f. The sum frequency can be referred to as the upper sideband, and the difference frequency as the
lower sideband. Amplification takes place at the signal frequency and at both sideband frequencies. The
relationship between the pump and input signal frequencies determines the relative amplifications produced at the
resulting frequencies.
g. One frequently used parametric amplifier is the up-converter amplifier. In the up-converter, so
named because the output is taken at a higher frequency than the input signal frequency, the pump frequency is
many times the signal frequency. Consequently, both the upper and lower sidebands are much higher than the
signal frequency, and most of the amplification takes place in the sidebands. Either the upper or lower sideband
in the up-converter can be used.
h. Figure 29 is a block diagram of a typical up-converter parametric amplifier using the upper sideband.
The signal arriving at the antenna is designated fs. It is mixed with the pump frequency fp in the up-converter, and
the amplified output frequency is then at a frequency fs plus fp. This is then mixed in a conventional crystal mixer
with the pump frequency, and the difference frequency, fs, is selected and passed on to a converter. The converter
heterodynes the signal down to a frequency where a conventional communication receiver can process the signal.
344 L1
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