Generation of FM from PM : In PM along with the phase variation, some frequency variation also takes place, higher modulating voltages produce greater phase shift which in turn produces greater frequency deviation it also produces a faster rate of change of modulating voltage hence greater frequency deviation is produced, in correct this problem modulating signal is passed through LPF.

Operation of Armstrong method :

• The crystal oscillator generates the carrier at low frequency typically at 1MHz, this is applied to the combining network and a 90 degree phase shifter.

• The modulating signal is passed through an audio equalizer to boost the low modulating frequencies, the modulating signal is then applied to a balanced modulator.

• The balanced modulator produces two side bands such that their 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 we get FM wave, this FM wave has a low carrier frequency fc and low value of modulation index mf.

• The carrier frequency and the modulation index are then raised by passing the FM wave through the first group of multipliers, the carrier frequency is then raised by using a mixer and a local osc and then they are raised to req value using second group of multiplier.

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