What is an FM Transmitter?

Generally speaking, an FM transmitter is an abbreviation of an FM broadcast transmitter, which is mainly used to wirelessly transmit voice and music programs of an FM broadcast station.

Chinese name: FM transmitter

Foreign name: frequency-modulation transmitter

FM transmitter

FM transmitter : firstly modulate the audio signal and high-frequency carrier into FM waves, so that the frequency of the high-frequency carrier changes with the audio signal, and then the generated high-frequency signal is amplified, excited, power amplifier and a series of impedance matching, A device that outputs signals to an antenna and sends them out. The generation of high-frequency signals includes frequency synthesis, PLL and other methods. The frequency range of commercial FM broadcasting in China is 88-108

FM transmitter rear view MHZ, campus is 76-87MHZ, Western countries are 70-90MHZ.

Any FM radio station, regardless of its size (national radio, provincial radio, municipal radio, county radio, township radio, village radio, campus radio, enterprise radio, army barracks radio, etc.), All will be composed of audio broadcast control equipment, transmission equipment, FM transmitter and transmitting antenna feeder. A radio station with a large coverage area needs a FM transmitter with a high transmission power and a high-gain transmitting antenna and is set up high from the ground; a radio station with a small coverage area needs a FM transmitter with a small transmission power and an appropriate gain antenna And erected at the appropriate height. Generally, the power levels of FM transmitters are 1W, 5W, 10W, 30W, 50W, 100W, 300W, 500W, 1000W, 3KW, 5KW, 10KW. Special power FM transmitters can also be customized according to actual needs.

There are many types of FM transmitters:

According to the use occasions of FM transmitters, it can be divided into professional-level FM transmitters and amateur-level FM transmitters. Professional-level is mainly used for professional radio stations and places with high requirements on sound quality and reliability. Amateur-level is mainly used for Non-professional radio stations and occasions with general requirements for sound quality and reliability;

Divided by broadcasting mode, it can be divided into stereo broadcasting and mono broadcasting;

According to the original circuit principle of the FM transmitter circuit, it can be divided into analog FM transmitter and digital FM transmitter:

Digital FM Transmitter

With the rapid development of electronic technology, especially professional FM transmitters, digital FM transmitters are gradually replacing analog FM transmitters. It is easy to distinguish between digital and analog, depending on whether it uses software radio technology (DSP + DDS). Design.

Digital FM Transmitter: A digital FM broadcast transmitter from audio to radio frequency. It uses software radio technology to implement FM broadcast transmitter. It receives digital audio signals (AES / EBU) or analog audio signals (into A / D), audio signal processing, and stereo coding are all performed by a DSP (digital signal processor), while the frequency modulation process DSP controls DDS (direct digital Frequency synthesizer) to achieve the digitization of the modulation process. Discrete digital FM waves are D / A converted to produce conventional FM waves for the RF amplifier to amplify to the specified power. Referred to as "DSP + DDS".

FM transmitter

Analog FM transmitter: can only receive analog audio signals, audio signal amplification, limiting and stereo coding are all analog; especially VCO (voltage controlled oscillator) + PLL (phase-locked loop) is used to generate FM carrier frequency signals, modulation Of course, the analog composite audio signal is also used to directly modulate the varactor diode of the VCO. This circuit is a typical analog FM transmitter, but there may be LED or LCD digital display transmitter working frequency, but the whole process is analog.

FM transmitter application

As a simple communication tool, the FM transmitter is popular because it can carry out effective mobile communication without the support of a relay station. At present, it is widely used in small-scale mobile communication projects in the fields of production, security, and field engineering.

Generally, the transmitter includes three parts: a high frequency part, a low frequency part, and a power supply part. The high-frequency part generally includes a main oscillator, a buffer amplifier, a frequency doubler, an intermediate amplifier, a power amplifier driving stage and a final stage power amplifier. The role of the main oscillator is to generate a carrier with a stable frequency. In order to improve the frequency stability, the main oscillator stage often uses a quartz crystal oscillator, and a buffer stage is added after it to reduce the influence of the subsequent stage on the main oscillator. The low-frequency part includes a microphone, a low-frequency voltage amplification stage, a low-frequency power amplification stage, and a final low-frequency power amplification stage. The low-frequency signal is gradually amplified to obtain the required power level at the final stage power amplifier in order to modulate the high-frequency final stage power amplifier. Therefore, the final low-frequency power amplifier stage is also called a modulator. Modulation is the process of loading the information to be transmitted onto a certain high frequency oscillation (carrier frequency) signal. Therefore, the final high-frequency power amplifier stage becomes a modulated amplifier.

FM transmitter coverage

Generally speaking, the transmission distance of the FM transmitter is related to the transmitter power, the height of the transmitting antenna, and the local transmission environment (geographical conditions). Generally, the coverage radius of the transmitter below 50W is within 10 kilometers, and the 3KW FM transmitter can cover 60KM.

Reference table of FM transmitter wireless propagation radius

Transmitter power (W)	Coverage radius (km)
5W	0.5-1
10W	1-1.5
20W	1.5-2
30W	1.5-3
50W	2-5
100W	3-6
300W	8-10
500W	10-15
1KW	20-25
3KW	25-35

FM transmitter propagation distance

1. The coverage radius of wireless broadcasting should be determined according to the actual local conditions. In relatively open terrain, the transmission distance in flat areas is relatively long, and the transmission distance in hilly and mountainous areas will be reduced.

2. The selection principle of the transmitter power: the distance from the transmission center to the furthest distance, the density of surrounding obstacles, and whether the height of the antenna is the highest point in the surrounding area.