The switch which is controlled by the
CPU feeds the signal from the certain side of the bridge to the
detector chip. When the load is totally active and its resistance is 50
Ω, the bridge is balanced and the switch outputs the same
both positions. This eliminates the need of calibration of the
analyzer, also giving very good precision near SWR=1.
The detector compares the output of the switch with a reference
signal. This avoids "dead zones" in the phase characteristics of the
AD8302 chip. The outputs of the chip are connected to the
analog-to-digital converters which are built into the microcontroller.
For the good performance at high frequencies (above 50 MHz) is it
important to built almost an ideal, symmetrical bridge. However, the
load is plugged into the analyzer through the connector (non-ideal
electronic part), which slightly unbalances the bridge. To compensate
parameters of the connector, a simple model was chosen:
The microcontroller takes into account
tiny capacitance and inductance of the connector, correcting the result
of each measurement.