Using GPS 10.00000Mc Output for Frequency Measuring Test (FMT)

k3Tim

How effective would it be to use the 10.000Mc GPS signal from the 6500 / 6700 (assuming the optional accesory is installed) in the following manner?

Using FLDIGI one has to calibrate the PC and a procedure to do so has been published.  A variant method follows:

Feed the GPS-10.0Mc output into one of the antenna ports and adjusted FLDIGI audio offset with LSB tuned to 10.001.  This results in a 1.00Kc tone to FLDIGI.  Use this to calibrate the entire received system (DaX, PC etc) with highest frequency accuracy possible.  The assumption is the GPS signal is much more stable / accurate wrt frequency than WWV over HF would be. 

Once calibrated, to perform FMT, a slice / DaX / FLDIGI instance would be set to the test frequency in the normal method.  A second slice / DaX / FLDIGI instance would be tuned to the GPS reference to track drift in the local system. 

After the test, the two frequency measurements are "combined" to remove the drift from the PC side.  Admittedly, "combined" is some hand waving at this point and perhaps the PC doesn't drift enough in the short term to be a concern - not sure. Granted also this is really splitting hairs - or more like a sub-Hz measurement. 

Comments?

_..--
K3Tim

elan

14.995 Mhz test calibration for all digi mod  
elan g0uut

Neal_K3NC

I think the calibration routine is for the soundcard. Since you are not really using a soundcard, is it even necessary to calibrate the PC?

k3Tim

Hi Neal,

This is a good point.  The signal to the PC is digital so the PC itself shouldn't need calibration.  I believe the GPS disciplines the main oscillator in the Flex but I am not sure if this reaches the DSP / ARM clock domains.  If these are from an internal crystal then a calibration run would be required.

When calibrating FLDIGI it appears an offset was required.

I'll try the GPS - 10Mc signal with no offset and check the FLDIGI results; will report back.

Thanks for your excellent point...

k3Tim

k3Tim

Follow up:  Tested using the GPS out, luckily reading the manual and see the output signal as a 3.3 V level!  Used an attenuator of 40 dB although more would have been preferred.  The GPS signal was observed on the FLDIGI "output scope" to be an ultra stable flat line.  A Rx offset (-38) was required to place this flat line on zero axis.  This would seem an easier method to tune the offset setting than over the air signals. The offset matched the over the air setting.

Since the signal shown on the frequency measurement window was so stable, confirming the "audio soundcard" in the system (flex and PC side) are stable there is no reason to use the GPS signal to check for drift during an FMT.

EA4GLI

I was actually thinking on using the GPS out signal to correct all my other radios. The 1500 has a 10Mhz IF input and there is a part you can get here http://www.vk3hz.net/XRef/XRef_Home.html to input a 10 MHz reference signal from a GPS reference into an FT857/897, TS2000, IC-706MkIIG, IC-910H, IC-9100 and FT-847. That together with a signal hub and I can use the GPS reference of the 6700 on all the other radios.... Right?

k3Tim

From what I read it looks like that part should work with the 6k GPS out.  It accepts up to 3.6V input and the signal out of the 6K is 3.3 V. That is an interesting device and I may use on in a TT Omni 6 that I have.  The TT needs a 20.0Mc reference and suspect this could be custom ordered.

Thank you for the link.

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Tim

EA4GLI

I will add another piece I have also on the list for this, a 10Mhz reference signal distribution hub.

http://www.downeastmicrowave.com/product-p/10-4.htm



Take the 10 Mhz signal from the 6500-6700 into this device and distribute among the other devices. making sure to use 50 Ohms terminations if not using the port.