New low-distortion band-switching techniques for SAW antenna duplexers used in ultra-wide-band cellular phone

A new low-distortion band-switching technique for a surface-acoustic-wave (SAW) antenna duplexer used in ultra-wide-band cellular-phone systems has been developed. Both a transmitter (Tx) portion and a receiver (Rx) portion of the duplexer consist of not only wide-band SAW bandpass filters (BPFs), but also band-rejection SAW filters (BRFs) whose rejection bands can be shifted by switching circuits. The spurious-response sensitivity caused by the third-order nonlinear distortion of the switching circuits was dramatically improved by the new above configurations, i.e., the combination of BPFs and BRFs. The developed duplexer for the 900-MHz Japanese cdmaOne system has a size of 10 * 8 * 2 mm. It also has frequency characteristics of equivalent transmitted/received frequency bands as wide as 38 MHz and the guard band as narrow as 17 MHz. The insertion losses in both the low and high bands of the Tx portion are less than 2 dB, and those of the Rx portion are less than 3.3 dB. The other required specifications, e.g., attenuation at harmonic frequencies, attenuation at the image frequency, etc. are also satisfied.