Bird Technologies 3171B020 - Manual (Page 19)

7
Chapter 2 Theory of Operation
Input Signal
Direct current signals representing operating power levels are brought into the RF monitor from the sensing ele-
ments through the rear panel jacks labeled RF Power Input Forward and Reflected. These currents are shunted to
ground through 1400 ohm resistors. The voltage developed across these resistors is amplified by a pair of opera-
tional amplifiers to approximately one volt (meter full scale).
The drive current to the main indicating meters is taken from the amplifier outputs through series resistors of 33.2k
ohm each. The drive voltage for remote meters also comes from the amplifier outputs and is buffered by a pair of
op-amps configured for unity gain. This protects the 3170B/71B circuitry against accidental shorting of the remote
meter line.
Adjustment
The calibrate/operate switch and the two reference potentiometer allow the user to substitute an adjustable cur-
rent in place of the signals from the sensing elements to facilitate adjustment of the set-points.
The three set-point potentiometer (forward, reflected, and confirm) provide DC voltages which are compared to the
amplifier outputs by three comparators.
The LED to the left of each potentiometer indicates whether the incoming signal is higher (LED ON) or lower (LED OFF)
than the set-point of the potentiometer.
Operations
The output of the confirm comparator goes directly to the confirm output drive transistors. The output of the
reflected comparator goes directly to the alarm circuitry and to the reflector monitor tripped LED latch. The output
of the forward comparator is gated by the activate forward monitor signal AFM before going to the alarm circuitry
and LED latch.
In other words, the signal is blocked if the activate forward monitor signal is not present. The AFM signal passes
through a delay circuit before reaching the gate. The purpose of the delay is to allow for any delays in the user’s equip-
ment between the initial excitation of the carrier ON signal and the presence of power on the transmission line.
Reset
The reset signal clears the horn latch and the two trip LED latches. This signal can come from three sources: the push
button switch on the front panel, the rear panel input/output, or from the power-up reset capacitor when the
device is first turned on or after a power interruption.