There are 2 ways of generating FM waves.

1] Direct method.

2] Indirect method.

FM generation using indirect method:

• The indirect method used for FM generation is called as Armstrong method.

• The working of Armstrong method can be divided into 3 parts.

1] Phase modulation and pre requisites:

• In phase modulation, due to phase variations, few frequency variations also tale place.

• Higher modulating v/g produce higher phase shift.

• Higher phase shift produces greater frequency deviation.

• Similarly higher modulating frequency produces a faster rate of change of modulating v/g which results into greater frequency deviation.

• Thus in PM, the carrier frequency deviation is proportional to modulating v/g as well as node frequency.

• But in FM, the frequency deviation is only proportional to the modulation v/g regardless of its frequency.

• To rectify this issue, the modulation s/g is passed through a FC filter (LP)

• The o/p of this filter is applied to phase modulation along with the carrier.

2] Implementation of phase modulator:

• The crystal oscillator produces a stable unmodulated carrier which is applied to 90 degree shifter as well as combining through a buffer.

• The 90 degree phase shifter, shifts the carrier by 90 degree and gives it to the balanced modulator.

• At the o/p of balanced modulator double side band suppressed carrier s/g is obtained.

• If we increase the modulation index, the amplitude of side bands also increases, thereby increasing the angle ? mode by the resultant with unmodulated carrier.

3] FM generation:

• Crystal oscillator generates carrier at 1MHz, this is applied to combining network and a 90 degree phase shifter.

• The modulating signal is passed through an audio equalizer to boost the low frequencies.

• The modulating signal is then applied to a balanced modulator.

• The (BM) balanced modulator produces two side bands whose resultant is 90 degree phase shifted wrt unmodulated carrier.

• The unmodulated carrier and 90 degree shifted sidebands are added in the combining network.

• At the o/p of combining network, fm s/g is generated. This s/g has low carrier frequency fc and low value of modulation index mf.

• The FM signal with high fc and high mf is then passed through a class c power amplifier to raise the power levels.