Page 334 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
8

Support Circuit Design

Most of today’s support circuits are so useful that a modern transmitter or
receiver could not satisfactorily function without them. Support circuits
include electronic switches, attenuators, frequency multipliers, automatic
gain control, the power supply, for example. Other circuits can only be called
“bells and whistles.” They are not essential for proper systems operation and
are only present for added operator convenience; these circuits will not be
discussed.

8.1 Frequency Multipliers

8.1.1 Introduction
Because sinusoidal crystal oscillators can rarely be designed to operate reli-
ably at frequencies above 200 MHz, even on a crystal’s overtone, frequency
multipliers, or sometimes SAW oscillators, must be employed for this purpose.
The frequency doublers and triplers used in FM transmitters to increase a sig-
nal’s frequency, as well as in microwave local oscillator stages, are either a
basic tuned-output nonlinear (Class B or C) amplifier or a diode multiplier cir-
cuit. These multipliers are not only able to increase an FM or CW signal’s fre-
quency, but also any FM deviation present. This is required in an FM
transmitter system, as a carrier oscillator’s frequency and its deviation may
need to be multiplied by 30 or more times. For instance, a modulated RF car-
rier that began at 6 MHz, with an FM deviation of 150 Hz, could be altered, if
fed through a 30 chain of multipliers, into an output frequency of 180 MHz,
with an FM deviation of 4500 Hz.
Because of their nonlinear nature, common Class C amplifiers (Fig. 8.1)
work quite well as frequency multipliers, especially when run into their satu-
ration curve. Since any distortion of a continuous wave produces harmonics of
the fundamental, an output of such an amplifier/multiplier can be rich in har-
monics. The input tank is tuned to the fundamental frequency that must be

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