Abstract: A duplexer for bidirectional communication systems includes tunable band reject filters. The tunable band reject filter on the receiver side can be tuned to reject the transmit signal frequency, and the tunable band reject filter on the transmitter side can be tuned to reject the receive signal frequency. Thus, the band reject filters allow for simultaneous tuning of the band reject frequencies on the transmission side and the reception side. The tunable band reject filters can be implemented as resonators including barium strontium titanate (BST) capacitors of which the capacitance can be tuned according to bias voltages applied to the BST capacitors.

Abstract: An identification token and reader circuit for an electronic identification and latch/alarm system, the token comprising an LC resonant circuit, a digital counter, an analog or digital demultiplexer and at least one analog switch. The token's supply voltage is derived from the token's resonant LC circuit by a rectifier means when the token is proximal to a suitable oscillator as found in a reader/controller. The device is useful for access control in an electronic lock and key system. When the token is brought near to the lock part, an oscillator contained within the lock part oscillates at a high frequency. The oscillation is picked up by the key part where it is modulated with a frequency or pulse pattern serving as a key identifier, then transmitted back to the lock part where the enclosed carrier is demodulated by means of synchronous detection and processed by recognition means at the lock side.