| 摘要 |
There is disclosed a method and apparatus for demodulating an R.F. modulated signal having a known carrier frequency Fo. A first reference signal of a frequency matching approximately the carrier frequency Fo, is mixed with the input R.F. modulated signal to produce the demodulated signal. The first reference signal is generated by a reference oscillator which provides a second reference signal of a second distinct reference frequency, a voltage control oscillator for generating the first reference signal dependent upon the amplitude of an error signal applied thereto, a selectively variable divider circuit for receiving and dividing the first frequency by a selected number to provide a third divided signal, and a comparator for comparing the phase of the second reference signal with that of the third divided signal to generate and apply the error signal to the voltage control oscillator. The demodulated signal is applied to the reference oscillator, whereby the second frequency varies in accordance with the phase of the demodulated signal and the first frequency is phase-locked to the carrier frequency Fo. A selected spectrum window of the demodulated signal is provided by generating a fourth reference signal of a frequency offset from the carrier frequency Fo and by multiplying that offset reference signal by the demodulated signal to provide a difference signal indicative of the difference therebetween centered about the offset frequency. The difference signal is filtered to pass only those frequencies below a selected first frequency to provide a spectrum window whose bandwidth is limited to that segment of the demodulated or base band signal to be examined.
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