Page 300 - Analog and Digital Filter Design

Selecting Components for Analog Filters 297

terminals. When terminated, the output voltage will be half the generator’s
EMF. This is because the source impedance and the load form a potential
divider and, since they are equal in value, the output voltage must be half the
EMF. The other half of the voltage is dropped across the source impedance.
High-frequency passive filters are often tested in conjunction with a spectrum
analyzer and tracking generator. As the spectrum is scanned across the fre-
quency range set by the operator, the tracking generator generates a sine wave
at the same frequency. Connecting an RF filter between the tracking generator
and the spectrum analyzer allows the filter’s transfer function to be displayed
on the screen. If a tracking generator is not available, a white noise source will
perform the required function, although at a lower signal level. Many analyzers
have optional plotters to allow zi hard copy of the response to be made.

Reference

1. Winder, Steve. “The Real Choice for Active Filters.” Electronics Worid
mid Wireless World, Sept 93: pp. 758-760.

Exercises

10.1 Why are surface-mount capacitors preferred for hgh-frequency
circuits?

10.2 Why are ferrite pot-cores popular for making inductors? Why are
air-gaps sometimes used between the two halves?
10.3 Which type of resistor is preferred for radio frequency circuits? Why
are some types avoided?
10.4 A sixth-order Butterworth lowpass filter is to be built using operationai
amplifiers (op-amps). The filter requires a cutoff frequency of 20kHz.
What minimum gain-bandwidth product should the op-amps have?