Basic Theory of Operation
6
Analog Mode of
Operation
Receiving When the radio is receiving (see Figure 2), the signal comes from the antenna
connector to the radio board, passes through the RX/TX antenna switch and into
the receiver front end. A 15dB step attenuator is included in the front end to provide
additional protection against strong signals. The attenuator is controlled by a DSP
based algorithm that continuously monitors signal strength. When the ON
threshold is exceeded (approximately -95dBm), the attenuator is activated via a
DAC in the PCIC by the host. The attenuator remains activated until the signal
drops below the OFF threshold (approximately -115dBm including the 15dB
attenuation). Hysterisis and timer functions are included in the algorithm to
enhance performance. The algorithm controlling the attenuator is enabled via the
CPS for each personality. When the algorithm is disabled, the attenuator is
essentially a short circuit from input to output.The signal is then filtered, amplified,
Figure 1. XTS 2500 Overall Block Diagram
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