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四、调频解调器由于调频制比调幅制抗干扰能力强,故在卫星电视广播中,电视信号对高频载波采用了调频制。调频改善系数I_(FM)定义为调频接收机解调后的信噪比S/N与解调前载噪比C/N之比。即I_(FM)=(S/N)/(C/N)=1.47(Δf_m~2B)/(f_h~3)式中,Δf_m为信号的最大频偏;B为中频滤波器的通频带;f_h为鉴频器后低通滤波器的最高频率,一般设计成f_h为电视视频信号的最高频率成分。由上式可见,f_h愈低(以不低于视频信号的最高频率为限度),Δf_m愈宽,B愈大,则调频改善度愈好,这正是调频制的优点。但是,要获得这种改善度有个前提,就是接收机输入端的载波信号必须远大于混入噪声中的载波幅度,否则调频解调器的抗干扰性能会显著变坏。
Fourth, the FM demodulator system than the amplitude modulation system anti-interference ability, so in satellite television broadcasting, television signals on the use of high-frequency carrier frequency modulation system. The FM improvement factor I_ (FM) is defined as the ratio of the demodulated signal to noise ratio S / N of the FM receiver to the C / N C N before demodulation. That is, I FM = S / N / 1.47 Δf_m ~ 2B / f_h ~ 3 Where Δf_m is the maximum frequency offset of the signal; B is the passband of the IF filter; f_h is the highest frequency of the low-pass filter after the frequency discriminator, generally designed as f_h is the highest frequency component of the television video signal. As can be seen from the above equation, f_h lower (at no lower than the maximum frequency of the video signal for the limit), Δf_m wider, B is greater, then the better FM adjustment, which is the advantages of FM. However, there is a prerequisite for this improvement to be that the carrier signal at the input of the receiver must be much larger than the carrier amplitude in the noise mix or the anti-jamming performance of the FM demodulator can be significantly degraded.