Varactor-devices-cont.

In the figure below, view A, is a diagram of a typical nondegenerative parametric amplifier with the equivalent circuit shown in view B. The pump signal (fp) is applied to the varactor. The cavity on the left is resonant at the input frequency (fs), and the cavity on the right is resonant at the difference frequency (fp-fs).

The difference frequency is called the IDLER- or LOWER-SIDEBAND frequency. The varactor is located at the high-voltage points of the two cavities and is reverse biased by a small battery. The pump signal varies the bias above and below the fixed-bias level.

View A: Non-degenerative parametric amplifier CIRCUIT.

View B: Non-degenerative parametric amplifier ELECTRICAL-EQUIVALENT.

The pump signal causes the capacitor in the figure above, view A, 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. The next figure below illustrates an actual parametric amplifier.

Parametric amplifier.

PARAMETRIC FREQUENCY CONVERTERS

Parametric frequency converters, using varactors, are of three basic types. The UPPER-SIDEBAND PARAMETRIC UP-CONVERTER produces an output frequency that is the SUM of the input frequency and the pump frequency. The LOWER-SIDEBAND PARAMETRIC DOWN-CONVERTER produces an output frequency that is the DIFFERENCE between the pump frequency and the input frequency. The DOUBLE-SIDEBAND PARAMETRIC UP-CONVERTER produces an output in which both the SUM and the DIFFERENCE of the pump and input frequencies are available.

Parametric frequency converters are very similar to parametric amplifiers in both construction and operation. The figure below is a functional diagram of a parametric down-converter.

The parametric frequency converter operates in the same manner as the parametric amplifier except that the sideband frequencies are not reapplied to the varactor. Therefore, the output is one or both of the sideband frequencies a

nd is not the same as the input frequency. The output frequency is determined by the cavity used as an output. For example, the idler cavity in the figure below could be replaced by a cavity that is resonant at the upper-sideband frequency (22 gigahertz) to produce an upper-sideband parametric up-converter. Since input and output signals are at different frequencies, the parametric frequency converter does not require a ferrite circulator. However, a ferrite isolator is used to isolate the converter from changes in source impedance.

Lower sideband parametric down converter.

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