Phase-nulling telemetry system incorporating a remote passive transponder

A noncontact telemetry system using a remote passive transponder in a feedback loop has been developed. The transponder consists of a primary coil tuned to 9 MHz, a secondary coil having a variable resonant frequency fs ¿ 190 k Hz (dependent on a measurand), and a junction diode. Transmitter and receiver coils inductively link to transponder primary and secondary coils, respectively. The transmitter-coil voltage is 4.5 V r.m.s., and consists of a carrier at frequency 9 MHz, amplitude modulated at frequency fm, variable about 190 kHz. Detection occurs in the transponder so that a signal at fm appears in the secondary and receiver coils. The feedback loop contains a phase-sensitive detector and voltage-controlled oscillator, and operates so as to maintain fm = fs so that the feedback signal (and system output) is always directly related to fs and thus the measurand. The experimental system enables a transponder of volume about 1 cm3 to move about in a volume of at least 2000 cm3, with the transponder some 25 cm from transmitter and receiver coils. Depending on its position, the transponder dissipates between 0.25 ¿W and 10 ¿W of electrical power. With moderate transponder movements, measurement accuracies of 1% should be possible. Improvements to allow for any transponder orientation, and to provide for multichannel operation, are proposed.