[Elecraft] {k3} PSK31 tuning

[Kok Chen]

On Aug 12, 2010, at 8/12    9:09 AM, Lyle Johnson wrote:

> ... Tuning is critical and an error of more than 5 Hz changes from  
> solid copy to no copy.

With a binary PSK demodulator, you can be off tuned up to an amount of  
plus or minus 90 degrees of perfect phase coherency between bit  
periods (what is sometimes called a "modulation chip").

BPSK31 has a bit rate of 31.25 per second. That means the carrier or  
pseudo carrier can be off up to 90 degrees in 1/31.25 of a second,  
i.e., 7.8 Hz is how much a demodulator can be off tuned under good SNR  
conditions.

With QPSK31, everything is halved since the signal constellation is  
doubled but all still lie on the circumference of a single circle.

If a higher order DPSK demodulator is used (for example, an Okunev  
demodulator), then the tuning range will be reduced accordingly.  A  
second order Okunev for BPSK31 requires 5.2 Hz tuning accuracy, a  
third order Okunev requires 3.9 Hz, and so on.

Higher order demodulators can give very significantly fewer decoding  
errors, but you also need to tune more carefully. Higher order  
demodulators (ditto coherent demodulation schemes such as the Costas  
Loop) also perform poorly when there is HF multipath.

Under non-ideal propagation, the simplest non-coherent differential  
PSK demodulator often works best for copying BPSK31.  It is also the  
most forgiving when it comes to tuning accuracy.

73
Chen, W7AY