Datron PRC1099A, Техническое руководство

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2: System Overview
PRC1099A-MS 2-3
2.4 Receive Path
2.4.1 Antenna Tuner Board Receive Path
The receive path extends from the antenna to a handset or headset connected
to one of the Audio connectors on the front panel. The input receive signal is
received at the antenna and is then applied to the Antenna Tuner board that
matches the radio output impedance to the antenna to achieve the best RF
power transfer.
2.4.2 Audio/Filter Board Receive Path 1
From the Antenna Tuner board, the receive signal is applied to the
Audio/Filter board where it passes through one of six 5-pole, elliptic-function
filters, and then through a 1600 kHz high-pass filter to prevent overload from
FM broadcast stations. The receive signal is then routed to the Mixer board.
2.4.3 Mixer Board Receive Path
On the Mixer board, the receive signal is filtered using a 30 MHz low-pass
filter to prevent VHF responses and then applied to a high level,
double-balanced mixer where it is mixed with the first LO (LO1) from the
DDS on the Synthesizer board to produce a 75 MHz IF signal. The 75 MHz
output is amplified using a high dynamic range amplifier before passing
through the 4-pole, monolithic 75 MHz filter.
The 75 MHz filter maintains the +11 dBm intercept point through to the
output. Output from the 75 MHz filter is amplified by an AGC-controlled,
dual gate MOSFET amplifier and is then down-converted in a dual gate
MOSFET mixer to the second LO (LO2) to 1650 kHz and output to the
1650 kHz IF board.
2.4.4 1650 kHz IF Board Receive Path
On the 1650 kHz IF board, the receive signal is applied to a 6-pole crystal
sideband filter and then amplified by an AGC-controlled dual-gate MOSFET
amplifier. A fixed-gain bipolar amplifier provides the final IF amplification.
From the 1650 kHz IF board, the receive signal is routed back to the
Audio/Filter board.
2.4.5 Audio/Filter Board Receive Path 2
Back on the Audio/Filter board, the receive signal is applied to the product
detector that removes the 1650 kHz IF frequency leaving just the audio signal.
From the product detector, the audio split into two paths; one passes through
the squelch circuit where it is processed and applied to a pulse counter that
detects the low-frequency FM component in human speech. This output
controls the squelch which opens when speech is detected and closed for
background noise.