2-49
Figure 2-45B.—Nondegenerative parametric amplifier. ELECTRICAL EQUIVALENT.
The pump signal causes the capacitor in figure 2-45A to vary at a 12-gigahertz rate. The 3-gigahertz
input signal enters via a four-port ferrite circulator, is developed in the signal cavity, and applied across
the varactor. The nonlinear action of the varactor produces a 9-gigahertz difference frequency (fp-fs) with
an energy-level higher than the original input signal.
The difference (idler) frequency is reapplied to the varactor to increase the gain and to produce an
output signal of the correct frequency. The 9-gigahertz idler frequency recombines with the 12-gigahertz
pump signal and produces a 3-gigahertz difference signal that has a much larger amplitude than the
original 3-gigahertz input signal. The amplified signal is sent to the ferrite circulator for transfer to the
next stage.
As with tunnel-diode amplifiers, the circulator improves stability by preventing reflection of the
signal back into the amplifier. Reflections would be amplified and cause uncontrollable oscillations. The
ferrite circulator also serves as an isolator to prevent source and load impedance changes from affecting
gain.
Typically, the gain of a parametric amplifier is about 20 dB. The gain can be controlled with a
variable attenuator that changes the amount of pump power applied to the varactor.
Parametric amplifiers are relatively simple in construction. The only component is a varactor diode
placed in an arrangement of cavities and waveguides. The most elaborate feature of the amplifier is the
mechanical tuning mechanism. Figure 2-46 illustrates an actual parametric amplifier.
