Dear LF Group,
Regarding portable receivers for 9kHz, I have now added a BFO/product
detector/AF output to my 9kHz loop/preamp/filter circuit. This effectively
converts it into a small, self-contained fixed-tuned upper sideband receiver
for headphone aural reception around 9kHz. The sound card preamp function is
still available of course. The main idea of this was to have a portable
receiving system for investigating noise sources at 9kHz, but it would also
work well for near-field transmission experiments that people are trying. I
also have an up-converter which can be used in conjunction with an FT817 as
a portable 9kHz RX, but the dedicated circuit is much smaller and simpler,
and more convenient. It only consumes about 20mA or so from the 12V supply,
so only a small battery is needed.
The attachment shows the complete circuit - The 7.5kHz BFO and product
detector uses a 4053 CMOS analogue switch IC as a combined mixer and
RC-tuned oscillator. There is a bit of an explanatory diagram at the bottom
of the schematic as to how this works... The BFO frequency stays within
about 100Hz, which is fine for aural reception - for narrow band modes
needing higher stability, down conversion can instead be performed by a PC
and sound card of course.
The overall audio gain between loop antenna input and headphone output is
very high, and when the "wide band" 3 - 22kHz bandwidth is selected,
oscillation can occur due to internal ground loops, or external coupling
between loop antenna and headphones. But for listening purposes, the wide
band setting is not very useful anyway, and feedback is not a problem when
the bandpass filter is selected since then input and output are in different
frequency ranges. The complete circuit is contained in a small diecast box -
however, I found this is not very effective as a shield against magnetic
fields at 9kHz - best to keep it some distance from any lap-top or PC which
it is connected to.
It is quite interesting to walk around with loop and RX listening to the
wierd buzzing and whirring man-made noises and mains hum that is
superimposed on the background QRN around 9kHz, and a good way of checking
the suitability of a receiving location for future 9kHz tests. I have found
that if you stand near overhead power lines, the noise present sounds quite
different depending on the orientation of the loop with respect to the
overhead wires. I guess this is due to different levels of 50Hz harmonics
being present in the differential-mode and common-mode currents flowing in
the lines. If you try this, you can expect very strange looks from passers
by ;-)
Cheers, Jim Moritz
73 de M0BMU
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