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Demodulate and Restore the Original Information from FM Signals

Key Takeaways

  • The goal of the demodulation process is to recover original information from the carrier signal, ensuring no loss of data in the transfer. 

  • The slope detector utilizes the technique of slope detection to demodulate the FM signals. A slope detector can be constructed from a tank circuit tuned to some frequency higher or lower than the carrier signal.

  • The phase-shift detector, ratio detector, and gate-beam detector are three FM detectors that are most often used in FM receiver systems.

FM signals

Figure 1. FM signals store information in frequency variations

In radio, TV audio, cellphones, cordless phones, and marine communication information is transmitted using frequency modulation (FM). Since FM signals are more immune to propagation-induced selective fading, transmission power loss, interference, and noises, frequency modulation is more widely applied than amplitude modulation (AM). 

FM improves the Signal to Noise Ratio (SNR) and provides a dynamic range of modulating signals with fewer distortions than AM does. However, frequency modulation has some cost disadvantages. The demodulation of FM signals is not possible using simple diode detectors that are used in AM. Instead, FM demands some complex detector circuits, making the demodulation of FM signals costly compared to amplitude modulation. However, for some projects, the costly disadvantages are worth it, given the numerous benefits FM provides.

Voltage versus frequency plot

Figure 2. Frequency to voltage conversion in FM demodulators are based on this plot

Demodulation of FM Signals

Once information is transmitted as FM signals, it needs to be retrieved at the receiving end. The demodulation process recovers the original information (modulating signal) from the carrier signal. The process of demodulation is also called detection or discrimination, and the device constructed for demodulation can be called a demodulator, detector, or discriminator. 

FM transmitters send modulated FM signals to be filtered, amplified, and demodulated at the FM receivers in order to retrieve crucial data from them. In FM, information is stored as frequency variations in the carrier signal. As information gets modulated to the carrier wave, the frequency varies above and below the center frequency. The FM detector should be capable of making the variations in output amplitude according to the variations in the frequency of the input FM signal. The FM demodulator can be simplified as a circuit used to convert frequency variations into voltage variations in FM communication receiver systems. The main objective of FM detectors is to reproduce the signal that is given as input to the FM transmitter. The various types of  FM detectors use different techniques to convert frequency variations into amplitude variations. The most basic frequency demodulation technique is slope detection. 

Tank circuits

Figure 3. Tank circuits are followed by a diode detector in FM demodulators

Slope Detector

A slope detector utilizes the technique of slope detection to demodulate FM signals. A slope detector is the simplest and most basic FM demodulator. The slope detector can be constructed from a tank circuit, tuned to some frequency higher or lower than the carrier signal. Figure 2 shows the voltage-frequency plot of the slope detector. The frequencies marked as 1 and 2 in the plot show the lower and upper frequencies in the FM signals, respectively. The resonant frequency of the slope detector is always made greater than the FM carrier frequency. The components of the slope detector are selected so that this frequency constraint is satisfied. The frequency variations in the FM signals should be on the lower slope side of the curve. Frequency variations of FM signals are closer to the resonant frequency of the tank. When the FM signal is given to the tank circuit as input signals, the amplitude of the output signals varies corresponding to the frequency variations. The AM output signals from the tank circuit are sent to the diode detector and the original information is retrieved. 

Types of FM Demodulators

The phase-shift detector, ratio detector, and gate-beam detector are three FM detectors that are most often used in FM communication systems. 

Phase-Shift Detector

Foster-Seeley response curve

Figure 4. In a phase-shift detector, the amplitude and polarity of the output varies with the input frequency

The phase-shift detector, also known as Foster-Seeley discriminator, employs a double-tuned RF transformer for the frequency to amplitude conversion. Both the amplitude and the polarity of the output signal varies corresponding to the frequency variations in the input FM signal. The so-obtained output signal is rectified and filtered to restore the original information. The amplitude of the detector output is zero when the FM signal frequency is equal to the carrier frequency (fr). Figure 4 shows the response curve of the Foster-Seeley discriminator. The frequencies f1 and f2 represent the lower and upper-frequency values in the FM signal.  

Ratio Detector

The ratio detector is somewhat similar to the phase-shift detector in operation. The ratio detector also uses a double-tuned RF transformer to convert the instantaneous variations in frequency to amplitude signals. However, the transformer in the ratio detector needs to have a tertiary winding to generate a phase-shifted additional signal. Due to the presence of tertiary winding in the transformer, ratio detectors are more costly than phase-shift demodulators. However, the ratio detection technique avoids the use of limiter stages, which are required in phase-shift detectors to prevent the influence of FM signal amplitude. 

Gate-Beam Detector

The gate-beam detector makes use of a gate-beam tube to convert FM signals to AM signals. It is also called a quadrature detector.  In this technique, the gate-beam tube is made to operate either in saturation or cut-off region by the input FM signals. The output voltage is equal to zero when the FM frequency matches the carrier frequency. The amplitude polarity is positive for frequencies greater than center frequency and negative for lower frequencies. 

Advantages of Different Types of FM Demodulators

Slope detector

Phase-shift detector

Ratio detector

Gate-beam detector

Simple and can be used for FM demodulation with AM detectors

Good performance and simple circuit construction

No influence of the FM signal amplitude on the output signal


 

Extremely simple and requires relatively few components in the detector circuit


 

FM demodulation is achieved without any additional circuitry

Higher output and lower distortion compared to ratio detectors

The output of the detector automatically adjusts to the average FM signal amplitude

Only one gate-beam tube employed in the detector provides an effective limiter operation with linear detection.

Table 1. Advantages of various FM demodulators 

Wireless transmission is a crucial aspect of communication systems. Amplitude modulation, frequency modulation, and phase modulation are some of the modulation techniques used for transmitting these wireless signals. In FM signal transmission, the receiving-end demodulators are incorporated to restore original information, ensuring a smooth transition without losing any data.