I didn't see the start of this thread, but I think that Oyvind is looking for a 'simple' receiver with a PLL / digitally steppable tuner so that he can scan known R/C control freqs, and by looking at an 'S-meter' output (signal strength), he can digitally decide whether that channel is in use or not...

The analogue detail that he needs to know is the RF channel spacing (the .XXX in the original note) - then he can start looking for a receiver that has the prerequisite master oscillator and divider circuitry to hit all those R/C channels while under digital control.

Any more than that, and sorry, I'm not an analog person !

Regards
MC


-----Original Message-----
From:   Mal Goris [SMTP:mgoris@NFRA.NL]
Sent:   Thursday, July 31, 1997 3:48 PM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        Re: R/C RF scanner design. Suggestions?

Oyvind Kaurstad writes:
> I'm planning on designing a "channels in use" indicator
> for R/C purposes. I know there are commercial units for
> sale, but they are expensive.=20

[snip]

> The hard part is obviously the RF design, and I wonder if anyone
> has any cheap and clever ideas on this.
>
> I live in Norway, and the RF frequency in question is 35.xxx MHz.
>
> The channel spacing is 10 kHz.

Everyone else who responded to this seems to think you were asking how
to build a superhet receiver. I sort of thought you were asking how to
cheaply build 40 or 80 receivers. If that is what you were asking,
I'll give you my ideas on how to do it. I'll assume you have 80
channels like in the US. To do this job cheaply you must reduce the
hardware. That is, build one single receiver that mixes the entire
35.XXX band down to baseband. You can decide whether it is best to use
a heterodyne or superhet receiver. Once the signal is at baseband, low
pass filter it to 80*10=800 kHz, sample and hold and 8-bit digitize
with at least 1600 kHz sampling. Store, say, 512 samples in your
micro's memory and then FFT it. You could probably choose a window
function that gives a broad mainlobe and low sidelobes because you are
not really interested in resolution but in detection. Use a threshold
scheme to decide whether there is a signal each channel. I am
assuming, with all of this that the R/C carrier always exists. That
is, a space is not indicated by no transmission. If it is then maybe
wavelet transforms would be a better choice than the FFT.

This is a pretty brief outline of what I would do but I think you
would need to understand the theory pretty well before attempting
it. One thing I can say is that I don't think I would attempt the
512-point FFT on a PIC.  Speed is not a concern so I would just choose
any old micro that has a proper stack. I think that an FFT of that
size can be most easily done with a recursive function that would go 9
or 10 levels deep.

Mal Goris
--
http://www.nfra.nl/~mgoris/