[Lowfer] WE2XGR/2 MFSK8, MFSK16

[Jay Rusgrove]

Thanks to W1ATT, W3NF, KA9YCB, KN8AZN, W1LE, VE1VDM, VE3EAR, N8TL, K3DJC,
W1TAG and W0RPK for the reports on last night's test. Most, but not all,
had 'good' sync in both modes with MFSK16 offering slight to moderately
better sync performance.

I conducted local reception tests last night of the two modes - at a signal
level about 15 dB above the receiver noise floor.   No antenna was connected
and therefore the decoder had no noise or fading to contend with -probably
close to ideal conditions for the decoder. BTW, the transmitter and receiver
frequency accuracy here are about .01 Hz (both TX and RX are GPS
disciplined).

In MFSK8 (with program AFC off), once sync was established I could dial the
receiver off in 1 Hz steps and retain copy out to 4 Hz either side. At 5 Hz
and somewhat beyond the decoder acted as if it was happily receiving the
signal but printed garbage. A move back to 4 Hz and the decoder would
sluggishly resync. As I stepped the receiver closer back to the correct
frequency resync time improved. I did not notice a significant difference
between sync times at the startup of a new line or resync in the middle of a
transmitted line. Also, program AFC, when selected, was only marginally
effective - perhaps extending the sync range another Hz or two.

In MFSK16 all of the above held true but at about twice the allowable
frequency error. It was possible to tune off to about 8 or 9 Hz either side
before copy became garbled. Resync performed similar at the frequency
extremes and improved the closer the receiver was tuned to the correct
frequency. AFC action was likewise similar.

Keep in mind that these tests were made under ideal conditions. Throw in
fading, QRN, static and ionospheric instability and one can see that MFSK8
requires critical attention to frequency accuracy and stability. Luckily,
MFSK16 is somewhat more forgiving.

Suppose another worthwhile battery of tests would be to run the various
digital modes under both very weak and moderate signal levels to determine
tolerable frequency error. Not all stations on the band will have
'laboratory grade' frequency stability, especially during an emergency
communication period, so some digital modes may not be practical under those
conditions. Perhaps John (if he's not burned out from the last tests!) and I
can run tests over the near future to get a better handle on this issue.

Thoughts and suggestions are always appreciated.

Jay W1VD  WD2XNS  WE2XGR/2