In an attempt to compare the different PSK-like modes I built a kind of
'universal modulator' that could generate a PSK signal, BPSK signal and all
variations of these. Such a modulator must be able to change the phase and
amplitude of the RF signal in a flexible way. This can be done by
phase-shifting the RF signal by 910 degrees, then modulate the orgiginal -
and phase shifted signal in a DBM (double balanced mixer) and finnaly sum
both signals.
Sounds complicated, but an RF of 137kHz and slow baudrates (10Bd) makes it
a lot easier than it would be on HF/VHF and at fast baudrates.
In fact the complete modulator is no more than a few opamps, 2 cheap DAC's
(digital analog convertors) and a PIC processor to control the modulation.
It all fits on a 7 by 7cm PCB.
Changing modulation is done by changing a few lines of code in the PIC.
So far I 'played' with 3 types of modulation : real BPSK, PSK (amplitudes
enveloped) and what I call 'soft' PSK (gradual phase transitions instead of
brute phase jumps).
The spectra (and and block diagram of the modulator) can be found at :
http://www.qsl.net/on7yd/PSK_TEST/psk_test.htm
It confirms that :
- BPSK is far too wide to be used with high power on 136kHz (unless you
live in the middle of nowhere or you just don't care about others)
- that PSK has the best (most narrow) bandwidth
- that 'soft' PSK may have some potential as a class-D suitable mode with
an acceptable bandwidth.
I intend to test the different modes on the band in the near future, I got
hold of a linear 100W amp that goes up to 250kHz.
Unfortunaly strongs winds took down my antenna, so first I will have to
re-install the antenna.
73, Rik ON7YD
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