| 摘要 |
An apparatus and method for measuring the average or carrier frequency of a frequency shift keyed (FSK) signal which consists of sinusoidal tones that alternate regularly in frequency at a known rate between two unknown frequencies separated by a known frequency shift. The incoming signal is multiplied by two delayed versions of itself to yield to product signals. The delay employed in the first version is nominally equal to the FSK shift rate and the second delay differs from the first by an incremental amount. The two product signals are passed through substantially identical bandpass filters centered at the shift frequency and having a bandwidth approximately equal to the shift rate, and the phase angles of the filtered signals are differentially compared and averaged to produce a signal proportional to the average frequency of the FSK signal.
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