| 摘要 |
FM receivers for communications at UHF frequencies often use automatic frequency control (AFS) or phase lock to precisely control a local oscillator. By causing the local oscillator to track errors and slow variations in the frequency of the received signal, these control loops suppress DC and low-frequency modulation components in the intermediate frequency signal and interfere with demodulation of low frequency information, such as digital signalling data. This invention permits recovery of the full modulation spectrum with flat frequency response. A conventional FM demodulator recovers the high frequency modulation components; the frequency control signal for the local oscillator provides the low frequency components. Combining the components with proper weighting produces recovered modulation with bandwidth limited only by the IF bandpass filter of demodulator. The invention would find application in an FM receiver of the superheterodyne or direct conversion type that requires precise frequency control, such as one used for communications in a narrow channel spacing environment at ultra-high frequencies.
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