Dear LF group,
Good reception of SAQ here, another opportunity to get out the
electromechanical RX, and to try out SM6LKM's SAQrx program, also with good
results.
Small frequency errors are usual with SAQ's signal - if you watch on a
spectrogram during the tuning-up periods, it is often apparent frequency
adjustments are being made. PA0SE some time ago provided the circuit
schematic for an Alexanderson alternator TX. From this, the speed /
frequency control of the alternator seems to work in two ways. One uses an
AFC-type arrangement to generate a feedback signal which controls the motor
speed. The tuned circuit in the AFC system is effectively providing the
carrier frequency reference. The other is a feed-forward compensation that
increases the driving power to the motor when the key is down to compensate
for the increased mechanical load of the alternator. Also, tuning the
antenna would change the alternator output current, and so the loading on
the motor. I think some inter-dependent adjustments must be required during
each transmission to get the frequency correct, and at the same time
maximise antenna current and minimise "chirp" on keying the TX. But
considering there is no precision "clock" to set the TX frequency, and the
electro-mechanical nature of the control system, keeping it within 0.1% of
nominal frequency is pretty good.
Cheers, Jim Moritz
73 de M0BMU
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