(1) Tracking filters are used in some injection units to filter the synthesizer's input signal frequency
spectrum to improve the signal-to-noise ratio. The tracking filter is basically a voltage-controlled
oscillator that has a phase-lock-loop circuit. The output of the voltage-controlled oscillator is
locked to the input frequency through the action of the phase-lock-loop circuit.
(2) Basically, the oscillator-synchronizer functions in the same manner as the tracking filter. The
main difference is that the input signal is compared with a reference signal, but there is no
oscillator output signal. Instead of the conventional oscillator output, there is an output error
signal which is used to control another signal source (usually a klystron oscillator).
(3) Multipliers are used in some injection units to raise the frequencies to the values required for
efficient operation in the translator unit.
3-19.
TRANSLATOR
The translator provides the necessary mixing action required to prepare the modulated signal for
transmission. The translator unit is composed of isolators, buffer amplifiers, filters, mixers, and attenuators (fig.
109).
3-20.
TRANSLATOR OPERATION
a. Each isolator allows the energy that is propagated in a forward direction to pass through with
negligible opposition, but energy that is propagated in a reverse direction is shunted to a dissipating element
which effectively absorbs this reverse or backward-wave energy.
b. The overall translating process uses three mixing stages. If a single mixing stage is used with the 70-
MHz modulated signal mixing with the final-oscillator-injection voltage, two disadvantages are apparent. The
final oscillator-injection voltage would have to approach the final transmitted frequency. This is not too
important in itself, but it creates a major problem in that both the upper and lower sideband frequencies are very
close to the carrier frequency. Any attempt to reduce the amplitude of the undesired lower sideband without
affecting the upper sideband is difficult. The use of multiple mixers provides easier suppression of the undesired
sideband and other spurious signals.
(1) The first translator mixes the 70-MHz modulated signal with the 400-MHz injection frequency to
produce the output of 470 MHz. The filter removes the lower sideband from the output signal.
(2) The 470-MHz output of the first translator is mixed with the 1,800-MHz injection frequency to
produce an upper sideband of 2,270 MHz (lower sideband removed by filter). This output signal
is then mixed with the final injection frequency of 5.005 gigahertz (GHz) to 6.105 GHz to
produce the final modulated signal. This signal is in the range of 7.25 to 8.4 GHz.
344 L3
114
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