXO, but it's possible that WWV Doppler shifts can be greater than the difference between the TCXO and the Trimble. I'll run a few tests and see what kind of results I get.
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w-u-2-o
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Re: How to determine that an external reference is active?

Postby w-u-2-o » Mon Jun 07, 2021 6:18 pm

Ah, totally forgot about the TCXO upgrade board for the 100D. Absolutely worth the upgrade. It gets you a 100ppb TCXO and automatic switching. Without it you'd have to go inside and move jumpers to use the GPSDO.

All the info, schematic and photos of the board are right here: viewtopic.php?f=35&t=2339

Data sheet on the TCXO is here.

If you let your ANAN thermally stabilize and then you run a freq. cal. against WWV, the Conner Winfield part will keep things within a Hz or so as long as there are no temperature changes. And the Doppler will always be more than that. But there's no substitute for a GPSDO. I love mine because it lets me be totally lazy and not have to worry about any of that stuff :D
FrankO
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Re: How to determine that an external reference is active?

Postby FrankO » Mon Jun 07, 2021 7:03 pm

Thanks for the links.

Another thought is that, since it appears the 100D receives the AM broadcast band, I could use our most precise local IBOC station instead of WWV -- I've measured its signal's frequency with other gear as being correct +/- 2 milliHertz.

I'll have to wait a few days to try experiments, though. This week I'm using the Anan to collect data for a Hamsci campaign before/during/after the annular solar eclipse this Thursday.

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