The charts below are are showing the frequency reported by WSPR 2.11, which as I understand it will be the first frequency it decodes and the amout of drift from that frequency over the life of the transmission.
The amount of drift once the transmission begins is in the opposite direction and after some initial jitter assumes asymptotic behavior at a consistent 2 hertz downward drift as the device warms up.
While none of these figures is horrific, good engineering would indicate that for this kind of application, careful attention to thermal stability is important.
What kind of frequency reference are you using? Is it a crystal osc or have you something more exotic on there?
ReplyDeleteInteresting. I too observed downward drift which resulted in no spots at all after it drifted out of the WSPR sub-band. But after setting the initial frequency high enough it has been OK, which led me to believe it stabilises after it warms up a bit. I haven't measured the drift as conscientiously as you though, Jeff. I wonder if I will experience the same problem I found with my other beacon transmitters - that they are stable enough when operated as bare boards on the workbench but drift once they are mounted in a nice case that keeps the heat in!
ReplyDeleteThe propeller device does seem to stabilize if you have a beacon mode that leaves the carrier on all the time such as with FSK QRSS. With the carrier on/off modes where there is any pause between transmissions, an un-temperature-stabilized propeller device will drift. My thought on this is to build an insulated box that raises the temperature of the device above any of the temperature excursions that normally happen when in use and hold it there. I suspect these drifts are a result of the increased power utilized when generating RF, so raising the ambient temperature to something like 30C or so, may settle things down, we shall see. I also plan to study the power consumption of the device in different beacon modes modes.
ReplyDeleteWith regard to my frequency reference, I am doing nothing fancy. I have a nice commercial receiver with a high stability oscillator. I have performed the calibration procedure for WSPR 2.11 as described in Joe Taylor's documentation in Appendix C. I then just plot the frequency and drift information recorded on the main WSPR 2.11 application window over the sample time period in a spreadsheet application. Basically it is measuring the drift and frequency of the audio signal from the receiver.