Analog Communication FM Modulators - Analog Communication

What are FM Modulators? Define NBFM and WBFM waves?

In this chapter, let us discuss about the modulators which generate NBFM and WBFM waves. First, let us discuss about the generation of NBFM.

Generation of NBFM

Generation of NBFM

The block diagram of NBFM modulator is shown in the following figure.

NBFM modulator

Here, the integrator is used to integrate the modulating signal m(t)m(t). The carrier signal a1(5)is the phase shifted by −900−900 to get witha2(2)the help of −900−900 phase shifter. The product modulator has two inputs i.e

a3(1)

It produces an output, which is the product of these two inputs.

This is further multiplied with fm1 by placing a block fm2in the forward path. The summer block has two inputs, which are nothing but the two terms of NBFM equation. Positive and negative signs are assigned for the carrier signal and the other term at the input of the summer block. Finally, the summer block produces NBFM wave.

Generation of WBFM

The following are the two methods that generate WBFM wave.

  • Direct method
  • Indirect method

Direct Method

Direct method is used by generating a wide band FM wave directly. In this method, Voltage Controlled Oscillator (VCO) is used to generate WBFM. The frequency of an output signal which is produced by VCO is proportional to the input signal voltage. This is similar to the definition of FM wave. The block diagram of the generation of WBFM wave is shown in the following figure.

Direct Method

Here, the modulating signal m(t)m(t) is applied as an input of Voltage Controlled Oscillator (VCO). VCO produces an output, which is nothing but the WBFM.

WBFM.

Where,

fi is the instantaneous frequency of WBFM wave.

Indirect Method

This method is quiet opposite to Direct Method where wide band FM wave is generated indirectly. This means, first we will generate NBFM wave and then with the help of frequency multipliers we will get WBFM wave. The block diagram of generation of WBFM wave is shown in the following figure.

 generation of WBFM wave

This block diagram consists of mainly two stages. In the first stage, the NBFM wave will be generated using NBFM modulator. We have seen the block diagram of NBFM modulator at the beginning of this chapter. As we know that the modulation index of NBFM wave is less than one.

Hence, in order to get the required modulation index (greater than one) of FM wave, we need to choose the frequency multiplier value properly.

Frequency multiplier is a non-linear device, which generates an output signal with the frequency of ‘n’ times the input signal frequency. Where, ‘n’ is known as the multiplication factor.

If NBFM wave with modulation index ββ is less than 1 is applied as the input of frequency multiplier, then the frequency multiplier produces an output signal, with the modulation index ‘n’ times ββ and the frequency also ‘n’ times the frequency of WBFM wave.

At times, we might require multiple stages of frequency multiplier and mixers to increase the frequency deviation and modulation index of FM wave.

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