Testing FM Transmitter

Frequency Modulation (FM):

We know that in amplitude modulation (AM), the frequency is constant which varies only the amplitude. Where as in frequency modulation (FM), which varies the frequency and keeps the amplitude constant.

There are many advantages of FM (frequency modulation) over the AM (amplitude modulation). The most significant of these focal points is that a FM has more excellent flexibility from obstruction and static. FM gives preferred sound quality and constancy over AM. Frequency modulation (FM) is utilized within radio broadcasts, as well as in police and healing center interchanges, channel emergency, TV sound, and remote systems. The FM radio band is from 88 to 108 MHz. The FM transmitter accomplishes the most excellent range with least power.

The FM Transmitter:

The Transmitter of FM utilizes FM wave to transmit sound. It transmits audio signals over a carrier wave by varying the frequency, where carrier wave frequency is equivalent to the amplitude of the audio signal. The circuit generates frequency in the VHF band i.e. 88 to 108MHZ.

Creating an FM Signal:

There are two important components to form a FM signal, first is carrier frequency, and second is audio frequency to modulate the carrier frequency. We will get FM signal by varying the carrier frequency by allowing the AF. The transistor of FM consist an oscillator to form the RF signal.

Creating an FM Signal
Basic block diagram of a FM transmitter

From the block diagram, the FM transmitter circuit consists following components:

  1. Micro-phone
  2. Audio pre-amplifier
  3. RF oscillator
  4. Amplification stage
  5. Antenna

Components of FM Transmitter:

The Microphone:

Microphones are changes over audio signals into electrical signals of the equal recurrence and of amplitudes in the same extent as a force variation. It enhances the signal 100 times before transmitting the signal to the first stage. The supply voltage of microphone is less than 0.5V.

A variable resistor at micro-phone is utilized to alter the audio quality of amplifier and modify the variable resistor to get best quality. Alternately assuming that you need to utilize altered resistor as a part of the spot of variable resistor and would prefer not to change the sound quality then a 5K resistor might be utilized. A 22n capacitor on the output of the micro-phone couples the sign to the first sound pre-amplifier stage. This capacitor is intended to divide the DC voltage on the receiver from the voltage with respect to the transistor.

Electrets microphone & Circuit

Audio Pre-Amplifier:

Pre-amplifier is a self-biasing emitter fit for amplifying the signals got by the micro-phone. It conveys these to the oscillator stage. The capacitor disconnects the micro-phone from the base voltage of the transistor and permits just AC signs to pass through. The output waveform at the micro-phone is passes through a coupling capacitor to an emitter stage.

In this stage the signal is amplified moreover 70-100 times and is currently huge enough to be infused into the RF stage. Just an exceptionally self-biasing emitter stage is used for the sound enhancer. This stage is said to be AC coupled as it has a capacitor on both the input and output so the DC voltages of the different stages don’t impact the voltage on the stage.

RF Oscillator:

RF oscillator, it is a modulation stage. In this stage the amplified audio input signal is adjusted for transmission. Each transmitter circuit needs an oscillator part to create the RF waves. The transistor and its components encompassing the tuned circuit essentially keep the tuned circuit working at its resonant frequency.

Final Amplification Stage:

This stage amplifies the output RF signal. The signal handled by the oscillator stage is not exceptionally capable so we pass it to an amplifying stage called an output stage to increase the amplitude. The FM transmitter circuit is enhanced by including this buffer or output stage so that oscillator is not driving antenna. This will give the circuit more dependability and more output.

FM Antenna:

The last/final stage of any FM transmitter is the FM antenna. This is the place where the electronic FM signal is changed over to electromagnetic waves, which are transmitted into the atmosphere. A 22 measure copper wire is suitable for antenna. We have to keep this wire in vertical. In that capacity you can utilize a telescopically extendable antenna for example ones discovered in radios. Its length ought to be give or take 1/4 the FM wavelength; review that duplicating frequency and wavelength equivalents the speed of light. For about 30-50 meters range, a 15cm antenna is sufficient but if you need to get a most extreme range you can utilize a half-wave antenna.

A telescopically extendable antenna
A telescopically extendable antenna

Testing the FM Transmitter:

The voltages around the oscillator stage cannot be measured with an ordinary multimeter as the leads of millimeter will act an antenna when the circuit is operating and kill the operation of the circuit. This is certainly the case on the emitter of the second transistor, where the leads of a multimeter will draw off so much energy that the stage will stop working. Therefore, a field strength meter is used to test the output of FM transmitter. A field strength meter shows the strength of the actual field being radiated from your antenna. It is used to determine the basic radiation pattern of your antenna and see which direction your signal is strongest. You can make changes to your antenna and instantly know if it radiates better or worse.

A field strength meter by RadioShack
A field strength meter by RadioShack
Applications of FM Transmitter:

Broadcast music to a nearby radio receiver: A FM transmitter can be used to broadcast music stored in memory of a phone on FM frequencies to a nearby compatible FM receiver such as a car radio or home stereo systems, thus eliminating the wire clutter. Some Nokia n-series phones have this FM transmitter feature.

Hearing Assistance: FM transmitter assists in hearing by transmitting the audio of speaker directly to the listener’s hearing aid. It is used in classrooms and noisy environments.

Panic button: FM transmitter is used in panic button devices for the elderly. When the panic button is pressed, a signal is transmitted to a nearby receiver to summon a nurse or relative. This allows patients to get immediate attention without having to call out.

Micro broadcasting: Low-power FM transmitters are also sometimes used for neighborhood or campus radio stations.

Snooping: FM transmitters have been used to construct miniature wireless microphones for surveillance purposes.


From the block diagram, the block diagram consists mainly three blocks VFO, class-C driver stage and class-C final power amplifiers which are main blocks of FM transmitter. A Micro-phone is utilized to nourish audio amplifiers to modulate a carrier sign of around 106 MHz frequency. Then this carrier signal is amplified with a RF power amplifier that is associated with a tuned receiver antenna to cover a viewable pathway separation of 2KM.

Application diagram

From the above article you can clearly understand the FM transmitter testing if any queries on this topic or from the electrical and electronic projects leave the comments below.


  1. Anwary shaikh says:

    speed control of single phase induction motor using v/f in closed loop is our project please help us.

    1. Team ElProCus says:

      Sent a mail to you check once and let me know if you have any questions.

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