The “RFID tag” receiver seen in the figure has been tested for proof-of-concept at 445 MHz. Some of the familiar receiver techniques, such as direct conversion, super regeneration, or superheterodyne, consume far too much supply current for long battery life. A better method involves a technique borrowed from simple field-strength meters: a tuned circuit and a diode detector.
Tuned circuits aren’t easily constructed or controlled at UHF frequencies, so a short section of transmission line is used to match the detector diode (1N5712) to a quarterwave whip antenna. The diode’s 1-pF junction capacitance is transformed into a virtual short circuit at the base of the antenna, while the received antenna current is transformed into a voltage loop at the diode, producing excellent receiver sensitivity.
Biasing the detector diode can improve sensitivity under load1, but the additional bias current presents a serious power drain. Special zero-bias diodes are available. But these are optimized for low-impedance circuits, and their low breakdown voltage (2 V) makes them susceptible to damage when placed close to the transmitter. The 1N5712 diode, which is rated for 20-V breakdown, can achieve high sensitivity with zero bias—provided its output isn’t loaded.
The nanopower comparator used draws only 300 nA (yes, nanoamperes) and its CMOS input presents an open circuit to the output of the detector diode. When the rectified signal reaches a threshold of approximately 18 mV at the comparator input, a rising edge at the comparator output triggers a one-shot. This temporarily enables the answer-back and other pulsed functions.
Total supply current for the comparator circuitry is a mere 400 nA, consuming just 7 mA-hr of battery life in continuous operation over a period of five years. The circuit will also work just as well with a 9-V battery as with a single lithium cell. With 200 mW from a reference dipole, the receiver can be triggered up to a distance of about 100 feet.
- Eccles, W.H., Wireless Telegraphy and Telephony, Second Edition; Ben Brothers Ltd., London, 1918, pg. 272.