FM Modulation System

FM transmitter

FM Transmitter Block Diagram Direct Method
FM Transmitter Block Diagram Direct Method

Using Reactance modulator direct method

The FM transmitter has three basic sections.

  1. The exciter section contains the carrier oscillator, reactance modulator and the buffer amplifier.
  2. The frequency multiplier section, which features several frequency multipliers.
  3. The power output section, which includes a low-
    level power amplifier, the final power amplifier, and the impedance matching network to properly load the power section with the antenna impedance.

The essential function of each circuit in the FM transmitter may be described as follows.

  1. The Exciter

    1. The function of the carrier oscillator is to generate
      a stable sine wave signal at the rest frequency, when no modulation is applied. It must be able to linearly change frequency when fully modulated, with no measurable change in amplitude.
    2. The buffer amplifier acts as a constant high-
      impedance load on the oscillator to help stabilize the oscillator frequency. The buffer amplifier may have a small gain.
    3. The modulator acts to change the carrier oscillator
      frequency by application of the message signal. The positive peak of the message signal generally lowers the oscillator's frequency to a point below the rest frequency, and the negative message peak raises the oscillator frequency to a value above the rest frequency. The greater the peak-to-peak message signal, the larger the oscillator deviation.
  2. Frequency Multiplier

    Frequency multipliers are tuned-input, tuned-output
    RF amplifiers in which the output resonant circuit is tuned
    to a multiple of the input frequency. Common frequency
    multipliers are 2x, 3x and 4x multiplication. A 5x

    Frequency multiplier is sometimes seen, but its extreme low efficiency forbids widespread usage. Note that multiplication is by whole numbers only. There can not a 1.5x multiplier, for instance.
  3. Power output section

    The final power section develops the carrier power, to be transmitted and often has a low-power amplifier driven the final power amplifier. The impedance matching network is the same as for the AM transmitter and matches the antenna impedance to the correct load on the final over amplifier.


A special form of class C amplifier is the frequency. multiplier. Any class C amplifier is capable of performing frequency multiplidàtion if the tuned circuit in the collector resonates at some integer multiple of the input frequency.

For example a frequency doubler can be constructed by simply connecting a parallel tuned circuit in the collector of a class C amplifier that resonates at twice the input frequency. When the collector current pulse occurs, it excites or rings the tuned circuit at twice the input frequency. A current pulse flows for every other cycle of the input.

A Tripler circuit is constructed in the same way except that the tuned circuit resonates at 3 times the input - frequency. In this way, the tuned circuit receives one input pulse for every three cycles of oscillation it produces Multipliers can be constructed to increase the input

frequency by any integer factor up to approximately 10. As' the multiplication factor gets higher, the power output of the multiplier decreases. For most practical applications, the best result is obtained with multipliers of 2 and 3.

Another way to look the operation of class C multipliers is .to .remember that the non-sinusoidal current pulse is rich in harmonics.  Each time the pulse occurs, the second, third, fourth, fifth, and higher harmonics are generated. The purpose of the tuned circuit in the collector is to act as a filter to select the desired harmonics.

Frequency Multiplier
Frequency Multiplier

In many applications a multiplication factor greater than that achievable with a single multiplier stage is required. In such cases two or more multipliers are cascaded to produce an overall multiplication of 6. In the second example, three multipliers provide an overall multiplication of 30. The total multiplication factor is simply the product of individual stage multiplication factors.

Reactance Modulator

The reactance modulator takes its name from the fact that the impedance of the circuit acts as a reactance (capacitive or inductive) that is connected in parallel with the resonant circuit of the Oscillator. The varicap can only appear as a capacitance that becomes part of the frequency determining branch of the oscillator circuit. However, other discrete devices can appear as a capacitor or as an inductor to the oscillator, depending on how the circuit is arranged. A colpitts oscillator uses a capacitive voltage divider as the phase-reversing feedback path and would most likely tapped coil as the phase-reversing element in the feedback loop and most commonly uses a modulator that appears inductive.