# Balanced Modulator Circuit for DSB-SC AM Generation

In the field of electronics, the Balanced Modulator Circuit for Double-Sideband Suppressed Carrier Amplitude Modulation (DSB-SC AM) is a vital component for communication systems. It is a technique used to transmit information over long distances, and it involves the process of modulating the amplitude of a carrier wave. This article aims to provide a detailed explanation of the Balanced Modulator Circuit for DSB-SC AM, its working principle, advantages, and applications.

#### What is DSB-SC AM?

DSB-SC AM is a type of amplitude modulation in which the carrier signal is suppressed. In this type of modulation, the modulating signal is multiplied by the carrier signal, resulting in two sidebands. The carrier signal is then suppressed, leaving only the two sidebands. The DSB-SC AM is mainly used in communication systems to reduce bandwidth requirements and increase transmission efficiency.

#### What is a Balanced Modulator Circuit?

A Balanced Modulator Circuit is a type of modulator that produces an output signal that is the product of two input signals- the message signal and the carrier signal. Balanced Modulator Circuit consists of two or more active devices circuit arranged in a balanced fashion so as to suppress the carrier signal. A DSB-SC balanced modulator can be either implemented using either transistors or using balanced ring diode modulator. Transistor based balanced modulator uses two AM modulators, a phase shifter circuit and differential amplifier. The block diagram of transistor based Balanced Modulator for DSB-SC AM wave generation is shown below.

#### Working Principle of the Balanced Modulator Circuit

The Balanced Modulator Circuit takes two input signals, the carrier signal and the modulating signal. In the upper arm of the modulator the message signal and the carrier signal are multiplied to get standard AM signal at the output of the product modulator.

Let m(t) be the message signal and let c(t) be the carrier signal. The output of the upper AM modulator is,

x(t) = [1+ka*m(t)]c(t)

where ka is the amplitude sensitivity of the modulator.

At the lower arm of the balanced modulator, the message signal is passed first through a 180 degree phase shifter circuit that changes the phase of the message signal. That is we get -m(t) at the output of the phase shifter circuit. Phase shift circuit can be realized using all pass filter implemented either using op-amp or passive components. This 180 degree phase shifted message signal is multiplied with the carrier signal from the local oscillator at the lower AM modulator circuit. The output from the lower AM modulator is,

y(t) =[1-ka*m(t)]c(t)

where ka is the amplitude sensitivity of the modulator.

The standard AM signal from the upper arm x(t) and from the lower arm y(t) are applied to BJT differential amplifier with the result that the output is DSB-SC AM signal.

s(t)=Av[x(t)-y(t)]

s(t)=Av[[1+ka*m(t)]c(t) - [1-ka*m(t)]c(t)]

or, s(t)=Av[c(t) + ka*m(t)c(t) - c(t) + ka*m(t)c(t)]

or, s(t)= Av[2*ka*m(t)c(t)]

s(t)=2Avkam(t)c(t)

Thus the output s(t) is DSB-SC signal.

DSBSC Balanced Modulator Circuit

Below shows the implementation of the DSB-SC balanced modulator circuit.

In the above circuit diagram, the phase shifter circuit is implemented with LM358N op-amp as all pass filter. The two standard AM modulator using the AD633 analog multiplier integrated circuits(IC). The difference amplifier which amplifies the difference between the standard AM signal generated in the upper and lower arm is implemented using BJT differential amplifier in dual input unbalanced output.

#### Advantages of the Balanced Modulator Circuit

The Balanced Modulator Circuit has several advantages over other modulation techniques. One of the main advantages is that it produces a DSB-SC AM waveform that is suitable for transmission. The DSB-SC AM waveform has a reduced bandwidth requirement, making it more efficient for transmission. Additionally, the Balanced Modulator Circuit is a simple and low-cost circuit that is easy to implement.

#### Applications of the Balanced Modulator Circuit

The Balanced Modulator Circuit is used in a variety of applications, including communication systems, radar systems, and medical devices. In communication systems, the Balanced Modulator Circuit is used to produce a DSB-SC AM waveform that is suitable for transmission over long distances. In radar systems, the circuit is used to modulate the signals that are transmitted and received. In medical devices, the circuit is used to modulate the signals that are used for imaging.

Video demonstration

Following is video demonstration of DSB-SC AM generation using balanced modulator circuit.

Conclusion

The Balanced Modulator Circuit for DSB-SC AM is a crucial component in communication systems, radar systems, and medical devices. The circuit operates on the principle of signal multiplication, producing an output signal that is the product of two input signals. The circuit has several advantages over other modulation techniques, including a reduced bandwidth requirement and increased transmission efficiency. Overall, the Balanced Modulator Circuit is a simple and low-cost circuit that is easy to implement, making it an essential component in many electronic