Silicon Labs puts out an application program that will calculate the various settings for the chip given a desired frequency. It will also calculate a new frequency and the associated settings based on a ppm change.
What becomes apparent from this tool is that Silicon Labs considers the 3500 ppm frequency change to be based on the output frequency rather than the DCO frequency. My interpretation of the datasheet therefore has been incorrect as I had concluded that the 3500 ppm frequency excursion limit was calculated based on the DCO frequency.
Based on this, the following table shows the amount of frequency excursion that can be done taking into account the Minima transceiver IF frequency of 19.997000 Mhz.
Band + IF 3500 ppm (kHz)
1.8 21.797 76.2895
3.5 23.497 82.2395
5 24.997 87.4895
7 26.997 94.4895
14 33.997 118.9895
18 37.997 132.9895
21 40.997 143.4895
24 43.997 153.9895
28 47.997 167.9895
29 48.997 171.4895
30 49.997 174.9895
50 69.997 244.9895
I have also worked up a spreadsheet that illustrates the amount of change in RFREQ for a given output frequency increment of 0.01, 0.1, 1, 10, etc Hz.
fOut (MHz) fOut RFREQ
change (Hz) change
14.00000000 0 0
14.00000001 0.01 9
14.00000010 0.1 82
14.00000100 1 818
14.00001000 10 8177
14.00010000 100 81763
14.00100000 1000 817625
14.01000000 10000 8176246
14.10000000 100000 81762455
15.00000000 1000000 817624545
Based on this, it should be possible at these operating frequencies to set the frequency to within .01 Hz for ham bands up through and including the 6 metre band.
You would not however be able to keep the same divisors over more than the 3500 ppm range specified for the band of interest, contrary to my table above.
More to come...
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