SAW/FET Programmable Transversal Filter with 100-MHz Bandwidth and Enhanced Programmability

The authors previously reported on a SAW/FET filter that uses air-gap coupling of a LiNbO/sub 3/ SAW delay line to a Si integrated circuit, thus implementing a programmable tapped delay line. The device incorporates edge-bonded transducers for wide bandwidth and short (15 ns) pedestal of delay. Modifications to the Si circuit are reported here, which both increase the programmable RF bandwidth to 100 MHz and provide means of increasing the programmable tap on/off ratio to 30 dB. This filter has 350 metal-oxide-silicon (MOS) analog sampling fingers spanning 1/5 microsec. of delay. The varactor inherent in each sampling finger provides programmable tap weighting; a digital shift register controls programming FETs that connect each MOS varactor tap to a common programming voltage line. The 100-MHz bandwidth results from doubling the number of taps by the addition of a 2-to-1 analog multiplexer to the FET array.