FM circuits, FM wireless transmitters

FM circuits generate and transmit radio signals using frequency modulation, a method of carrying information on a radio wave. In frequency modulation the amplitude of the carrier wave stays constant while its frequency is shifted up and down in step with the signal being sent, such as an audio waveform. Encoding the sound as small frequency changes rather than amplitude changes makes the transmission relatively resistant to noise and interference, which is one reason FM is used for high-quality broadcast radio and for short-range wireless links.

At the heart of a simple FM transmitter is an oscillator, a circuit that produces a steady radio-frequency carrier. The audio signal is applied so that it slightly varies the oscillator's frequency, producing the modulation. The resulting signal can be radiated from a short antenna and picked up by an ordinary FM receiver tuned to the same frequency. Many introductory designs achieve this with very few parts — sometimes a single transistor, a coil, a few capacitors and a microphone — which is why FM circuits are popular learning projects.

Common examples include miniature wireless microphones and low-power FM transmitters that send audio to a nearby radio. Building one demonstrates oscillation, tuning by means of a coil and capacitor, and the principle of modulation. Practical considerations include keeping the oscillator stable so the signal does not drift, limiting transmitted power, and operating only on frequencies permitted by local regulations, since unauthorized transmission can interfere with licensed services.

Frequently asked questions

What is frequency modulation?: A method of carrying information by varying a radio carrier's frequency while keeping its amplitude constant, which resists noise well.
What is at the core of an FM transmitter?: An oscillator that produces a radio-frequency carrier, whose frequency is varied slightly by the audio signal to create the modulation.
Why are FM circuits popular projects?: Simple designs use very few parts and clearly demonstrate oscillation, tuning and modulation, making them good learning exercises.

Electronic circuits:			Verticaal
1 wire circuits	Light dimmer circuits	RF-Crystal radio
Analog circuits	Magnetics	RF- FM transmitter
Audio	Measurement	Sensors
Audio amplifiers	Microcontroller, PIC	Sensors-fluid
Battery chargers	Microcontroller projects	Sensors-Light
Computer	Motor control circuits	Sensors-sound
Digital circuits	Opto electronics	Sensors-temperature
Fluorescent Lamp	Oscillator circuits	Sensors-weather
General	Power - control	Solar power
Infra Red Circuits	Power supplies	Telephone technology
LED circuits	Power supplies-SMPS	Video circuits
	RF circuits

FM transmitters

2-3W FM transmitter wireless microphone, single transistor, wireless FM transmitter circuit

3 Watt FM Transmitter schematic for an FM transmitter with 3 to 3.5 W output power that can be used between 90 and 110 MHz

4 Transistor Transmitter This circuit provides an FM modulated signal with an output power of around 500mW. The input Mic preamp is built around a couple of 2N3904 transistors, audio gain limited by the 5k preset

80 Watt FM stereo transmitter 80 Watt FM stereo transmitter

9V FM transmitter pdf file

9V FM transmitter 9V FM transmitter (Tx) with a range of up to 1 kilometer in the open, pdf file

Crystal Controlled FM Transmitter The circuit follows that of a simple textbook synthesiser comprising a Voltage Controlled Oscillator (VCO), a damped Loop Filter, a Reference Oscillator (Crystal) and a Frequency Comparator

Emetteur FM pdf file, en Français, a tip

FM band monaural transmitter a high quality monaura FM transmitter, pdf file

FM Broadcast Audio Transmitter The circuit consists of a frequency modulated oscillator, an audio preamplifier with pre emphasis to supply the frequency modulating signal, and a buffer amplifier to drive the antenna connector

FM radio pirate Introduction to Community Radio Electronics, A Design for a 40W broadband VHF RF Power Amplifier for FM broadcast, How to make an Weatherproof Vertically Polarised Omnidirectional Aerial

FM stereo transmiter FM wireless stereo transmiter circuit, A Simple FM Stereo Transmitter using an AVR microcontroller

FM stereo transmiter BA1404

FM Remote Speaker System A high quality, noise free, wireless FM transmitter/receiver may be made using the LM566 VCO and LM565 PLL Detector. The LM566 VCO is used to convert the program material into FM format, which is then transformer coupled to standard power lines. At the receiver end the material is detected from the power lines and demodulated by the LM565, pdf file

FM Telephone Bug a simple transmitter that when connected to a phone line, will transmit anything on that line (execpt the dial tone) to any FM radio. The frequency can be tuned from 88 to about 94Mhz and the range is about 200 feet

FM transmitter FM transmitter, wireless microphone, single transistor, wireless FM transmitter circuit

FM transmitter a good fm-transmitter (88Mhz-110Mhz)

FM transmitter this is an old one, not so stable

FM transmitter

FM transmitter pdf file

FM Transmitter schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet

FM transmitter FM transmitter circuit, power output 200-250 milliwatts, Long-range FM Transmitter

FM transmitter wireless microphone, wireless FM transmitter

FM transmitter wireless microphone, wireless FM transmitter, pdf file

FM transmitter a tip, zip file, Scan of PCB

FM transmitter 0,4 W

FM transmitters a tip

FM transmitter that fits on top of a 9v battery

FM transmitter with a single transistor Simple 30 meters range FM Transmitter

FM Transmitter with Opamp An low power FM Transmitter using an op-amp as the audio preamp and a single transistor as the RF amplifier

FM wireless microphone This article describes the theory of operation and construction details of a wireless microphone capable of picking up sounds within a 20 meter radius and transmitting them to an ordinary FM radio at distances of up to 2 kilometers, pdf file

FM transmitter, RDS encoder, antenna FM transmitter, RDS encoder, antenna circuits, DTMF decoder, PLL digital tuning unit

Low power FM transmitter VHF FM wireless microphone circuit

MEDIUM-POWER FM TRANSMITTER the range of this FM transmitter is around 100 metres at 9V DC supply, pdf file

PLL bug transmitter the frequency of this transmitter is PLL controlled which makes it very stable. The frequency is programmed in digitally way and can be changed very easy. Frequency range is about 50 to 150 MHz and the output power 100mW

Portable FM transmitter 1 W This small FM transmitter includes a limiter, a microphone amplifier and a PLL digital tuning. All the parts are placed on one circuit board. The RF power is switchable between 1 W (HI) and 0,2 W (LO)

VHF transmitter - design Very stable FM transmitter - VCO

Voice Activated FM Transmitter A sound activated switch and an FM transmitter joined together to make a voice activated FM transmitter. Set the level of sound that it will respond to. Two stage transmitter needs tuning to get best results, pdf file

WASP any frequency from 85MHz to 110MHz, depending on the number of turns in the oscillator coil and the spacing of the turns

Wireless FM transmitter Frequency Modulation Wireless Microphone

Wireless microphone or "bug" a wide band FM wireless microphone circuit

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