Write short notes on the following:
(a) Foster-Seeley Detector
(b) Stereophonic FM transmitter and receiver a neat circuit diagram, describe the direct method of generating FM. Also explain feedback scheme for frequency stabilization of a frequency modulator in direct method.
The correct answer and explanation is :
(a) Foster-Seeley Detector
The Foster-Seeley detector is a type of frequency modulation (FM) demodulator used to extract the audio signal from an FM radio wave. It is a phase discriminator circuit that converts the frequency variations in the FM signal into amplitude variations, which can be processed by a subsequent amplifier.
Working Principle:
- The detector consists of a balanced mixer with two diodes, and it is designed to handle the nonlinearities of the FM signal.
- The FM signal is applied to the detector’s input, where it is split into two branches.
- Each branch contains a tuned circuit (LC circuit), where the frequencies are demodulated by the diodes.
- As the frequency of the FM signal varies, the phase shift between the two branches changes, resulting in an output signal proportional to the instantaneous frequency deviation of the FM signal.
- The output is passed through a low-pass filter to remove the high-frequency components, leaving only the demodulated audio signal.
Advantages:
- Simple and effective for extracting the modulated signal.
- It provides a linear relationship between the FM signal’s frequency deviation and the output amplitude.
(b) Stereophonic FM Transmitter and Receiver
Stereophonic FM Transmitter:
A stereophonic FM transmitter is used to broadcast stereo audio signals (left and right channels) over an FM carrier. The two audio signals are modulated onto a single FM carrier, enabling stereo sound reproduction on FM receivers.
Working Principle:
- The left and right audio signals are first mixed with a pilot tone (a 19 kHz tone) to provide synchronization information.
- The resulting signals are then modulated onto the FM carrier. The left channel is typically amplitude-modulated (AM) onto the carrier, while the right channel is frequency-modulated.
- The combined signal is transmitted as a single FM signal.
Stereophonic FM Receiver:
A stereophonic FM receiver detects the FM signal and extracts the stereo audio. It demodulates the FM signal and separates the left and right audio channels by detecting the pilot tone.
Circuit Diagram:
In an FM stereo system, a multiplex (MPX) decoder is used in the receiver to separate the audio channels.
Direct Method of Generating FM
Direct Frequency Modulation (FM) involves directly modulating the frequency of the carrier signal. This is done by varying the input voltage to a frequency modulator, which adjusts the frequency of the carrier signal accordingly. A common method of generating FM directly is by using a voltage-controlled oscillator (VCO).
Working Principle:
- The modulating signal (such as audio) is applied to the VCO.
- The VCO adjusts the output frequency in direct proportion to the input modulating signal’s amplitude.
- The carrier frequency changes in response to the amplitude of the modulating signal, thus generating the FM signal.
Feedback Scheme for Frequency Stabilization of FM Modulator
Frequency stabilization is crucial in FM transmission to avoid frequency drifts that may lead to distortion or loss of signal quality. A feedback scheme is used in direct FM modulation to maintain a stable output frequency.
Working Principle:
- The output of the VCO is fed back into a feedback loop that compares the output frequency with a reference frequency.
- If any frequency deviation occurs, the feedback loop adjusts the VCO to bring it back to the desired frequency.
- This ensures that the frequency modulation process remains stable, providing a clear and consistent FM signal.
Feedback systems often include phase-locked loops (PLLs), where the loop automatically corrects frequency deviations by locking the output to a reference signal, thereby stabilizing the FM transmitter’s output.