Balanced Modulator

Introduction

A balanced modulator is a circuit that generates a DSB signal, suppressing the carrier and leaving only the sum and difference frequencies at the output. The output of a balanced modulator can be further processed by filters or phase-shifting circuitry to eliminate one of the sidebands, resulting in a SSB signal.

Balanced modulator is a modulator in which the carrier and modulating signal are introduced in such a way that the output contains the two sidebands without the carrier. The term 'Balanced' represents suppressed carrier AM modulated signal.Double side band suppressed carrier modulation is simply AM without the broadcast carrier.

 Recall that the AM signal is defined by:

The carrier term in the spectrum can be eliminated by removing the dc offset from the modulating signal. A band pass filter can be used to select any one of the AM signals. The number of different output frequencies can be significantly reduced if the multiplier accepts sinewaves at the carrier input.

Removing the DC component from the input eliminates the carrier signal and creates DSBSC modulation

An analog multiplier is a type of integrated circuit that can be used as a balanced modulator. Analog multipliers are often used to generate DSB signals. The analog multiplier uses differential amplifiers operating in the linear mode. The carrier must be a sine wave and the multiplier produces the true product of two analog inputs.

simplified diagram for a modulator

Properties of Balanced modulator:

1. There is a 180 phase reversal at the point where +A(t)=+m(t) goes negative. This is typical of DSB-SC modulation. 

2. The bandwidth of the DSB-SC signal is double that of the message signal, that is, BWDSB-SC =2B (Hz). 

3. The modulated signal is centered at the carrier frequency ωc with two identical sidebands (double-sideband) – the lower sideband (LSB) and the upper sideband (USB). Being identical, they both convey the same message component. 

4. The spectrum contains no isolated carrier. Thus the name suppressed carrier. 

5. The 180 phase reversal causes the positive (or negative) side of the envelope to have a shape different from that of the message signal, This is known as envelope distortion, which is typical of DSB-SC modulation. 

6. The power in the modulated signal is contained in all four sidebands.