As far as I know this may be the easiest and high quality FM radio circuit that could be built by any new hobbyist. In addition, this circuit's FM reception quality is pretty impressive, although the level of sensitivity may not be that good.
The theory of this receiver may seem a little bit unconventional to many. It is according to a 'synchronizable oscillator' made up of transistors T2 and T3 which is synchronized to the captured FM stations through T1.
This transistor works as a wide band HF preamplifier for the FM band.
Actually, this amplification stage could possibly be removed and the antenna hooked up straight to capacitor C4, though the sensitivity would likely then be significantly lowered.
The oscillator is tuned by capacitor C5 to approximately 87 ... 108 MHz and, as a result of synchronization mentioned previously, the frequency exhibits exactly the same oscillations as the transmission captured by the antenna.
These frequency oscillations, obviously, signify the AF data.
The AF data is acquired by the basic reason of looking at the T2/T3 oscillator as an upside down transmitter then a tiny modulating voltage across P1/R5 could be ample to bring about the frequency modulation.
The opposite of this is that if the transmission is previously frequency modulated through an alternate method this will result in a modulated voltage to be created around P1/R5.
This voltage signifies the primary data sent. Therefore following low-pass filtering R6/C6 and amplification (T4 to T6) the demodulated AF transmission is introduced on the circuit.
The winding specifications for the various coils are furnished below the circuit diagram. The radio is tuned to different stations by means of tuning capacitor C5. Potentiometer P1 should subsequently be adjusted to allow the ideal FM reception of the transmitter.
When integrated with a loudspeaker and an amplifier (for example an LM386 amplifier), this simple FM radio circuit could very easily be constructed into a extremely sleek and stylish pocket radio.
Test Point Voltages
- 3.4 V
- 0.2 V
- 0.66 V
- 1.2 V
- 0.73 V
L1 = 10 turns CuL, ⌀ 0.5 mm (SWG 25), d = 3 mm
L2 = 13 turns CuL, ⌀ 0.5 mm (SWG 25), d = 5 mm
L3 = 10 turns CuL, ⌀ 1.2 mm (SWG 25), d = 5 mm