Frequency Modulation (FM) is a widely used modulation technique in communication systems due to its advantages in signal quality and noise immunity. To demodulate FM signals and extract the original information, various techniques are employed such as and one such technique FM slope detector, balanced slope detector, ratio detector FM detector, pulse averaging discriminator, quadrature detector, phase locked loop etc. One of the practically used FM demodulator is the Foster-Seeley discriminator. The Foster-Seeley discriminator, also known as the phase-shift discriminator, is a frequency demodulation circuit that converts frequency variations in an FM signal into amplitude variations. It was invented by R.A. Foster and L.W. Seeley in the 1930s and has found widespread use in radio and television receivers, frequency synthesizers, and other communication systems.Here, we will delve into the Foster-Seeley discriminator, its working principle of operation, advantages, and applications.
Foster-Seeley Discriminator
The Foster-Seeley discriminator was one of the first FM demodulators used in FM receivers and televisions receivers in 1940s. The Foster-Seeley discriminator operates based on the principle of phase discrimination. It consists of center tapped RF transformer whose primary and secondary sides are tuned to the carrier frequency of the FM signal. The center tap of the secondary tuned circuit is connected back to the primary tuned circuit using a coupling capacitor and the center tap of the transformer to the common ground using a RFC(radio frequency choke). The output from the two secondary sides are connected to the envelope detectors. The Foster-Seeley discriminator circuit described converts the FM signal into phase difference signals at each arm and then applies these signals envelope detectors circuits that then when combined discriminates the phase difference to produce original demodulated signal. The primary advantage was its linearity improvement over the balanced slope detectors and the main disadvantage was its sensitivity to amplitude variation and stability because it had to be tuned periodically for signal alignment.
Circuit Diagram of Foster-Seeley discriminator for FM demodulation
The circuit diagram of Foster-Seeley discriminator for FM demodulation is shown below.
The signals produced at the secondary, Vsa and Vsb are out of phase by angles between 180 degree. That is,
Vsa = -Vsb
It can be shown that the voltage at the input voltage of the diodes D1 and D2 are:
Vad = Vp + Vsa/2 = Vp + Vs/2
Vbd = Vp-Vsb/2 = Vp - Vs/2
The output voltage is,
Vout = Va - Vb
Since Va and Vb are proportional to Vad and Vbd we have,
or, Vout = Vad - Vbd = Vp + Vsa/2 - (Vp-Vsb/2) = 1/2( Vsa + Vsb)
That is Vout is proportional to Vsa+Vsb and we can assume for simplicity ignoring the 1/2 magnitude factor,
Vout = Vsa + Vsb
Three cases arises which are as follows.
Case 1: fin = fc
When the input signal frequency is equal to fc, Vsa leads the primary voltage Vp by 90 degree and Vsb lags Vp by 90. See the phasor diagram depicting this case.
Vsb = -Vsa
and therefore, Vout = Vsa + Vsb = Vsa - Vsa = 0
That is when the input frequency is equal to carrier frequency, the output voltage is 0.
Case 2: fin > fc
When the input signal frequency is greater than carrier frequency fc, Vsa leads the primary voltage Vp by less than 90 degree and Vsb lags the primary voltage Vp by more than 90 degree. This implies that Va is greater than Vb. See the phasor diagram depicting this case.
The result is that the output of the discriminator, Vout, which is vector addition of Vsa and Vsb, is positive.
and therefore, Vout = Vsa + Vsb = +ve since Vsa > Vsb
That is when the input frequency is greater than the carrier frequency, the output voltage is positive.
Case 3: fin < fc
When the input signal frequency is less than carrier frequency
fc, Vsa lags the primary voltage Vp by more than 90 degree while Vsb lags the primary voltage Vp by more less 90 degree.
This implies that Va is less than Vb. See the phasor diagram depicting this case.
that is, Vout = Vsa + Vsb = -ve since Vsa < Vsb
The tuned circuit ensures that the amplitude of the output signal from the diodes are linearly proportional to the frequency deviations. As a result, the output of the Foster-Seeley discriminator is an AC voltage with an amplitude that is proportional to the frequency deviation of the input FM signal from the resonant frequency of the tuned circuit. This output can then be filtered and amplified to obtain the demodulated signal, which contains the original information.
Video demonstration of Foster-Seeley FM Demodulator
The following how the above described FM Demodulation with Foster Seeley Discriminator circuit works.
Advantages of Foster-Seeley Discriminator
The Foster-Seeley discriminator offers several advantages as a frequency demodulation technique:
- Wide Frequency Range: The Foster-Seeley discriminator can operate over a wide range of input frequencies, making it suitable for demodulating FM signals with varying carrier frequencies.
- Simple Circuitry: The circuitry of the Foster-Seeley discriminator is relatively simple, consisting of only a few components and making it cost-effective. The FM signal is tuned to carrier frequency and it simplies alignment problem with the balanced slope detector and thus Foster-Seeley discriminator is easier to implement.
- Good Linearity: The Foster-Seeley discriminator exhibits good linearity in converting frequency variations into amplitude variations, resulting in accurate demodulation of the FM signal.
- Wide Bandwidth: The Foster-Seeley discriminator has a wide bandwidth, enabling it to handle signals with large frequency deviations and high modulation indices.
Applications of Foster-Seeley Discriminator
The Foster-Seeley discriminator finds applications in various communication systems, including:
- Radio and Television Receivers: The Foster-Seeley discriminator is commonly used in AM/FM radio receivers and television receivers for extracting the audio or video information from the FM signals.
- Frequency Synthesizers: The Foster-Seeley discriminator is used in frequency synthesizers, which are used in communication systems, radar systems, and other applications that require precise frequency generation.
- FM Demodulation in Data Communication: The Foster-Seeley discriminator can be used in data communication systems that employ FM modulation, such as frequency shift keying (FSK) and audio frequency shift keying (AFSK), to demodulate the data signals.
In conclusion, the Foster-Seeley discriminator is a reliable and widely used demodulation technique for frequency modulation signals. Its simple circuitry, wide frequency range, good linearity, and wide bandwidth make it suitable for various applications in communication systems. It overcomes the disadvantage of Slope Detector for FM demodulation. Whether in radio and television receivers, frequency synthesizers, or data communication systems, the Foster-Seeley discriminator plays a significant role in extracting the original information from FM signals, making it an essential component in modern communication technology.
Further Readings and References
[1] Difference and Similarities between FM and PM
[2] Frequency Modulation(FM) vs Amplitude Modulation(AM)
