On the wireless transmitter module in the radio, television, communication, remote control, telemetry device or system, in addition to using amplitude modulation, but also widely used in the frequency modulation mode. The following are the main properties of these two modulation compared:
1, the transmitter power
Frequency modulation transmitter, the instantaneous frequency of the FM carrier wave amplitude of the FM wave with the modulation signal change, it is the maximum power is equal to the average power; AM Built transmitter is amplitude modulated signal, when mAM = 1, the maximum transmit power is equal to the average power (carrier power) 4 times if the final stage of the AM transmitter and FM transmitter using the same type of power tube, according to the maximum power estimation, the FM transmitter emits power AM transmitter transmitter the average power of 4 times.
2, anti-jamming performance
FM FM main advantage of its strong anti-jamming performance is so-called anti-jamming, mainly refers to the input signal to noise ratio (referred to as the signal-to-noise ratio S / N) under the same conditions, the output terminal of the FM receiver signal-to-noise ratio is greater than AM The signal-to-noise ratio of the receiver output.
FM than AM anti-interference ability of the system from the transmitted signal power size of the two formats for analysis. The FM wave edge frequency components of the power is out from the carrier power grab. The modulation coefficient MFM is bigger, the edge of the frequency number and the amplitude increases, this means the carrier power is converted into a large proportion of the edge-frequency power, the edge frequency of the amplitude modulated wave power is equal to only half of the carrier power (when the modulation Index MAM = 1). Accordingly, the amplitude modulated wave of the FM table may have a greater side frequency power, which means that it is also the ability to overcome the noise and interference in the channel or inside the machine.
Secondly, from the mediation of the reception signal to be compared and analyzed. Contains the information of the amplitude modulated signal in the amplitude modulated signal amplitude proportional to the amplitude of the change in amplitude of the modulation signal (), the demodulation of the envelope detector (also called amplitude detector) can not suppress a parasitic amplitude modulation interference; FM signal included in the change of the instantaneous frequency of the high frequency oscillation, therefore, the interference caused by the parasitic amplitude modulation can be removed by the slicer (or Limiting the proportion of the frequency discriminator).
Therefore, no matter from the edge of the transmit signal frequency power from the received signal demodulation analysis, frequency modulation anti-interference performance is superior to the amplitude modulation system.
3 demanding requirements of the AM transmitter power amplifier tubes
AM transmitter wave with the amplitude of the modulation signal changes, which have high requirements for the reverse voltage of the amplifier tubes, should adopt the higher breakdown voltage of the tube or pipe susceptible to breakdown and FM transmitter work in the state of equal amplitude, the tube is not easy to breakdown.
4, the production cost is relatively
Production FM family of devices is a complex process of debugging, so need to be equipped with the sum parameter tester, shielded room, expensive production debugging equipment and commissioning the equipment directly determine the stability of product performance, also need professional technical workers. The production amplitude modulation device is relatively a lot easier, so the production of FM threshold is much higher than the production of AM products, both the difference in the cost of production more than doubled.
5, the width of the frequency band occupied by
Due to the frequency band occupied by the wideband FM systems, FM only suitable in the VHF band (30Hz-30GHz) use, long-wave band (medium wave :1000-100M, long-wave 10-1km), using AM or SSB modulation way.
