Types of Frequency Demodulation Circuits

Frequency modulation (FM) is a popular method used in radio communication to transmit information signals. The FM signal is transmitted through an FM transmitter, which uses a frequency modulation circuit. To recover the information signal from the FM signal, a frequency demodulation circuit is used. In this blog post, we will explore different types of frequency demodulation circuits, such as FM slope detector, balanced slope detector, Foster-Seeley discriminator, and ratio detector.

FM Slope Detector

The FM slope detector is a simple and popular demodulation circuit used to demodulate FM signals. It consists of a high-pass filter and a diode. The high-pass filter removes the carrier frequency, and the diode rectifies the resulting signal. The output of the diode is then passed through a low-pass filter, which smooths out the signal and removes any remaining high-frequency components. The resulting signal is the information signal.

Balanced Slope Detector

The balanced slope detector is a modification of the FM slope detector that improves its performance. It consists of two high-pass filters and two diodes. The two high-pass filters are connected in series, and the diodes are connected in parallel across the output of the filters. The two diodes are connected in opposite polarity to each other. The output of the diodes is then passed through a low-pass filter. The balanced slope detector provides better frequency response and improved linearity compared to the FM slope detector.

Foster-Seeley Discriminator

The Foster-Seeley discriminator is a frequency demodulation circuit that is based on the principle of phase shift. It consists of a transformer, a tuned circuit, and two diodes. The FM signal is applied to the primary winding of the transformer, which induces a voltage in the secondary winding. The tuned circuit is tuned to the carrier frequency, and the voltage induced in the secondary winding is applied to the tuned circuit. The two diodes are connected to the tuned circuit in opposite polarity. The output of the diodes is then passed through a low-pass filter. The Foster-Seeley discriminator provides good linearity and frequency response.

Ratio Detector

The ratio detector is a frequency demodulation circuit that is based on the principle of a balanced bridge circuit. It consists of two diodes, two resistors, and a capacitor. The FM signal is applied to one input of the bridge circuit, and the output of the bridge circuit is taken from the junction of the two diodes. The two resistors are connected to the other two inputs of the bridge circuit. The capacitor is connected between the output of the bridge circuit and ground. The output of the ratio detector is then passed through a low-pass filter. The ratio detector provides good linearity and frequency response.

Zero-crossing FM detector

The zero-crossing FM detector is another type of frequency demodulation circuit used to demodulate FM signals. It is based on the principle that the frequency of the FM signal is proportional to the rate of change of the phase of the signal. The zero-crossing FM detector detects the zero-crossings of the FM signal and uses them to determine the frequency of the signal. It consists of a comparator, a phase shifter, and a low-pass filter. The FM signal is applied to the comparator, which generates a square wave signal with a frequency proportional to the frequency of the FM signal. The phase shifter is used to shift the phase of the square wave signal by 90 degrees. The two signals are then applied to the inputs of the comparator, which generates a pulse at each zero-crossing of the FM signal. The output of the comparator is then passed through a low-pass filter to remove any remaining high-frequency components. The resulting signal is the information signal. The zero-crossing FM detector is simple, low-cost, and provides good frequency response.

Conclusion

In conclusion, frequency demodulation circuits are essential components of an FM receiver. Different types of frequency demodulation circuits, such as FM slope detector, balanced slope detector, Foster-Seeley discriminator, zero-crossing detector, and ratio detector, are used to recover the information signal from the FM signal transmitted by FM transmitter. The selection of the appropriate frequency demodulation circuit depends on the specific requirements of the application.