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FM stereo transmitter, FM transmitter schematic


Tanınmış Üye
Süper Moderatör
Input impedance: 47kOhm
Input level: less than 2Vrms
Output level: maximum 2Vrms into 50kOhm

Stereo encoder: tuning and calibration:

  1. Adjust your FM receiver to the frequency of your transmitter.
  2. Select Mono reception mode.
  3. Adjust POT1, POT2, POT3, POT4 and POT5 to minimum.
  4. Connect a TAPE or CD player Left (L) channel to the encoder.
  5. Play a sound track.
  6. Adjust POT3 to 3/4 of the value.
  7. Adjust POT1 until the signal distorts on your monophonic receiver.
  8. Turn POT1 back until there is no distortion.
  9. Disconnect the (L) channel and connect the (R) channel.
  10. Adjust POT2 until the signal distorts on the monophonic receiver.
  11. Turn POT2 back until there is no distortion.
  12. Adjust C7 until the frequency output (PIN3) of the 555 is 76 kHz (here you'll need a frequency counter).
  13. Select Stereophonic reception mode on your receiver.
  14. Adjust POT5 until the pilot tone indicator switches on.
  15. Adjust POT4 until there is no signal on the left (L) channel and there is a signal on the right (R) channel of the receiver.
  16. All adjustments required are now done.
  17. Re-connect the left (L) channel to the encoder.
  18. Should the callibration be correct you will hear the sound track in stereo on you receiver.

Figure 2: FM stereo transmitter: FM transmitter schematic
(click on the image for a larger version)
The transmitter output power is approx. 80mW, which is sufficient to cover an area of about fifty metres.
The RF portion consists of a totally screened oscillator which is operated by a stabilised voltage and a loosely coupled buffer stage. With this two-stage lay-out a good frequency stability and low harmonic interference radiation is achieved. The frequency of the oscillator can be adjusted in the commercial range (88-108MHz) by a trimmer capacitor Cl.
The FM-modulation of the transmitter is achieved by a varicap BA 138. The frequency ratio can be adjusted by the capacitor C6.
To enlarge the transmission range, the output circuit which is tuned with capacitor C14, can be terminated into a full wavelength antenna.
RF filters (RFC1-3) minimize power supply hum in the transmitted signal.
Operating voltage: 9-12V
Operating current: 8mA
Frequency (adjustable): 88-108MHz

L1: 3 turns, 5mm coil diameter (air-core), 1mm copper wire with a tap-off at a wire length of 12 mm, measured from ground terminal.
L2: 2 turns, 5mm coil diameter (air core), 1.5mm copper wire.
RFC1-3: 5 turns, 6-hole ferroxcube, thin Copper wire.

Antenna construction:
The antenna consists of a full wavelength dipole configuration. The construction details are indicated in figure 3.

Figure 3: FM stereo transmitter: antenna construction