Dear LF Group,
With some 136kHz tests I did with Andy last year, it was apparent that,
as well as accurate bit timing, a more or less constant carrier phase
difference could be maintained over periods of hours using a GPS-derived
carrier frequency. I suppose that the receiver would in principle then
only have to determine the initial phase difference due to the
propagation path, and no further clock recovery would be required. This
might not be true over sky-wave paths with more rapidly varying
characteristics though. Also, the indifferent accuracy of sound card
sampling clocks would be a limitation if a sound card was to be used for
reception.
These tests used PSK at 0.1 bits per second - an advantage of a bit rate
this low is that the "key clicks" generated by the abrupt phase
transitions of the BPSK modulation are not a significant issue, because
the phase transitions occur so infrequently. It also makes it a simple
matter to manually synchronise the various clock circuits.
Cheers, Jim Moritz
73 de M0BMU
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