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:
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 $F_m$ wave. This wave has a low carrier frequency $f_c$ and low value of the modulation index $m_f$ .
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 $f_c$ and $m_f$ both are raised to required high values using the second group of multipliers.
The FM signal with high $f_c$ and high $m_f$ 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. This method can be understood by dividing it into four parts as follows:
1.Generation of FM from phase modulator:
The modulating signal is passed through a low pass RC filter.
The filter output is then applied to a phase modulator along with carrier.
Hence the extra deviation in the carrier $f_c$ due to higher modulating frequency is compensated by reducing the amplitude of the high frequency modulating signals.
Hence the frequency deviation at the output of the phase modulator will be effectively proportional only to the modulating voltage and we obtain an FM wave at the output of phase modulator.
2.Implementation of phase modulator:
- The crystal oscillator produces a stable unmodulated carrier which is applied to the “90° phase shifter” as well as the “combining network” through a buffer.
The 90° phase shifter produces a 90° phase shifted carrier. It is then applied to the balanced modulator along with the modulation signal.
At the output of the balanced modulator we get DSBSC signal i.e. AM signal without carrier. This signal consists of only two sidebands with their resultant in phase with their resultant in phase with the 90° phase shifted carrier.
3.Combining parts 1 and 2 to obtain The FM:
- Combining the parts 1 and 2 we get the block diagram of the Armstrong method of FM generation
4.Use of frequency multipliers and amplifiers:
- The FM signal produced at the output of phase modulator has a low carrier frequency and low modulation index. They are increased to an adequately high value with the help of frequency multipliers and mixer. The power level is raised to the desired level by the amplifier.