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卫星电视广播一般均采用调频制,这主要由于频率调制相对于振幅调制具有较强的抗干扰性能。在相同的输入载噪比情况下能够得到较大的输出信噪比(即有较大的制度增益)。这对于卫星发射功率有限,距离地面接收站十分遥远来说,有尤其重要的意义。调频改善因子,即调频解调时的制度增益为: K_(FM)=输出信噪比/输入载噪比若取Δf=m_fF_m可得:K_(FM)=3m_f~3。式中,Δf为解调器前接收机带宽的一半,F_m为调频信号基带最高频率,m_f为调频指数。由于通常m_f>1所以调频信号鉴频解调后输出信噪比较解调前载噪比会有较大提高。
Satellite television broadcasting generally adopts FM system, which is mainly due to frequency modulation has relatively strong anti-interference performance compared with amplitude modulation. In the case of the same input carrier-to-noise ratio, a larger output signal-to-noise ratio (ie, greater system gain) can be obtained. This is of particular importance for the limited transmit power of satellites, which is very far from the ground receiving station. The FM improvement factor, that is, the system gain when FM demodulation is: K_ (FM) = output signal to noise ratio / input carrier to noise ratio If Δf = m_fF_m is available: K_ (FM) = 3m_f ~ 3. Where, Δf half of the receiver bandwidth before the demodulator, F_m FM baseband signal for the highest frequency, m_f FM index. Because usually m_f> 1 Therefore, the FM signal demodulation frequency demodulation output signal-to-noise ratio demodulation before the carrier-to-noise ratio will be greatly improved.