Frequency Demodulation

Modulation is the process of encoding information in a transmitted signal, while demodulation is extracting information from a transmitted signal. This demodulation is also called FM detection or FM discrimination. Moreover, this term is often used in older circuits and technology. FM demodulation plays an integral part in the reception of frequency-modulated signals. This article elaborates on the concept of frequency demodulation. 

What is the Meaning of Frequency Demodulation?

Frequency demodulation is the inverse of frequency modulation. The original modulating signal is obtained as output following demodulation. After the signal has been received, filtered, and amplified, the original modulation from the carrier must be recovered. This is known as demodulation or detection.

FM demodulator circuits can be found in any FM receiver, including broadcast receivers, two-way radios such as walkie-talkies and handheld radios that use FM, and any receiver that uses frequency modulation.

How to Demodulate a Wave?

We need to follow a few techniques to demodulate a wave. The methods are mentioned below. 

Generating a Waveform

We cannot create an FM waveform with an arbitrary behavioural voltage source and a simple mathematical relationship like we did with AM. However, it turns out that creating an FM signal is simpler. 

Passing through the High Pass Filter

The first demodulation method we will examine starts with a high-pass filter. We will suppose we are talking about narrowband FM (discussed briefly on this page). Use a high-pass filter design so that the attenuation varies considerably within a frequency band twice the width of the baseband signal.

The frequency response of our high-pass filter must be such that the lowest frequency in the modulated signal is attenuated substantially more than the highest frequency in the modulated signal. 

We converted frequency modulation to amplitude modulation by using the filter. This is a useful way of FM demodulation because it enables us to use envelope-detector circuitry designed for use with amplitude modulation. The filter used to generate this waveform was simply an RC high-pass with a cutoff frequency close to the carrier signal.

Using the Phase-Locked Loop Demodulator

A phase-locked loop (PLL) is used to build a complicated but high-performance FM demodulation loop. The intensity of an incoming waveform can be “locked onto” by a PLL. This is accomplished by combining a phase detector, a low-pass filter (also known as a “loop filter”), and a voltage-controlled oscillator (VCO) into a negative-feedback system.

The phase detector generates a signal proportional to the phase difference between the incoming waveform and the VCO output. The loop filter smoothens this signal, which then serves as the VCO’s control signal. As a result, if the intensity of the input signals is constantly increasing and decreasing, the VCO control signal must also increase and decrease for the VCO output frequency to remain equal to the input frequency.

Types of Demodulator

Several types of demodulators can be used to generate a demodulated wave. Some of these waves were used when radios were made from discrete devices. However, nowadays, PLL demodulators are more famous. We have listed a few of them below. 

• Slow Detection Demodulator

This is an elementary form of FM demodulation that depends on the receiver’s precision to provide demodulation. It is ineffective and only used when the receiver does not support FM.

This method of FM detection has several drawbacks: the radio’s selectivity curve will be nonlinear, resulting in distortion; the receiver will be sensitive to amplitude variations, and so on.

• Radio Detector Demodulator

When discrete components were used in transistor radios, this type of detector was widely used. The ratio detector required the use of a transformer with a third winding to generate an additional signal that was phase-shifted for the demodulation process. The ratio detector was built with two diodes and a few resistors and capacitors.

Despite its excellent performance, the ratio FM detector was expensive due to the transformer being employed. These FM demodulators were expensive to manufacture because all wire-wound elements are more expensive than resistors and capacitors. After the introduction of integrated circuit technology, where distinct circuits could be used, the ratio detector was rarely used. Nonetheless, it was a good performer back then.

• Quadrature FM Detector

In FM radio integrated circuits, the quadrature FM detector is now widely used. It is simple to set up and delivers outstanding results. The quadrature coincidence form of FM demodulator is straightforward to incorporate into an integrated circuit and can be added at almost no extra cost.

As a result, it is a highly appealing option for modern receiver designs. Many integrated circuits designed to provide the functionality of a complete receiver or an IF strip include a quadrature detector/ coincidence detector, allowing FM demodulation to be added to the final receiver at almost no cost. 

Conclusion

Frequency demodulation is a crucial part of the modulation process. It is used in the reception of a frequency-modulated wave. Once the signal is received, filtered, and modified, this process is necessary to recover original modulation from the carrier.