Among the advantages FM is free from interference air, the bandwidth (the width of the tape) is greater, and fidelitas high. If compared with the PM system, the FM has several advantages, including:
More noise resistant
The frequency allocated to FM broadcasting are among the 88 - 108 MHz, where the frequency of this region relatively free of interference from both the atmosphere and interference that is not expected. Reach of the modulation system is not far, if compared to the modulation system where the PM gelombangnya long term. So that the noise caused by the decrease in power almost no effect because it is in LOS (Line of Sight).
The width of the Bandwidth
Siar standard FM channels occupy more than ten times the width of bandwidth (the width of the tape) channel Siar AM. This is caused by the sideband nonlinear structure that is more complex with the effects (deviation) that require more bandwidth than the width of a simple linear distribution of sideband-sideband AM in the system. Siar FM band is located on the VHF (Very High Frequency) of the frequency spectrum where the available bandwidth wider than the length of the medium wave (MW) on the AM band Siar.
High Fidelitas
A uniform response to the audio frequency (at least at intervals of 50 Hz to 15 KHz), distortion (harmonic and intermodulasi) with amplitudo very low, the noise level is very low, and a good response transien is required for the performance of Hi-Fi is good. The use of the FM channel to provide an adequate response to the audio frequency of the radio and provides links with low noise. Characteristics of the other problems are only determined by the design of instruments only.
Stereo transmission
Allocation of the channel width and the ability to integrate FM harmony with some audio channels on one carrier wave, the system allows the development of the broadcast that stereos practical. This is a way for the broadcasting industry to provide quality reproductive health as well as or even better than that available on tape recordings or stereo. The emergence of compact discs and other digital audio devices will continue to encourage the industrial equipment and broadcast engineers to further improve the performance of the FM broadcast chain as a whole.
Additional communication rights
The wide bandwidth on the FM channel Siar also possible to load two channel audio data or additional, often called the subsidiaries Communication Authorization (SCA), together with the transmission of stereo. SCA channel provides an important source of revenue for most radio stations and media as well as the provision of services and digital audio, which is useful for the audience.
Theory of Frequency Modulation (FM)
Both FM (frequency modulation) and PM (Phase modulation) is a special case from the angle modulation (angular modulation). In the system of frequency modulation and phase angle of the waves against the carrier changed by the time the signal from the dimodulasikan (ditumpangkan). The carrier wave equation formulated as follows:
UC Ac = sin (WC QC)
In amplitudo modulation (AM), then the value 'Ac' will change according to the functions of the signal ditumpangkan. Meanwhile, in the corner of the modulation is changed is one of the components' toilet QC '. If that is changed is the 'toilet' so-called frequency modulation (FM), and if the components' QC 'that changed so-called Phase modulation (PM).
So in the FM system, the signal modulation (the ditumpangkan) will cause the carrier frequency of the waves change according to changes in the frequency of signal modulation. Meanwhile, the AM signal modulation of the changes will alter the phase of the carrier wave. Relations between the frequency changes from the carrier wave, the wave changes phase from the carrier, and frequency modulation signal modulation index is expressed as (m) where:
m = Changes in the frequency (Hz peak to peak) / frequency modulation (Hz)
In the broadcast FM carrier waves must have changes in accordance with the frequency of the signal modulation amplituda, but free frequency signal modulation, which is governed by the frequency modulator.
Pre-Emphasis
Pre-emphasis is used in aircraft to prevent transmitting the influence of disability in the signal received. Because iru component pre-emphasis placed on the initial signal before it was entered on the modulator. The influence of disability is derived from the differential gain (DG-strengthening different) and differential phase (DP-phase, which is different). Pre-emphasis will be pressing amplitudo from the FM signal frequency the lower the input.
With the use of this tool ketidaklinearan (disability) due to the nature of DG and DP in the transmission can be reduced. Later in the end accept the demodulator installed components de-emphasis function that has the reverse of the pre-emphasis.
FM transmitter
The objectives of the FM transmitter is to change one or more input in the form of audio frequency (AF) to termodulasi waves in the RF signals (Radio Frequency), which is intended as the output power and diumpankan to the antenna system is applied to. In simple form can be separated on the FM modulator and an RF power amplifier in one unit. Actually FM transmitter consists of a series of sub-blocks that have distinctive functions, namely:
- FM exciter alter the audio signal into a frequency RF is termodulasi
- Intermediate Power Amplifiers (IPA) is required in some radiator to increase the level of RF power to be able menghandle final stage
- Power Amplifiers at the end of the power to raise the appropriate signal from the antenna required by the system
- Power (power supply) from the input power source into AC voltage and AC or DC currents needed by each subsystem
- Transmitter Control System monitor, protect and provide for each of the subsystems so that they can work together and provide the desired results
- RF lowpass filter that does not limit the frequency of the output relay diingikan
- Directional coupler, which indicates that power is being sent to or received from the antenna system
FM Exciter
The heart of the FM broadcast transmitter is located in the exciter it. Function of the exciter is to generate and memodulasikan carrier wave with one or more input (mono, stereo, SCA) in accordance with FCC standards. Carrier wave that has been strengthened by dimodulasi and wideband amplifier to the level required by the next level.
Direct FM is a technique where the frequency modulation of oscilator can be changed in accordance with the voltage used. Like oscilator, called a voltage tuned oscilator (VTO) is possible by the development of diode Tuning varaktor that can change according to changes kapasitansi reverse bias voltage (also called a voltage controlled oscillator, or VCO).
The stability of the frequency of direct oscillitor FM is not good enough, it needed to automotic frequency control system (AFC) that uses a crystal oscillator as a stable frequency reference. Component AFC role as regulator raised the frequency of the local oscillator to dicatukan to the mixer, so that the frequency oscillator becomes stable.
Boosting microphone with the level tone Dynamic Compressors
In this transistor design BC547C apply as early brace of 20 dB for signals from the microphone. Kolektornya set the voltage level DC voltage to the input op-AMP is less than half the ration of voltage.
The output of the audio signal from the op-AMP disearahkan by the diode D1 and D2, which dole capacitors C1 and C2 respectively positive and negative. Different voltage between the C1 and C2 cause disposal of cargo passing through R3, D3, D4, and R4. Capacitor C3 and C4 have a dual function, namely to reduce the ripple from the AC-ripple currents through the D3 and D4 and provide pembumian (ground) for the voltage divider, which consists of r5 and impedansi of diode D3 and D4 (parallel). Impedansi in the second diode depending on the amount of cargo disposal by the capacitors C1 and C2, which pass through the second diode. The greater flow in the series diode, the small impedansinya, and also means the small voltage input to the op-AMP pin noninverting (positive).
At the signal voltage at the input op-AMP small, ketidaklinearitasan diode create distortions that small, the 2.5 V pp output in the op-AMP.
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