User Manual

SECTION 1: THEORY OF OPERATION
~\Technical Documentation\System Manuals\FCC-Reports\DR4B-FCCRpt.doc Page 7
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example, if a fault in the base station site produces a contact closure on one of the alarm inputs, the radio
transmits a fault message to the Communication Center. Examples of faults include site door open, high
temperature, high VSWR, etc. Upon receipt of the fault message, a technician is dispatched to the base
station site to correct the fault. In order to obtain these capabilities, monitoring software must be loaded
into the System Controller Board CPU and the Internet Protocol Network Controller (IPNC).
Another header (J1) routes RS232 data, clock, and handshake signals from the System Controller Board
to DB25 interface connector (J3). This connector is physically mounted on the rear panel of the base
station.
This section displays the level shifters for the three (3) receivers. Each receiver provides a DC voltage
received signal strength indication (RSSI) corresponds to the signal strength of an incoming signal. The
RSSI output from receiver 1 appears at RSSI1. It passes through low pass filter R13 and C16. The low
pass filter capacitor (C16) minimizes RF pickup by the op amp. An op amp (U11B) provides amplification
and a DC level shift. An additional op amp (U11C) provides additional amplification. Resistors (R3 and
R4) and a pot (R11) set gain of the op amp. The DC offset is injected into U11C via a pot (R12) and a
resistor (R5). The amplified and level shifted output leaves the op amp as SHIFT_ RSSI1 and goes to a
comparator circuit.
The remaining level shifters work in the same manner.
Individual gain and DC offset pots are provided so that minor RSSI performance differences between the
three receivers can be trimmed out.
Receive Signal Strength Indication Comparator
This section displays the RSSI comparator circuitry. It uses a two-step selection process to determine
which receiver has the strongest signal, as follows:
 The receiver 1 and receiver 2 RSSIs are compared, and the strongest receiver is selected.
 The selected receiver’s RSSI and receiver 3 RSSI are compared, and the strongest is selected. The
circuitry provides two (2) digital outputs and an analog RSSI voltage from the selected receiver.
A comparator (U5) looks at the RSSI1 and RSSI2 inputs. When RSSI1 is greater than RSSI2, the
comparator output goes high, and a LED (D1) lights to indicate receiver 1 was selected. At the same
time, the RX1/RX2_SELECT line goes high, which activates the solid-state switch (U8). This causes the
receiver 1 RSSI signal to be routed to a second comparator. Otherwise, if the receiver 2 RSSI is greater
than receiver 1, the RX1/RX2_SELECT line goes low, and another LED (D2) lights to indicate receiver 2
was selected. U8 simply routes receiver 2 RSSI to a second comparator (U10). The digital
RX1/RX2_SELECT output controls a switch.
U10 looks at the selected receiver (RSSI1/RSSI2) and RSSI3 inputs. Should RSSI3 be greater than the
other input, the comparator output goes high, and LED (D3) lights to show receiver 3 is selected. At the
same time, another switch (U6) connects RSSI3 to the analog SEL_RSSI output. The digital
RX3_SELECT output controls a switch. Otherwise, if the selected receiver’s RSSI is greater than RSSI3,
the comparator output remains low making the RX3_SELECT output low. U6 simply connects the
selected receiver’s analog RSSI signal to the SELRSSI output.
A chip (U3) forms a comparator circuit. When the RSSI voltage exceeds a threshold, a LED (D4) lights.
Like the other three (3) LEDs, this circuit is intended as a diagnostic tool. It provides a go/no go indication
that an RF signal has been received. A pot (R74) sets the turn-on voltage.