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Theory of Operation—492/492P Service Vol. 1 (SN B030000 & up)
« 8 2 0 p F
Fig. 5-5. Amplifier signal path.
829 MHz 1st IF signals from the 829 MHz Amplifier, en
ters the converter through an 829 MHz bandpass filter. The
filter blocks unwanted inputs, primarily the 609 MHz image
signal. A 1.3 GHz printed element lowpass filter blocks high
frequency signals that would otherwise be admitted at the
re-entrant frequencies of the bandpass in excess of 2 GHz.
The function of the 1.3 GHz lowpass filter is shared by the
1.2 GHz lowpass filter located on the 829 MHz Diplexer
board. A 3 dB attenuator on the 829 MHz Amplifier board
and one following the 1.3 GHz lowpass filter help ensure
consistent 50 Ω interfaces for the 829 MHz bandpass filter.
The 829 Mhz bandpass filter is composed of four quar-
ter-wave, coaxial-type resonators mounted on the 829 MHz
2nd Converter board. The end resonators are tapped near
their grounded end to facilitate the filter's input and output
coupling. Inter-resonator coupling is provided by printed
"through-the-board” capacitors that connect between the
resonators at their high-impedance end. A bendable tab is
located at the high-impedance end of each resonator for fine
adjustment of resonant frequency. The bendable tab acts as
a small, variable capacitance from the end of the resonator
to ground, making fine adjustments of resonant frequency
possible. When properly tuned, the filter presents an input
return loss of at least 12 dB at 829 MHz and an insertion
loss of about 2 dB.
Fig. 5-6. Am plifier signal path.
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2 7 2 7 - 1 0 6
REV AUG 1981
5-15
