RF ID tags, circuits that detect a “wake-up” call and return a burst of data, must operate on a very low quiescent current for weeks or months, yet have enough battery power in reserve to answer an incoming call. If the system operates in the ultra-high frequency range, designing a receiver for micropower current consumption can be problematic. Familiar receiver techniques such as direct conversion, super-regenerative, or superheterodyne consume far too much supply current. A better method involves a technique borrowed from the simple field strength meter—a tuned circuit and a diode detector.
The circuit,which was tested at 470 MHz, contains a couple of improvements over the standard L/Cwith-whip field strength meter (see the figure). Tuned circuits aren’t easily constructed or controlled at UHF, so a transmission line is used to match the detector diode (1N5711) to the approximately 6-in.-long whip antenna.
The 0.4-wavelength section presents an efficient, low-impedance match to the base of the quarter-wave whip, but transforms the received energy to a relatively high voltage at the diode end of the transmission line for good sensitivity. Sensitivity is improved 10 dB by biasing the detector diode. The forward threshold is reduced essentially to zero, so a very small voltage can generate a meaningful output change.
When a signal at the resonant frequency of the antenna is received, the left-most Schottky diode rectifies the incoming carrier and creates a negative-going dc bias shift at the noninverting input of the comparator. Note that the bias shift is sensed at the base of the antenna where the impedance is low, rather than at the Schottky diode where the impedance is high. This introduces less disturbance into the tuned antenna and transmission-line system. Total current consumption is approximately 5 mA, including the CMOS one-shot. The one-shot output is used to temporarily enable other circuitry, which in turn answers the interrogation.
Sensitivity of the detector is excellent. The finished circuit can detect 200 mW from a reference dipole at 100 ft. Of course, range depends on operating frequency, antenna orientation, and surrounding obstacles. But, in the clear, a more reasonable distance (e.g., 10 ft.) can be achieved at 470 MHz with only milliwatts of output power.
All selectivity is provided by the antenna. A quarter-wave stub (shorted with a capacitor) can be added to the base of the antenna for better selectivity and improved rejection of low-frequency signals.