The "brain" of the analyzer is Atmel ATMEGA128 8-bit RISC microcontroller. It controls the dual-channel AD9958 DDS running at 500 MHz and generating two sinusoidal signals with 90° shift. Both signals are low-pass filtered. The first channel is fed to the amplifier to get the +10 dBm signal at the antenna connector. Resistive brigde was chosen to measure parameters of a load because of its simplicity and good frequency response. The switch commutates two outputs of the bridge. The detector, AD8302, measures the manitude ratio and the phase difference at its two inputs. The resulting signals are then fed into the 16-bit analog-to-digital converter for further processing.

The 128x64 graphical LCD and the 6x3 keys keypad are connected directly to the CPU. Also, it is equipped with 512K of external Flash memory to store measurement results. The USB interface chip allows connecting the analyzer to the PC.

The power circuits are responsible for generating a number of voltage levels needed for the rest of the analyzer.  It also charges the built-in battery when the external 12V DC source is applied.
 

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 signal in 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 90° 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.


Connector issue

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: