FM Transmitter Using Reactance Modulator Direct Method

In FM modulation, one method to achieve frequency variations is through a reactance modulator using the direct method. Let's describe the process of an FM transmitter using this modulation technique.

The reactance modulator in FM transmission utilizes a varactor diode, which is a diode with a capacitance that varies with the applied voltage. Here is a step-by-step explanation of how an FM transmitter using reactance modulation operates:

Audio Signal Input: The audio signal, which could be from a microphone or any other audio source, is fed into the FM transmitter.

Preemphasis: In some cases, the audio signal may go through a preemphasis stage. Pre-emphasis boosts the higher frequencies of the audio signal to improve its transmission and reception quality.

Voltage-Controlled Oscillator (VCO): The VCO generates the carrier wave. It typically consists of an oscillator circuit controlled by a voltage. The voltage applied to the VCO determines its frequency.

Reactance Modulator: The reactance modulator stage introduces frequency variations to the carrier wave based on the audio signal. It uses a varactor diode, whose capacitance changes with the applied voltage.

Audio-to-Voltage Conversion: The audio signal is converted into a corresponding voltage signal that controls the varactor diode's capacitance. The stronger the audio signal, the greater the voltage, and consequently, the larger the capacitance variation.

Capacitance Variation: The varying capacitance of the varactor diode causes a corresponding variation in the resonant frequency of the oscillator circuit. This results in frequency deviations of the carrier wave.

RF Amplification: The modulated signal from the reactance modulator is then amplified using RF (Radio Frequency) amplifiers to increase its power level.

Antenna and Transmission: The amplified FM signal is sent to an antenna, which radiates the signal as electromagnetic waves, propagating it through the air for reception by FM receivers within the transmission range.

At the receiving end, an FM receiver demodulates the received signal to recover the original audio signal.

It's worth noting that while the reactance modulator using the direct method is one way to achieve FM modulation, there are other techniques, such as the phase modulation method and the indirect FM modulation method, which utilize different circuit configurations to achieve frequency variations in the carrier wave.

FM transmitter Block Diagram

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 buffer amplifier.
  2. The frequency multiplier section 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

    A frequency multiplier is sometimes seen, but its extremely low efficiency forbids widespread usage. Note that multiplication is by whole numbers only. There can not be 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 by 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 at 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 tap coil as the phase-reversing element in the feedback loop and most commonly uses a modulator that appears inductive.