A direct-conversion receiver for 900 MHz (ISM band) spread-spectrum digital cordless telephone

Direct conversion (or zero intermediate-frequency) is an attractive architecture in radio design because it eliminates expensive filters at both radio frequencies (RF) (for image rejection) and intermediate frequencies (IF). Instead, an inexpensive low-pass filter is used for channel selection, and no image rejection is required. A digital code that has negligible energy below 30 kHz is used, so that a combination of ac coupling and dc servo feedback prevents propagation of dc offsets in the IF chain. A class AB servo stage reduces the turn-on time of the receiver to 200 /spl mu/s, so it may be used in a time-division duplex (TDD) system. The typical phase accuracy of the I/Q output is one degree. The IC is designed for use in the 902-928 MHz industrial-scientific-medical (ISM) Band. It operates on a supply voltage of 2.7-5.0 V and draws 50 mA. It uses a 25 GHz f/sub T/ silicon-bipolar process.