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TCXO frequency calibration offset - Flex 6600M

Geoff_W8GNM Member ✭✭
edited February 2018 in FLEX-6000 Signature Series

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

Geoff - W8GNM


  • N7IPY
    N7IPY Member
    edited February 2018
    Thanks for the information Geoff, I was going to ask the question today and you answered it for me.  I will be attaching my Lab 10Mhz reference to my new 6400 this evening and was wondering if I needed the offset when using an external clock source.  As this is my first Flex Radio and I have only had it 6 days I still have much to learn.  Will - N7IPY
  • Tim - W4TME
    Tim - W4TME Administrator, FlexRadio Employee admin
    edited February 2018
    This is most likely a defect where the offset is not properly reset after selecting a different clock source.  I have entered this into our bug tracker as defect #5960.  Thank you for the detailed bug report.
  • Geoff_W8GNM
    Geoff_W8GNM Member ✭✭
    edited February 2018

    Hi Will,  Yes you will need to manually set the frequency offset to 0.0 at the bottom of the radio panel for the radio frequency to be correct with your external 10MHz GPS reference attached.

    Be careful to check the external reference frequency p-p voltage level and keep it in the range specified in the Flex hardware documentation.

    The frequency offset function should be changed to only apply to the internal TCXO.  That way if your are operating with the external GPS detected automatically, the radio will switch back to the corrected, internal, TCXO frequency if the GPS is lost.  73, Geoff

  • Geoff_W8GNM
    Geoff_W8GNM Member ✭✭

    This bug in the frequency offset being applied to both the internal TCXO and the external 10MHz GPSDO frequency reference still exists in SSDR ver. 2.8.1 and should remain on the bug list.

    The external 10MHz input should never have a frequency offset applied to it.


    Geoff - W8GNM

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