How it works?


Structure diagram

The structure diagram of the AA-500 Antenna Analyzer is located below:




The "brain" of the analyzer is Atmel ATMEGA128 8-bit RISC microcontroller. It controls two PLL synthesizer chips which produce signals in 1.2-1.4 and 1.4-1.7 GHz range. The signals are mixed to form a  signal of lower frequency of 1 to 500 MHz, which is low pass filtered and amplified.  Resistive brigde was chosed to measure parameters of a load because of its simplicity and good frequency response. The switch commutates two outputs of the bridge. The detector, AD8307, measures differential power level at its two inputs. The resulting signal is then fed into the CPU 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.
 

Close look at the bridge

This is a diagram of the resistive bridge and its connection to the detector:



The detector chip measures signal levels in three of four possible positions of the switches which are controlled by the CPU. First, voltage at the unknown load is measured. Then voltage at the 50 Ω resistor and offset voltage is taken. When the load is totally active and its resistance is 50 Ω, the bridge is balanced and there is no offset voltage. This eliminates the need of calibration of the analyzer, also giving very good precision near SWR=1.

The output of the detector chip is connected to the analog-to-digital converters which are built into the microcontroller.




 
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