Converting FM Transistor radio to 350khz

[Camerart]

Hi,

I would like to know if it is possible to modify (simply) a transistor
radio so it can pick up frequencies on other ranges. I particularly
want 300 to 400 khz FM.

Cheers, Camerart.




--
Camerart

[Ralph Mowery]

"Camerart" <Camerart...@radiobanter.com> wrote in message
news:Camerart...@radiobanter.com...

It would not be very easy to do that. It would be easier to take some of
the ICs that are designed around those frequencies and build a FM receiver.

It is easy to modify a FM receiver to go from about 50 to 120 MHZ. Outside
this range requires more work.

[Michael Black]

On Mon, 13 May 2013, Camerart wrote:

>
> Hi,
>
> I would like to know if it is possible to modify (simply) a transistor
> radio so it can pick up frequencies on other ranges. I particularly
> want 300 to 400 khz FM.
>

What are you trying to do?

There's virtually nothing at 300 to 400KHz, and it won't be FM. So I
don't know if you typed that wrong, or have some other scheme in mind.

There was a time when retuning FM broadcast radios was't uncommon, move
them to the "public service band" or the amateur 2M band. Shorten
the front end coils (or spread them out more, they would generally be air
wound) or see if the trimmer capacitors are such that they can returne to
the other range (higher in frequency). But there were problems. The
most important is that FM broadcast is wideband FM, while the 2way of the
public service band or 2m amateur is narrow band. At best little audio
would be recovered. The FM broadcast signals are wideband, so the
receivers are wideband, so even if you could recover the modulation,
adjacent signals could interfere.

If you had a need for a wideband FM receiver, then yes, the front end
could be modified or a converter put ahead of it.

But it's still not clear what you want. The 300 to 400KHz in your post
suggests you want to add an FM detector to an existing radio, that happens
to have a 455KHz or so IF. If that's the case, find something with a
matching IF that is for FM. COrdless phones, better baby monitors, better
49MHz walkie talkies, old clunky cellphones, they all have FM IF strips,
and generally landed at 455KHz or 450KHz. You could extract the FM IF
strip and use it as a module. WHether it's 455KHz or 450Khz would vary on
the design, you might have to poke around, or make do witha 450KHz IF
strip (which may not matter, since many existing receivers in the solid
state age use 450KHz anyway). Then just connect it to the existing
radio's IF strip.

More recent equipment is likely to use some other scheme, or use such tiny
and high density components that you can't get anything useable out of it.

Michael VE2BVW

[Arid ace]

On Mon, 13 May 2013 10:53:43 +0100, Camerart <Camerart...@radiobanter.com>
wrote:

An FM radio usually has an IF of 10.7 MHz so in order to use it between 300 and
400 KHz would require to change the value of the LO to a fixed 11.05 or 10.35
MHz, and to change the input circuit to a bandpass for the desired range.
Problem is that the LO will be relatively close to the IF and could swamp it,
especially when the mixer doesn't suppress the LO (like in a DBM) and the IF
filters don't have a good enough stopband rejection. It probably is simpler to
use a dedicated FM demodulator IC like (antique) TBA120, CA3089 etc. as their
sensitivity will be high enough. At those low frequencies, in principle it is
also possible to use the four CMOS EXOR (or even NAND) gates, as preamp, phase
detector and VCO, for a comparable sensitivity.

Jan

[Rob]

You are sure you didn't mean to write "300 to 400 MHz FM"?

[Michael Black]

I'm certainly waiting for that clarification.

Michael VE2BVW

[raypsi]

On Monday, May 13, 2013 5:53:43 AM UTC-4, Camerart wrote:
> Hi,

It would be easier to build a dedicated radio for that frequenncy, but you could use a transconverter and you might have to make an AM to FM converter in there with a BFO. If you want to receive FM at any frequency all you need is just a transconverter.
Good luck and good dxing
73
de n8zu

[Jerry Stuckle]

The other problem here is the FM broadcast band (which is what I think
you're talking about) uses +/- 75khz deviation, would would be way too
wide for the 300-400 khz band.

It would be possible - you'd have to change the front end, local
oscillator, narrow the IF strip and redo the discriminator, but it could
be done. Probably much easier and cheaper to just get a radio for that
band.

--
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