[wsjt-devel] Dial calibration errors

[Joe T. <jo...@pr...>]

Hi all,

One of the fun things about WSPR is the frequency accuracies that
are involved.  Having WSPR mode in WSJT-X motivates some serious thought 
about how best to handle frequency calibration errors in transceivers.

Typical dial readout errors in modern radios are a few parts per million 
-- for example, a 20 Hz error at 14 MHz.  For JT65 or JT9 such 
discrepancies are not very important.  But the WSPR sub-bands in 
conventional use since 2008 are only 200 Hz wide, and we'd like to use 
all of that range effectively.  If my transceiver's dial reads 20 Hz 
low, and yours reads 20 Hz high, and we both set our dials to the 
conventional 14.0956 MHz for 20 meters, after setting our WSPR Tx 
frequencies at random within the 200 Hz sub-band there's something like 
a 20% chance that we won't decode one another.

Earlier production versions of WSPR have handled these issues in a 
rather sophisticated way.  The User's Guide includes detailed 
instructions for determining calibration constants for your transceiver 
using over-the-air signals (see Appendix C of
http://physics.princeton.edu/pulsar/K1JT/WSPR_3.0_User.pdf ).  The 
resulting accuracies can be better than 1 Hz.

If CAT control is in use and  "Enable frequency correction" is ticked on 
WSPR's "Advanced Setup" window, frequencies sent to the radio are 
adjusted so as to compensate for the dial errors.  For example, if 
14.0956 MHz has been requested, the command for 14095620 Hz may be sent 
to the radio.

I picture this being implemented in WSJT-X in a similar way.  In this 
example, the radio dial would read 14.096520.  I'm suggesting that the 
frequency readout on the WSJT-X screen would read 14.095600, the 
supposedly "true" frequency.

Comments and suggestions would be welcome.

	-- 73, Joe, K1JT