by Harry Lythall - SM0VPO
Here is a very simple VHF FM receiver which is little more than a single IC and a "slack handfull" of capacitors. Note that an external amplifier is a really necessity since the unit only delivers about 70mV of AF.
The 10K resistor (*) is only required if you want the receiver to mute (squelch) under no-signal conditions. You could add a 100K in series with this resistor to get an adjustable squelch. This circuit will JUST drive a crystal earphone or high impedance headphones directly, but an output isolating capacitor (100nF) is needed for any other device. L1 is 6 turns No 18 SWG enamelled wire on a 5mm former, but you may have to play with the values a bit. I used a coil fabricated on the PCB itself, tuned with a trimmer capacitor.
All the other components are just a bunch of capacitors which are fitted to the board at the other side of the chip, just to make it look a bit prettier.
|Sensitivity (3dB S/N)||-||1.5uV||-|
|Sensitivity (26dB S/N)||-||5.5uV||-|
|AFC Range (110MHz)||-||+/-300KHz||-|
|Max I/P Signal||200mV||-||-|
As you can see, this receiver is VERY sensitive, small and seems to work very well indeed. It is in fact a full superhet receiver with a very low RC tuned IF. The Image signal is rejected by the action of the AFC which functions to push it away. With suitable antennas and terrain, with this receiver you could easily get the full 500 meters from the FM wireless microphone v5. I am offering this receiver in kit form, including solder, antenna wire etc. All you will need to provide is the battery, tools and soldering iron. Here is the kit version fully assembled.
The total size is 45mm x 48mm. If there is any interest in the kit then I may even add an AF amplifier kit so that you can make a full bedside/table radio.
Original by Harry Lythall - SM0VPO
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