Well, I obtained a nice temperature compensated 10 MHz clock for my propeller board, courtesy of Eldon WA0UWH. Here you can see the little daughter board that Eldon designed. The normal 5 Mhz crystal has been removed and the software adjusted to set the system clock at times 8 instead of times 16, so we are still running at 80 MHz.
After spotting it on 10.140.200 MHz I captured my WSPR decode and as you can see in the last spot after touching up the pot a little, I am spot on frequency (10.140.200) and zero drift. Yeah! The waterfall display shows me swinging the pot through its range over several samples.
The board has a nice, though a little touchy pot to net the frequency where you want it. It currently has a range of about 100 Hz. In the final version, I think this will be tweaked to slow the tuning down to about 1/2 that amout or so. Once adjusted it stays put and doesn't seem to be affected by temperature as expected.
The oscillator output looks like this:
So, I am quite pleased with this little modification to the propeller proto board.
I want to see how raising the chip temperature to about 50 degrees C will affect frequency stability of the propeller chip. I decided that for my initial testing, I would just put the board under a halogen lamp and monitor the chip surface temperature with a laser thermometer.
After a few minutes the surface temperature has stabilized at about 55,4 C or 131,7 F.
I fired up my QRSS beacon code and gave it a bunch of text to send. The initial results results are not encouraging:
When I turn off the heat source, the expected drift back upwards begins. I saw more than a 150 Hz change in frequency from heat on (50C) to heat off (20C).
One thing I notice is that the crystal is removable and therefore there is considerable mechanical instability in the crystal socket and the associated change in circuit capacitance and resistance that goes along with these connections being able to move. I noticed that as the board heated up there were more small excursions in frequency as evidenced by the jitters in the waterfall display of the line than there are when the board is at room temperature where the jitters tend to disappear.
So, I supect there is not much I can conclude here other than I have an incredibly crude setup for this kind of testing. My intuition and the deep dive of the frequency with the heat on indicates to me that it is likely that whilst the surface temperature appeared to have stabilized, the internal temperature of the chip may not have yet stabilized. I need a proper environmental chamber that can maintain temperature tolerances more closely and need to give the device sufficient time to completely heat up. I also need to address the crystal socket issues as it appears to introduce a lot of instability as it heats up.
More head scratching whilst I figure out a better setup.
