| 摘要 |
In a bandpass filter, a first cyclical analogue signal V3 at a centre frequency divided by integer N in divider (44) and the divided signal supplied to a digital filter (48) as its system clock; a second cyclical analogue signal at a frequency to be measured, differing by a small amount from the centre frequency, is delayed by N cycles by a dynamic store (54); a subtractor (56) subtracts the second signal from the delayed second signal and supplies any difference to the digital filter (48); the output of the digital filter is related to the frequency of the second signal, but the centre frequency is suppressed. The filter may be used in an interrogation for an electronic detection system in which an interrogation signal is modified in frequency by a small amount, characteristic of a device to be detected.
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