Page 155 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

Chapter 11

Inductor-Iess Circuits

In previous chapters there was extensive use of coils, inductors, or transformers for
radiofrequency (RF) and intermediate-frequency (IF) signals. This chapter presents
alternatives to coils or inductors.
Ceramic Filters

Ceramic filters emulate high-Q inductor/capacitor (LC) circuits as band-pass filters.
These filters generaUy are three-terminal devices with input, ground, and output
terminals. However, ceramic filters also are made as two-terminal devices.

The earliest use of ceramic filters for amplitude-modulated (AM) transistor radios
can be found in the 1960s. For example, the Motorola Model XP3CE and Model
XP4CE 9- and 11-transistor radios from 1965 to 1967 each used a two-terminal
ceramic filter in the first IF amplifier. Normally, the first IF amplifier has an RF

bypass capaCitor from the emitter to ground. In these Motorola radios, the RF
bypass capaCitor is replaced with a two-terminal ceramic filter, which acts like a
series resonant 455-kHz tank circuit. Thus at 455 kHz, the two-terminal ceramic
filter has a minimum impedance or resistance, and at frequencies outside 455-kHz
band, such as 470 kHz, the ceramic filter has a high impedance. By rep'lacing the
RF bypass capaCitor with the two-terminal ceramic filter at the emitter of the
common-emitter IF amplifier, the output current from the IF amplifier is peaked at

the IF frequency and lowered at frequencies outside the IF band.
Two-terminal ceramic filters should not be confused with the very common
two-terminal ceramic resonators. A ceramic resonator has a very narrow bandwidth
that generally is not suitable for an IF filter used in a standard AM radio. However,
because ceramic resonators have very narrow bands, they can be used as a

continuous-wave (CW) IF filter for receiving Morse code. Ceramic resonators
generally are used for generating frequencies such as seen in one of the generator
circuits in Chapter 4. For this chapter, we will concentrate on (wider-bandwidth)
ceramic filters instead because resonators exhibit very narrow band characteristics
that are not suitable for broadcast AM receivers.
Today, two-terminal ceramic filters are not that readily available, but three-terminal

ceramic filters are found commonly. Three-terminal ceramic filters for the AM band
could be found in stereo receivers in the 1970s, such as the 1973 Sony STR-222.
As the cost of these three-terminal devices dropped, they were very commonly
designed into portable or pocket radios by the middle to late 1970s or early 19805.

For example, in 1981, the Sony ICF-200 pocket radio had three-terminal ceramic
4S5-kHz and 10.7-MHz filters for AM and frequency-modulated (FM) bands.