Sunday, March 11, 2012

Stable Frequency for the Prop

More testing is needed, but it looks like the use of a 10MHz TCXO is going to work very well with the Propeller Software and Beacons. See previous posts, where the plans were made.

When I first installed the TXCO I could see that it was going work, as it did not produce the Hash (or fuzz) on the transmitted signal that was shown on previous blogs.

From turn-on the Frequency was very stable, after a few minutes no change could be detected. The output was NOT exactly on frequency, as I had not bothered to hook up the "frequency voltage control" line, I wanted to see the behavior without, and the single page manual does not provide much details of it's use.

The Frequency was low by about 1.4ppm as indicated on a previously calibrated Spectran. Within the Spin program I was able to correct the frequency and ran several more tests. All tests worked as expected (or hopped).

To complete the installation I hooked up a simple resister divider to drive the Frequency Control line; a 1K ohm fixed up to 3.3v, 2K ohm variable to Freq Control, and a 1K ohm fixed down to ground. Then I removed the previous Spin Freq Corrections and re-compiled the program.

The RF output frequency was within a few Hz of spot on, a quick twist of the variable Freq Control Pot put it on frequency. The pot is a little touchy for small adjustments; I will replace the two 1K ohm fixed resistors for maybe 2K ohms each.
Beacon Stability with TCXO
WSPR Above, QRSS Below 
as Seen by my Local (overloaded) Receiver
If you look very-very close, you my detect about a 1Hz drift between the two WSPR signals, but I am not sure that I can trust my receiver, or any part of my test configuration for that fine of resolution. Normally I check with and other external sites for conformation of details, but wsprnet was down when I checked.

More tests are needed, but I think this is a excellent fix for the RF Frequency Stability Problem for the Propeller Beacons.

I will add an installation photo here, later.


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