Telecom

Low loss quasitransversal microwave filters with specified amplitude and phase characteristics

F. Huang

Indexing Terms: Microwawfilters, Phase, Amplitude, Chirp devices

Abstract: A microwave filter synthesis procedure for almost arbitrary phase and amplitude responses is described. Although based on a passive transversal filter implementation, it contains an efficient numerical algorithm to allow for losses in the delay line and multiple reflections between taps; this permits large amounts of energy to be drawn from the delay line so that the filter can have reasonably low loss. The performance of chirp filters and a phase shifter implemented in copper stripline and microstrip are presented as a demonstration. A fractional bandwidth of 150% has been achieved for the chirp; this is about double that produced by previous design methods. It is intended that eventual implementations will include superconducting linear phase filters, chirp devices and amplitude or phase equalisers.

1

Introduction

ference leads to a smaller response. This is a bandpass characteristic. An alternative implementation employs a

I

I

matched loads

---

Many filters are designed to specified pass and stopband amplitude characteristics, because the phase characteristics are unimportant. In others, both amplitude and phase are specified. Examples are chirp (deliberately dispersive) filters, linear phase filters and amplitude and phase equalisers. Devices based on previous synthesis procedures may require multilayer patterning [13. A simpler implementation has been pioneered elsewhere for superconducting filters [2], but only for chirps whose impulse response is a sine wave with slowly varying frequency and amplitude. This superconducting filter is implemented as a pair of coupled striplines, shown in Fig. 1, slightly modified as part of the present work. The input wave propagates along one line towards a matched load which absorbs any energy that reaches it, At intervals...