| 摘要 |
An interrogator (10) for use with a passive transponder (12) has a Colpitts oscillator (30, 32, 34, 36, 38) arranged to vary in frequency in accordance with the frequency of a signal from the transponder; a voltage controlled oscillator VCO 42 arranged to vary in frequency in accordance with the Colpitts frequency after a delay of a number of signal cycles, the delay being provided by a loop filter in a phase discriminator (40); and a digital filter (48) arranged to detect the phase difference between the Colpitts oscillator and the VCO. Preferably the digital filter has an input (46) comprising an array of flipflop circuits (52, 54, 56); a delay line (53, 55, 57) associated with each flipflop and providing a time delay which increases along the array, so that when first and second clock signals are provided to each flipflop-delay line pair, if the phase difference between the clock signals is less than the time delay of that delay line the flipflop provides a logical 0 and if the phase difference is greater than the time delay the flipflop provides a logical 1. |
