External 10Mhz operation, I think I understand how it works?

Tom K3TF

I have a new Flex 6500 with the latest firmware VS 1.10.16

I connected my external house standard GPSDO 10Mhz. reference signal, about 2 volts P-P. Rebooted the Flex 6500 and it is working fine. I am now locked to my external reference standard, but when I measure 15Mhz. WWV I find the Flex is reading -264 ppb the offset I had the last time I calibrated the Flex without an external reference. 

From what I observed the Flex Software function, "Offset (in PPB)", is an additive effect to the external 10Mhz. input.

1. If I set Offset to 0 then the Flex 6500 is dead on the 15Mhz. WWV signal. If I type in an offset like +1000 or - 1000 then I can see on the Spectrum Display the Flex moved up or down a little in frequency. 

2. If I want to use the external 10Mhz. reference but also add any offset the Flex6500 can measure from 15Mhz. WWV then I hit the start button and I wind up with a small offset value +/- 5 or 6 ppb.

3. Bottom line, if you hook up an external 10Mhz. reference, in my opinion, make sure you type in a 0 in the Offset box or run another frequency calibration with the start button. 

4. My thoughts, which may or may not be correct, if I want to use only my house reference 10Mhz. reference it is best to just type in a 0 in the Offset (in ppb) box.

73,
Tom
K3TF.....

Tim - W4TME

Tom, your assumptions are correct.  The external reference OX (clock) just replaces the internal reference oscillator and any defined offsets are applied.  So it is a "best practice" to re-calibrate the radio after connecting an external clock source to "fine tune" it so that the master XO puts the radio dead on frequency.  Manually setting it to 0 before running the calibration isn't necessary because the test starts at a 0 offset and adjusts from that point.

The only time this is not necessary is when using the internal GPSDO option, as we have complete control over the module and disable the option to do a frequency calibration because it isn't necessary.

Phil - AA4MY

This article was extremely helpful; having scratched my head this afternoon as to why my new GPSDO wouldn't align properly with WWV on my Flex 6600.  Thanks Tom and Tim.