A stereo FM receiver has three major sections:
- Mono mode
- Stereophonic mode
- Section common to both mono and stereo modes
The section that is common to both mono and stereo modes is a standard FM receiver that recovers the modulating signal. The output of this section is routed to the remaining two sections. The output consists of both the left and right channel marked as (L + R) in Figure. This output is applied to the mono section and the speaker produces audio signals monophonic mode.
The stereo section is more complicated. It uses three filters to extract (L + R) and (L – R) signals and the pilot-carrier from the discriminator output. The (L + R) signal is obtained from the low-pass filter, which contains frequencies between 50 Hz and 15 kHz. This signal delayed for a fixed time before applying it to the matrix and the de-emphasis network. This is done to simultaneously get the (L + R) and (L - R) signals at the matrix. The matrix network separates the left (L) and right (R) channels. These are then de-emphasized and amplified by the audio amplifiers and are given to their respective speakers.
A band pass filter is used to extract the (L - R) signal varying between 23-53 kHz. It is a double-side band (DSB) signal. This signal is applied to an AM detector to demodulate. The transmitter uses a 38 kHz carrier signal to get a DSB-SC signal from the (L - R) signal. Thus, at the receiver, a carrier of 38 kHz is required to demodulate the received (L - R) signal.
The pilot carrier of 19 kHz is extracted using another band pass filter. This pilot carrier is given to the frequency doubler, which doubles its frequency to 38 kHz. After amplification of this The AM detector detects the (L - R) signal, which is carrier, it is applied to the AM detector matrix. As some time is taken for the (L –R) signal to demodulate, the (L + R) signal is delayed so that both (L + R) and (L – R) reach the matrix at the same time.
Trouble shooting FM Receiver
To locate, the faulty stage in the receiver, use the signal injection method and analyze the waveforms on the oscilloscope or examine the audio from the speaker. Before starting the fault-finding exercise in the receiver, consult the service manual. The manual will provide the voltages at the various key points or the circuit and the details or the various signals and their waveforms at the key points.
You may start injecting the signal from the last stage for a dead receiver. However, ensure that the dc power supply and the speaker are not faulty. Then start injecting the signal at the input of each stage and examine the audio output from the speaker. The faulty stage will not pass the signal. Once the faulty stage is identified, use the usual troubleshooting techniques to check to faulty components.