What's the Frequency, Kenneth?

The developers are heads down taking care of the stuff the beta team has provided as feedback and they are tackling the hard stuff first, so there were not too many updates that came out of the software factory yesterday and today other than the ability to key the radio using the FlexWire v2.0 port and some features that would be applicable to transverter use.
With a little time in-between the software feature/functionality testing, I decided to do some frequency stability tests of the FLEX-1500. It should be very good because it uses the same XO as the FLEX-3000.

I set up a test scenario where I used WWV as the frequency reference (yeah, I know, it is not the most stable reference due to atmospheric perturbation of the RF) and used the Frequency Analysis mode in Fldigi v3.20.0b8 to plot variations from a center frequency. In this mode the Fldigi decoder is merely a very narrow band AFC tracking filter. The filter bandwidth is set to 2 Hz and the tracking time constants to about 5 seconds. This reads variations in the frequency and logs them to a CSV file for further analysis.

Below is a graph of the frequency error for 32,000 data points that represents a little less than 4.5 hours of data collection.

(Click on image to enlarge)

What I have observed is that the frequency variation is approximately +/- 0.6 Hz @ 10 MHz over a long sample time (hour). That is a total variation of 0. 12 ppm. Not to bad for a $650 radio. How much is this the ionosphere induced Doppler effect and what is real error? Don't know. I'll let you know when I get my Rb atomic frequency reference.

But what is good is that the frequency drift is minimal when the ambient temperature is fairly constant. A more interesting test would be to observe the drift as a function of temperature during transmit. Maybe that will be the next science project once I can dig up a thermocouple.