| 摘要 |
A receiver for frequency modulated signals, having down-conversion to a baseband, zero intermediate frequency for selectivity, followed by up-conversion to a non-zero IF for amplification, limiting, and demodulation. A phase-lock loop locks the frequency of the down-conversion source to the center frequency of the signal coupled to the baseband IF, converting the signal to precisely zero frequency. This avoids the beat note often found in direct conversion systems. The phase-lock loop additionally provides inherent demodulation of the FM signal. The received FM signal is coupled to the baseband IF through a radio frequency amplifier for improved sensitivity and local oscillator isolation, or through a first IF comprising a mixer, local oscillator, filter, and amplifier.
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