A book for engineers who design and build filters of all types, including lumped element, coaxial, helical, dielectric resonator, stripline and microstrip types. A thorough review of classic and modern filter design techniques, containing extensive practical design information of passband characteristics, topologies and transformations, component effects and matching. An excellent text for the design and construction of microstrip filters.
The book reviews developments in the following fields: network fundamentals; reactors and resonators; transformations; filter losses; computer-aided strategies; lowpass structures; bandpass structures; highpass structures; bandstop structures; and PWB manufacturing.
This chapter is included for the novice. It provides a brief historical perspective
and a review of very basic analog, high-frequency, electronic filter terminology.
For this section, we assume that networks are linear and time invariant. Time invariant
signifies that the network is constant with time. Linear signifies the output is a
linear function of the input. Doubling the input driving function doubles the resultant
output. The network may be uniquely defined by a set of linear equations relating
port voltages and currents.
The distributed resonators we have previously envisioned consist of a resonant line
of uniform impedance. Later we will investigate loading a transmission line with a
lumped or distributed reactance to resonate a line which is less than a self-resonant
length. In this section we investigate the properties of distributed resonators formed
by cascading two lines with a different characteristic impedance. Electrical resonance
is achieved with a shorter physical length.
As we have seen, designing L-C lowpass filters from the lowpass prototype involves
only scaling of the resistance and cutoff frequency of the prototype by simple multiplication
and division. The transfer characteristics of the prototype are exactly retained and
no special realization difficulties are introduced. The design of other structures,
such as highpass, bandpass and bandstop filters, require transformation in addition
to the scaling. These transformations naturally modify the attributes of the prototype
and may introduce severe realization difficulties, especially for bandpass and bandstop
structures. The 'ideal' transformation does not exist, and it becomes necessary to
consider alternative transformations and how they relate to specific filter requirements
and applications. This chapter considers a number of network transformations and equivalences.
The ideal filter transfers all incident energy at passband frequencies to the filter
output termination. In practice, energy is lost by reflection at the filter ports,
dissipation within the filter and/or radiation from the filter. These topics are considered
in this chapter.
More than easing computational burdens, the digital computer has revolutionized the
way we design filters. Any modern treatment of filter design must address strategies
which have become practical including real-time tuning, statistical analysis, sensitivity
analysis, design centering and optimization. It is now feasible to optimize for desired
and customized characteristics while simultaneously considering component losses,
parasitics and discontinuities. Many filter synthesis theories which we use today
were developed in an age when computing tools were far less sophisticated. Wonderfully
elegant mathematical solutions were found for a variety of filter problems, but idealized
assumptions were required to make the process manageable. Today, these idealized symbolic
theories form a starting point which is followed by brute force numeric techniques.
The L-C lowpass is a direct application of synthesized prototypes and poses the fewest
implementation difficulties of all filter structures. Ideally the same would be true
for the distributed lowpass because the synthesis is based on the conversion of L-C
filters. Also there is the potential for tighter tolerance on element values. However,
difficulties are introduced by the unique characteristics of distributed elements
such as reentrance, discontinuities and the realizable range of line impedance. In
this chapter, distributed lowpass filters are studied. The effects of these limitations
are considered along with potential methods of mitigating these difficulties.
The introduction of a fractional bandwidth parameter for bandpass filters significantly
impacts performance and realizability. Over the years, a number of unique distributed
bandpass structures have been developed which provide the best possible performance
for certain characteristics at the expense of others. There is no one best solution
for all applications. The designer who attempts to apply a favorite structure to all
problems will not have the success of those who learn to match filter structures and
required specifications. Therefore, this chapter is a study of a range of distributed
bandpass structures and the advantages and disadvantages of each. We close with a
powerful technique for taming the tricky process of tuning bandpass filters of all
types.
This chapter describes a hybrid lumped-distributed highpass filter structure. Highpass
filters require series capacitors which are difficult to realize in distributed form.
The hybrid highpass uses distributed stubs and series lumped capacitors.
When the rejection of a signal is required, it is natural to think in terms of a notch.
Just as a true bandpass filter offers improved selectivity over a single resonator,
the bandstop filter offers improved rejection over a simple notch or even a cascade
of notches. However, the general realization difficulties of distributed structures
are worsened by particular difficulties associated with the bandstop structure.
Discusses PWB manufacturing. Requirements can be divided into two classes: prototype
and production quantities. Prototype boards have traditionally been constructed using
a photographic etching process, although milling is becoming popular for single-layer
prototypes due to the fast turn-around time and reduced setup costs. Production quantities
are seldom milled due to a large processing time per board and are more often etched
or deposited.
This appendix defines equation variables (symbols) used throughout the book. These
variables are listed in this appendix in alphabetical order. This appendix also briefly
describes =SuperStar= circuit file model codes and variables written by the =M/FILTER=
program to define physical dimensions of distributed filter structures. Model codes
are organized by process and by function. Circuit file dimensional variables are listed
in alphabetical order.