Bird Technologies 4391A - Manual (Page 16)

4
Elements, Analog Circuitry, and A/D Converter
These components are controlled by the computer. The Plug-in Elements in the
line section provide low level positive voltages related to the instantaneous
value of power (see Figure 3). The first group of solid state switches selects the
forward element, the reflected element, or ground as the input to the preampli-
fier which boosts these signals to 0.1 to 2.0 volt range. The remaining switches
shown as two groups direct the output of the preamp to the analog-to-digital
converter either directly or through a peak or negative peak detector. The ana-
log-to-digital converter converts the voltage to a 15 digit binary number.
Each reading output by the display is derived from up to three voltage readings
using the circuitry described above. Once these voltages are measured, all remain-
ing operations are performed within the computer chip as follows:
The voltages are corrected for error due to dc drift in the analog circuitry. Each
voltage is converted to square root of power using stored data tables. These values
are then combined mathematically to arrive at the final result in binary. This is
used to update the registers containing the last value, the maximum value, or the
minimum value as required. Finally, the result is converted to a decimal number
and placed into a register from which the display driving routine operates.
Figure 3 Plug-In Element Schematic Diagram
The coupling circuit that samples the travelling waves is in the Plug-in Element.
The circuitry of the element and its relationship to the other components of the
Thruline Watt-meter are illustrated in the schematic diagram. Energy will be pro-
duced in the coupling circuit of the element by both mutual inductance and
capacitance from the travelling RF waves of the line section. The inductive cur-
rents will flow according to the direction of the travelling waves producing them.
The capacitive portion of these currents is independent of the direction of the
travelling waves. Therefore, assuming that the Plug-in Element remains station-