Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

• The direct methods cannot be used for the broadcast applications. Thus, the alternative method i.e. indirect method called as the Armstrong method of FM generation is used.
  • In this method the FM is obtained through phase modulation. A crystal oscillator can be used hence the frequency stability is very high.
  • The block diagram of the Armstrong method is shown below:

Figure: Armstrong method (Indirect method of FM generation)

Operation:

• The crystal oscillator generates the carrier at low frequency typically at 1MHz. This is applied to the combining network and a 90° 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 produced two side bands such that their resultant is 90° phase shifted with respect to the unmodulated carrier.
• The unmodulated carrier and 90° phase shifted sidebands are added in the combining network.
• At the output of the combining network we get FM wave. This wave has a low carrier frequency fc and low value of the 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 then the fc and mf both are raised to required high values using the second group of multipliers.
• The FM signal with high fc and high mf is then passed through a class C power amplifier to raise the power level of the FM signal.
• The Armstrong method uses the phase modulation to generate frequency modulation.