Dear Mike, LF Group,
Mine is certainly not the only way of generating a BPSK signal, and
it is certainly true from experience that little trouble results from key
clicks on 136k, in spite of the fact that a lot of TXs on the band
have less than ideal keying. But in my own position, being in an
area relatively densely populated with LF stations, having a high
ERP compared to most, and transmitting beacon signals over
extended periods means I feel obliged to try and produce as clean
a signal as possible.
Actually, key clicks are never likely to be a huge issue on LF; when
compared to HF standards, even a full 1W ERP is a sub-QRP
signal, and the higher noise level will tend to swamp clicks from all
but the most local stations anyway.
Both "WOLF" and (usually) "Coherent" BPSK use a 10 bits/second
rate; so the number of phase transitions per second will be similar
to the number of on-off transitions of a 12wpm CW signal. Cutting
the drive before a phase transition would eliminate the click due to
the phase change, but would add 2 more, when the drive was
switched off and then back on again, so little would be gained.
Having "redundant" envelope modulation when no phase transition
occurs would actually increase the sideband levels - the
explanation I gave was something of an over-simplification. It
would still be neccessary to synchronise the amplitude modulation
with the phase modulation somehow, so little saving in complexity
would be acheived.
A perfectly good way of generating BPSK would be to first low-
pass filter the logic level signal, and then mix it with the carrier in a
linear balanced mixer (rather than an ex-or gate). A linear PA would
be required. This would be a good option if you do have a linear,
but do not have an SSB exciter that will give output on 136k.
To put the complexity into perspective, my prototype modulator is
built on 2 100 x 160mm prototyping boards, plus a heatsink
assembly carrying the MOSFETs. Implementing the waveform
generator part on a PIC - type microcontroller would roughly halve
the number of components and interconnections. If a PCB were
designed, the small-signal parts of the circuit would be no more
difficult to build than, say, a fairly elaborate audio filter kit. The
MOSFET pass element obviously needs to be quite big to handle
the power. The Heatsink assembly could be simplified by using a
really big MOSFET power module instead of 8 small ones, but I
doubt if this would be cost-effective.
cheers, Jim Moritz
73 de M0BMU
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