## Tuesday, May 6, 2014

### Si570 Programming continued...

While I certainly didn't intend to write a novel on the topic of Si570 programming, it is a topic that at least for me is unclear from datasheet.  What follows are my current findings.

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...