[hpsdr] exact frequency determination

[Brian Lloyd]

Several of the things I like to do involve precise frequency measurement in
the HF spectrum using a Hermes and/or Angelia board, and PowerSDRmrx. Since
my frequency reference is either a GPSDO or an Rb reference, the potential
accuracy is very high. The actual limit has become the error in radix
conversion between the decimal frequency value and the binary frequency
control word used by the NCO in the Hermes/Angelia FPGA "firmware" plus the
error in PowerSDR's NCO frequency control word.

Now this error is not large, typically on the order of 20 or so mHz most of
the time, but there are some combinations of control words that produce
larger errors.

Now these errors are completely deterministic. When one enters a frequency
in decimal that produces a binary control word for the nearest possible
frequency, usually within a couple of handsful of mHz. But if you know the
binary control word, it is possible to work that backwards to the ACTUAL
frequency. So, yes, if I enter 7,150,000.000 Hz, I really DO want to know
that the radio actually tuned to 7,150,000.037 Hz.

What would be nice would be to have PowerSDR actually include this
calculation and display it, or, better yet, include it as the actual value
represented when queried with CAT. Still, just knowing the calculation so I
can do my own corrections by hand would be extremely useful.

Thanks in advance.

-- 
Brian Lloyd, WB6RQN/J79BPL
706 Flightline Drive
Spring Branch, TX 78070
brian at lloyd.com
+1.916.877.5067 (USA)
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