Electrically Tunable Narrow-Bandwidth SAW Filter

Abstract

SAW voltage-controlled oscillators are very easily implemented using a conventional hybrid junction and varactors. Taking advantage of this, a similar device may be designed for use as an electrically tunable filter. The device consists of a SAW delay line w i t h moderate insertion loss and closed through a low-noise amplifier i n order t o maintain the loop losses t o a few dB. In addition, a phase-shifter is inserted in the loop t o match the overall phase. The input is typically the input of the amplifier, while the output may be similarly the output of the amplifier, for l o w loss, or an auxiliary SAW transducer designed t o compensate for the frequency response o f the delay line. This is a very agile narrow bandwidth f i l t e r w i t h the capability of meeting the requirements of the future vehicular communication system w i t h frequency around 900 MHZ.This type of tunable SAW f i l t e r has been f i r s t implemented at about 200 M H Z on lithium tantalate and experimental results are reported. I -IntroductionThe development of future vehicular communicat i o n system requires the use of agile narrow bandwidth filters : the bandwidth is typically a few tens of kilohertz but the carrier frequency is tentatively fixed around 900 MHz. The conventional solution is t o use a frequency conversion t o obtain a fixed intermediate frequency and then the main problem consists in tuning very rapidly (in a few milliseconds) the frequency of the down frequency converter. In addition, since many adjacent channels (typically 1000) are available the second drastic problem is t o secure a good immunity against intermodulation. In this paper we suggest t o design an electrically tunable narrow-bandwidth in-frontfilter, directly at the carrier frequency. This obviates both shorthcomings of the conventional solution. I1 -Principle of electrically tunable f i l t e rThe device is built as a tunable SAW oscillator [1,2] (cf. Fig. 1). A SAW delay line is inserted in a closed loop together w i t h an amplifier and a phase shifter. If the gain of the amplifier increases up t o compensate for the loss, the device is indeed an oscillator operating at a frequency w0 defined by where 0 is the auxiliary p%ase shift, T the delay of the SAW line and k is an integer. F r o m the wellknown design rules of such an oscillator, the transducer lengths are chosen i n order t o have a single mode operation. If, next, the gain o f the amplifier decreases leading t o a linear closed loop gain r less than unity, the transfer @I+ U T = 2 k n AV, 1 T AV2 Fig.1. : Schematic o f the electrically tunable f i Zt e r . function f r o m the input t o the output is roughly : 1 3 1 -2 1 + 2jQ (-) 0 0 2 k n -@ T where Q =a=& and W O = (k is the effective number of wavelengths in the delayline accounting for both electrical and mechanical reflections). The voltage-controlled phase-shifter PS is conveniently realized using a quadrature hybrid junction and varactors : this leads t o a very agile tuning of the filter, especially at the high frequency of o...

New architectures for feedthrough SAW recursive devices

“…An example of a SAW device used in a superregenerative amplifier was described by Ash [15]; this device is associated with the feedback architecture that is analyzed below. Positive feedback reappeared as a useful option in recursive or infinite-impulse response digital filters in which it provided much improved selectivity compared to the finite-impulse response counterparts [16]. Due to the delays involved in signal processing, it became possible to keep poles of the transfer function (3) out of the self-excitation region.…”

“…The derivation is presented in [16], [24], controllable by changing k design for the SAW device is acceptable, the concept could be applied with some additional benefits. First, the electronic summer can be avoided by using a feedback IDT and arranging for the output IDT to sum SAWs generated by both transmitting IDTs [22].…”

“…However, most of these devices merely combine features of the acoustic and electronic pathways without adding much extra value. A more promising approach that uses feedback to control the frequency response of the SAW device was introduced by Henaff and Feldman [16]. Interaction between the acoustic and electronic pathways became essential for their recursive device to achieve the objective of a controllable frequency response.…”