A CMOS IC for RF programmable surface acoustic wave filter

Abstract

A 1 /spl mu/m CMOS integrated circuit has been designed and fabricated for an RF programmable surface acoustic wave filter. It has sampling, weighting, controlling and summing functions. There are eight identical cells in the chip. Each cell has a parallel sampling input which can get the RF signal from each tap of the delay line type (TDL) SAW filter or from each terminal in a set of the IDT filter. Two double aluminum layers and one poly-silicon layer sandwich capacitors C1 and C2 are used as the coupling capacitors in series for sampling. A low on-resistance NMOS transistor T1 with high W/L value (500) is used for weighting and summing. Its on-resistance can be changed in a large dynamic range with the variation of an analog weighting voltage (0-5 volt) in its gate. The output signal voltage will be changed by varying its on-resistance to realize the weighting function. All the drains of the T1 transistors in each cell are connected to realize the summing function. The input of the weighting analog voltage is controlled by another NMOS transistor (W/L=100) T2 and an 8 bit shift register. This IC has been fabricated and tested. The chip size is 3 mm /spl times/ 1 mm. A 16.6 dB on/off ratio and a 13.6 dB insertion loss in each bit of the IC is obtained in testing. This IC is connected with a set of multi-IDT SAW devices to realize a programmable SAW filter. A 13.0 dB on/off ratio and 17.3 dB insertion loss in each bit is obtained also in programmable SAW filter circuit. This CMOS IC chip is also used with a LiNbO/sub 3/ SAW chip to form a programmable SAW filter using MCM packaging.