This circuit is a simple fm stereo encoder. A method commonly used in (double side-band suppressed carrier) DSB-SC modulation to provide synchronisation between modulator and demodulator is to transmit a sinusoidal tone (pilot tone) whose frequency and phase are related to the carrier frequency. This tone is positioned at 19 kHz, outside the pass-band of the modulated signal. The carrier frequency is 38 kHz, double that of the pilot tone. The receiver circuitry detects the pilot tone and translates it to 38 kHz, which is then used to demodulated the encoded signal.
In stereo broadcasting it is necessary to transmit and receive both left (L) and right (R) audio channels while also providing the sum (L+R) to monophonic receivers. To serve both stereophonic and monophonic receivers, the (L+R) signal occupies the normal audio spectrum in the frequency range 20 Hz to 15 kHz and the (L-R) signal, also in the same frequency range, is shifted in frequency using DSB-SC modulation. The carrier frequency used in this process is 38 kHz. A typical block diagram of a FM stereo encoder is shown in figure 1 (a) and figure 1 (b) indicates the resultant composite spectrum.
In the receiver, the pilot tone is filtered out and is doubled in frequency which is then used to synchronise the demodulator to the modulator. Finally an addition and subtraction (matrixing) of the two signals yields the desired L and R audio signals.
Subtractor:
The subtracter consists of an op-amp configured as a one-to-one subtracter. The subtraction process yields the (L-R) signal which is the modulated with the carrier at a frequency of 38 kHz.
Adder:
The adder consists of an op-amp configured as a one-to-one adder. The addition process yields the (L+R) signal which is used in monophonic receivers.
Multiplier (modulator):
The multiplier consists of an analogue switch which chops the (L-R) signal at a frequency of 38 kHz.
Band-pass filter:
The band-pass filter is centred at 38 kHz and yields the desired DSB-SC signal.
Pilot tone generation:
An astable consisting of a 555 timer is set to generate a frequency of 76 kHz. This frequency is divided using two F/F's to produce 38 kHz and 19 kHz.
Carrier generation:
The carrier is generated by dividing the 76 kHz signal by two.
Low-pass filter:
The 19 kHz is passed through a low-pass filter to produce a sinusoidal pilot tone.
Mixer:
The final stage is the mixer. The mixer, by using an addition process, combines the monophonic (L+R) signal, DSB-SC (L-R) signal and pilot tone.
Specifications:
Input impedance: 47kOhm
Input level: less than 2Vrms
Output level: maximum 2Vrms into 50kOhm
Tuning and calibration:
- Adjust your FM receiver to the frequency of your transmitter.
- Select Mono reception mode.
- Adjust POT1, POT2, POT3, POT4 and POT5 to minimum.
- Connect a TAPE or CD player Left (L) channel to the encoder.
- Play a sound track.
- Adjust POT3 to 3/4 of the value.
- Adjust POT1 until the signal distorts on your monophonic receiver.
- Turn POT1 back until there is no distortion.
- Disconnect the (L) channel and connect the (R) channel.
- Adjust POT2 until the signal distorts on the monophonic receiver.
- Turn POT2 back until there is no distortion.
- Adjust C7 until the frequency output (PIN3) of the 555 is 76 kHz (here you'll need a frequency counter).
- Select Stereophonic reception mode on your receiver.
- Adjust POT5 until the pilot tone indicator switches on.
- 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.
- All adjustments required are now done.
- Re-connect the left (L) channel to the encoder.
- Should the callibration be correct you will hear the sound track in stereo on you receiver.
See more : FM Encoder - FM Transmitter - FM Stereo Transmitter
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