Dear LF Group,
Regarding frequency stability - for a long time for QRSS I used
simple VFOs, both for transmit, and in my homebrew RX. It was
easy to achieve TX stability within a few Hz using the VFO,
provided the temperature didn't vary too wildly; the main
requirement is just that your signal frequency does not drift into
someone else's, or a Loran line, carrier, etc. so a few Hz is
adequate. It is easy to keep an eye on the spectrogram display to
make sure this is not happening, and make manual adjustments if
neccessary - of course, much finer tuning adjustment is required
compared to normal CW. The exact frequency is not important,
because it is easy to monitor 100Hz or more of bandwidth at the
receive end. Having said that, some truly dreadful VFO's have
appeared on the air from time to time! The mixer-VXO circuits
seem to be somewhat better than VFO's in terms of stability and
are perfectly adequate for most things.
On receive, stability is even less important - the frequency has to
drift many Hz per minute to make any difference from the sensitivity
viewpoint, and this isn't hard to achieve. Drift just results in a bit of
a slant on the spectrogram display. I once tried an old HRO on
136kHz, and once warmed up, stability wasn't too bad for QRSS.
Selectivity left a lot to be desired, though, so I wouldn't recommend
it. Provided you don't mind checking and adjusting the TX and RX
frequencies every few minutes, QRSS operating with 3s per dot
can be done with crude equipment. Longer dot lengths, or long-term
monitoring, requires something a bit better, but any synthesised or
crystal controlled equipment should be adequate.
My experience of BPSK is currently very limited - but using a
locally generated signal with reasonable SNR, VE2IQ's "Coherent"
software seems to cope with a 1Hz frequency error without much
trouble, but 2Hz is too great, when using the MS100, 10 bit-per-
second data rate. As Andy points out, it isn't hard to get better
stability than this. The 1 in 10e-7 level of stability seems to be
more or less standard for the ovened references that are getting
cheaper in surplus test gear available at rallies and so on. This
should be OK for 1 bit-per-second BPSK speeds at LF - the main
problems seem to be knowing what frequency to tune to in the first
place, and the fact that the HF gear that normally gets pressed into
service wasn't really conceived with such narrow tuning resolution
in mind.
And loops- A receiving loop has it's own intrinsic signal to noise
ratio that, for a small loop, depends on the unloaded Q. Noise in
the loop arises from the thermal noise produced by the resistive
losses. The higher the unloaded Q, the greater the available signal
power, while the noise power stays the same. However, the overall
system signal to noise ratio depends very much on the type of
circuit the loop is connected to - the optimum matching for a
particular receiver input may or may not significantly alter the Q.
The FET preamps used tend to give their lowest noise figures
when fed from a high source impedance, and have a high input
impedance themselves, so don't significantly reduce the Q. This is
not generally true of other input circuits. A low impedance input
circuit will almost certainly acheive it's best SNR using matching
where the loaded Q is significantly lower than the unloaded Q. The
loaded Q therefore does not directly control the signal to noise
ratio. It is not easy to say what will give the best results, because
receiver input circuits vary, and their input impedance is likely to be
anything except 50ohms, especially at LF.
But usually obtaining the highest possible SNR is not that
important, because it is fairly easy to make the antenna/preamp
noise less than the external noise, of which there is plenty at LF,
with reasonably sized loops. G3LNP's design deliberately degrades
the Q to give a wider bandwidth, and eliminate the need for remote
tuning. Another reason why you might not want the highest possible
Q is that a high Q loop is more easily de-tuned, for example by rain
or movement.
Cheers, Jim Moritz
73 de M0BMU
|
