Haven't seen / played / even heard of them, but we
can get a rough idea of its possible performance just by looking
at the picture of the PCB on the web page. Image filterign is
teh biggest problem with this single conversion route.
Assuming all the image filtering is done by those three coils in
cans that are visible on the photograph:.
An AADE design for a suitable third order bandpass filter,
with 8kHz bandwidth and allowing a compromise 1dB ripple
suggests image rejection at 125kHz will only be around 30dB.
(Top coupled capacitor design, so low side rejectino is
better than high side) Not sure what signals are down there
to get through, and a highly resonant antenna will help a lot
more with image rejection - but if its for a grabber, you're
probably going to use a broadband active antenna.
Can't make out much else from the web page photos, and it
give no specification. But such a simple converter concept
can't go hopelessly wrong, if you accept the limited image
filter that would be achieved with three tuned circuits
If I were to try that approach, I'd got for an IF of 15 -
19kHz, at the top end of normal soundcard coverage, so
bringing the image down to 105kHz and making it easier to
filter - although there you would just be creeping into LORAN
area.
Andy G4JNT
