I have an external Trimble Thunderbolt GPS frequency reference. The Flex 6600M automatically detected its 10MHz precision frequency correctly and locked to it using the rear panel 10MHz input. I switched to the internal TCXO and did the frequency offset calibration at 10MHz using the GPS frequency reference as the input to the Flex receiver. The calibration process completed with a -546 ppb offset. I now checked the Tx output of the radio running on the internal TCXO only to be exactly on frequency using an external GPS locked frequency counter.
When I switched back to the external GPS reference, I found the frequency of the radio to be off frequency by -546ppb when locked to the GPS reference. I had to manually reset the offset to +/- 0 ppb to get the radio back on the exact frequency when locked to the external GPS, but now the radio is off frequency when switching to the internal TCXO. From this I deduce that the frequency offset is applied to both the internal TCXO and the external GPS 10MHz reference frequency. I believe the frequency correction offset should only be applied to the internal TCXO and never applied to the external 10MHz GPS frequency reference. Any frequency calibration offset to the GPS reference frequency would be the responsibility of the external GPS frequency source.
I am running SmartSDR version 18.104.22.168
Geoff - W8GNM