RF and Microwave Circuit Design

8RF Filters

8.1 Introduction

Filters are essential circuits in all RF and microwave systems. Their primary function is to control which frequencies can propagate through a particular part of a system. Signals should be transmitted within the desired pass-band of the filter with minimum loss, and with minimum change to the phase relationships between the frequency components passing through the filter. Whilst filters are relatively straightforward circuits, it is important that they are designed correctly to avoid unwanted transmission impairments.

This chapter will introduce the concepts of RF filter design using lumped components. It will subsequently be shown how the lumped designs can be extended into distributed formats, and how modern fabrication processes permit the design of compact, high-quality planar filters.

8.2 Review of Filter Responses

There are four common types of filters, indicated by the idealized amplitude-frequency responses in Figure 8.1, which show the insertion loss (L) in dB as a function of frequency (f). The four types are known as low-pass, high-pass, band-pass, and band-stop filters.

In practice, there are six key aspects of filter responses which must be addressed when selecting and designing filters:

1. The shape of the amplitude-frequency response within the pass-band.
2. The low insertion loss of low-pass and band-pass filters within their pass-bands.
3. The high insertion loss of band-stop and high-pass filters within their stop bands.
4. The phase-frequency ...