10-2
September 27, 2000
68P81093C75-O
Power Amplifier
Module Operation
The PDR 3500 uses the mid-power Spectra RF power amplifier (RF
PA) board, housed in a heavy-duty, shielded, aluminum heat sink.
The gain of the RF PA board is controlled by the power control
voltage from the Exciter Module.
The modulated RF signal is amplified by the RF PA and sent to the
site transmit antenna.
The transmit path inside the RF PA contains a directional coupler
(calibrated at setup), which feeds a DC voltage (proportional to the
output power) to the TX Power Control Circuitry in the Exciter
Module. This signal serves as the feedback signal in the power
control loop.
Receiver Circuitry
Introduction
The Receiver Circuitry performs the following functions:
• Accepts receive RF signals from the site receive antenna
• Filters and dual conversion
• Outputs a digitized receive signal to the Station Control Module.
Receiver Module
Operation
The receive signal is generated from the site receive antenna to a
multi-pole, preselector filter which provides highly selective
bandpass filtering.
The filtered signal is then amplified and fed to the RF input of the 1st
mixer.
The 1st mixer combines the filtered signal with an injection signal
generated by the Synthesizer/VCO. This results in a 21.45 MHz
(VHF), or 73.35 MHz (UHF) 1st IF (intermediate frequency) signal.
(The injection signal is determined by frequency programming data
from the Station Control Module via the SPI bus.)
The 21.45 or 73.35 MHz 1st IF is filtered and routed to a custom
receiver IC. This component contains the following circuitry:
• 2nd injection and mixing
• Amplification
• A/D (analog to digital) conversion
This results in a digitized receive signal. This signal is routed to the
Station Control Module as differential data.
