Can You Trust Your RX For a FMT?

My go to ham rig is my K3s, which includes the KTCXO3-1 Temperature Compensated Crystal Oscillator, a device specified to maintain 0.5 to 1 PPM,  One might think that in a reasonably controlled environment that the K3s could be used in CW mode coupled with an audio spectrum analyzer like Spectrum Labs to participate in the ARRL Frequency Measuring Tests. 

While it is useful for ham purposes, the reality of the K3s/KTCXO3 is that the frequency will vary considerably even within a few minutes as oscillator frequency is adjusted by the radio’s firmware.  This is NOT a temperature related thing.

Below is a screen capture from Spectrum Labs showing variability in the the received audio tone (CW mode, pitch 300 Hz) tuned to a Rubidium 10.0 MHz source.  The rig had been powered on for more than five hours in a room that was temperature controlled to 71.5 degrees +/- 0.8 degrees.

The audio tone shown below over an interval of 45 minutes, wobbles from a low of 299.65 Hertz to a high of 300.56 Hertz.  While certainly capable of measuring a frequency to within 1 Hz, it isn’t capable of getting “Green Band <0.1 Hz” FMT results.

The upper half of the waterfall shows the K3s in CW mode detecting a 10.0000 MHz rubidium source.  The detected audio wobbles significantly.

The bottom half of the waterfall shows the K3s in AM mode detecting a 10.0003 plus 10.0000 MHz signal, which create a precise 300 Hz beat-tone.  This indicates that Spectrum Labs and the soundcard in the K3s are rock solid in that mode, recording a signal of 300.000000 Hz for hours.

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