Wednesday, March 7, 2012

Propeller frequency stability

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.

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