Brenda Ann wrote:
"Slow Code" wrote in message
nk.net...
"Ridgh" wrote in
ups.com:
Hello,
I need to convert a domestic analog FM receiver to the 108-140 MHz
band.
I've bought a Sony ICF-S10MK2 receiver and found that the oscillator
tank has a 4 turns of 22 gauge. I've turned them apart, so the distance
between the turns will be a little greater than the factory adjusted
coil, and could receive up to about 115MHz, no more.
Is this the way to do it? Maybe to cut one turn will be better?
I'm not stucked to this receiver, I'd like to know if other cheap
receivers may be converted, and if this is the way, or if I have to
adjust anything else. I have all the gear needed, oscilloscopes,
counters etc. - but no much knowledge on this particular field.
I'll be very thankful for your help.
Ridgh
You might be able to get it higher with more inductor adjustments, I don't
know the circuit. Maybe have to add some capacitance to the tank.
Here's your real problem though. Broadcast FM is wideband. FM
communications 135-175 MHz are narrow band. That means more modifying is
needed to the receiver in order to hear the narrow band FM better.
It would be easier just to buy a cheap VHF receiver. Talk to the people
in rec.radio.shortwave for ideas on cheap plastic receivers. They're
experts.
Couple problems with your answer. Let's take them one at a time:
You might be able to get it higher with more inductor adjustments, I don't
know the circuit. Maybe have to add some capacitance to the tank.
To raise the frequency of resonance, you would decrease either inductance or
capacitance, or both. I.E. you would remove turns from a coil, or use a
smaller series or parallel capacitor. I have successfully modified a GE
Superadio II to receive as high as 155MHz by removing a turn from each of
the RF, Antenna and Oscillator coils and realigning for proper tracking.
Here's your real problem though. Broadcast FM is wideband. FM
communications 135-175 MHz are narrow band. That means more modifying is
needed to the receiver in order to hear the narrow band FM better.
True, that broadcast FM is wideband (150KHz total, 75KHz deviation), but
most of the frequencies the OP wishes to listen to are in the aircraft comms
band, and are not FM at all, but fairly broad band AM (IIRC, 12KHz). These
can be heard by slope detection on an FM radio, and the discriminator (or
other detector circuit) can be detuned a bit to allow for reception with the
majority of the radio's circuits tuned to the center frequency of the
carrier. The detector circuit could be modified to detect only AM (and
therefor slope detect FM) fairly easily, actually by removing some
components and wiring around them, but overall this is not absolutely
necessary.
I believe you have the slope detection application backwards, i.e. it
allows FM to be demodulated on a radio set up for AM demod.
A slope detection FM demod puts the FM signal through a filter that is
not centered properly, i.e. the signal's frequencies are on the slope
of the filter, not the passband. Then as the frequency changes, the
attenuation of the signal also changes, which effectively has turned FM
into AM. Then you rectify and low pass filter, as you would do with an
AM signal.
Note also that the FM demodulator rejects AM by the use of a limiter.
Thus FM radios reject AM quiet well, but the opposite is not true, i.e.
FM can be demodulated by an AM demodulator that is tuned off center a
bit. High end FM radios will spec their AM rejection:
http://www.fanfare.com/fta100-specifications.html
The AM rejection in this application relates to the ability to reject
atmospheric effects on the FM signal strength, but it would reject
broadcast AM too if somehow the AM was on an FM broadcast frequency.