Page 310 - Analog and Digital Filter Design

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CHAPTER

TRANSMISSION LINES AND PRINTED

CIRCUIT BOARDS AS FILTERS

This chapter describes how transmission lines and printed circuits boards can
be used to produce filters. Both of these topics are wide-ranging, and it will not
be possible to provide more than an introduction here. The references provided
should allow the interested reader to pursue the subject further.

Transmission lines can be used to filter signals. Quarter-wavelength lines of
either short- or open-circuit termination can be used to pass some frequencies
while stopping others. One application of this is to allow a radio carrier signa!
into a receiver from an antenna while preventing internal signals, from the
receiver, from radiating back to the antenna. Connecting a short-circuit quarter-
wavelength line across the antenna input will short circuit low-frequency signals
but not interfere with signals at the quarter-wavelength frequency.

Transmission lines of less than a quarter wavelength at the passband cutoff
frequency can be used to replace inductors and capacitors. The design process
starts by producing a conventional lumped element filter design. Short-circuit
lines then replace inductors and open-circuit lines replace capacitors. Each of
these short- and open-circuit lines is a quarter wavelength long at the stopband
frequency.

Transmission lines can be produced on a printed circuit board (PCB) as tracks.
This is only significant when the signal frequency is high, so that the track length
is about 2/20 or longer. A short-circuit line produces inductors, but this is
difiicult to produce on a PCB. A special mathematical transformation of the
transmission line design is needed to overcome this problem. After transforma-
tion, an open-circuit line combined with a matching series quarter-wavelength
line replaces the short-circuit line.

Printed circuit board LC filters will also be described. This type of filter is not
the same as the quarter-wavelength line filter. All sections of PCB filters have