18.
In a backward-wave oscillator, amplification occurs when
a.
bunching occurs in the beam.
b.
the signal on the helix reaches the gun end of the tube.
c.
the cathode voltage exceeds the voltage on the accelerating anode.
d.
the velocity of the signal on the helix is greater than that of the electrons in the beam.
19.
In both the traveling-wave tube and the 0-type backward-wave oscillator, the axial velocity of the RF
signal is reduced by using a helix. In a cross-field oscillator (carcinotron) the same function is performed
by the
a.
sole of the tube.
c.
matching termination.
b.
accelerating anode.
d.
interdigital delay line.
20.
The primary reason for using a parametric amplifier as the preamplifier in a microwave receiver is that
the parametric amplifier has a
a.
wide bandpass.
c.
low-power requirement.
b.
low-noise figure.
d.
high-resonant frequency.
21.
The parametric amplifiers used in communication receivers are either cooled or refrigerated. The purpose
of this temperature reduction is to
a.
overcome the atmospheric noise.
b.
reduce the resistance of the amplifiers.
c.
eliminate the pump-frequency requirements.
d.
reduce the noise figures of the amplifiers.
22.
a.
c.
b.
d.
23.
If the incoming signal frequency to the parametric amplifier shown in figure 29 is 8 GHz and the pump
frequency is 32 GHz, what frequency is applied and utilized by the VHF-UHF converter stage?
a.
8 GHz
c.
40 GHz
b.
32 GHz
d.
48 GHz
344 L1
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