Figure 27. M-type backward-wave oscillator (carcinotron).
f. The M-type backward oscillator is most unusual in that it can be amplitude- or frequency-modulated.
To amplitude-modulate this tube the accelerator voltage is modulated, and to frequency-modulate the tube, the
sole voltage is modulated. The tube can also be amplitude- and frequency-modulated simultaneously.
Section VIII. PARAMETRIC AMPLIFIERS
1-30.
INTRODUCTION
a. The gain of a device can be controlled by the use of a source (pump) frequency to control the device's
inductive or capacitive parameter. Devices that produce signal gain through the control of the inductive or
capacitive parameter are called parametric amplifiers. Instead of using dc power, as does a conventional-type
amplifier, the parametric amplifier uses ac power to build up signal power. This principle is not new. Increased
attention has been given to it in recent years as a result of the advanced developments of solid-state diodes.
b. With the long trunks used for communications, there is always considerable attenuation in the
transmission path. The parametric amplifier allows us to make up for a weak signal by using an extremely low
noise amplifier as the receiver preamplifier. A receiver with a parametric preamplifier may have a noise figure 12
decibels (db) below that of a conventional receiver. This 12-db improvement is equivalent to having increased
transmitter power 16 times to achieve the same improvement in carrier signal-to-noise ratio.
344 L1
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