Bird Technologies 3129 Display - Manual (Page 23)

7
Chapter 2 Theory of Operation
General Theory
The enhanced Broadcast Power Monitor (BPM-E) is an instrument that is
inserted directly into a transmission line to monitor the forward power, reflected
power, and VSWR of the system. The BPM-E consists of a line section with for-
ward and reflected directional couplers and an additional housing that contains
the external circuitry. The external circuitry consists of a detector circuit and a
control circuit (Figure 3, page 7).
The directional couplers sample, through a known coupling factor, the forward
and reflected wave in the transmission line. The coupled RF signal is passed to the
detector board and converted into a dc voltage. The dc voltage, which is propor-
tional to the RF power in the line seciton, is then sent to the control circuit.
In the control circuit, an analog-to-digital converter digitizes the incoming signal
and passes it to a micro controller that manages input and output processing.
The control circuitry comminicates by sending signals to the alarm and power
LEDs, receiving a user initiated signal from the reset button, and both sending
and receiving signals via the RS-232 connector (DB9), the ethernet connector
(RJ45), and the power/alarm connector (DB15).
Factors that Affect Power Reading Accuracy
Many sources can affect the accuracy of any in-line power reading. These
sources include directivity of the directional couplers, noise in the system, and
the insertion loss of various system components. To more fully understand how
these and other sources cause the power readings to be out of specification,
consult with a Bird Applications Engineer.
Figure 3 BPM-E Functional Block Diagram
Detector Board
Control
Board
Alarm
LED
Power
LED
Reset
Button
DB-9
Connector
DB-15
Connector
Rj45
Connector
and Xport
Ethernet
Controller Forward
Coupler Reflected
Coupler
RF IN RF OUT Line Section