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RigExpert AA-54 – How it works?

Structure diagram

The structure diagram of the RigExpert AA-30/AA-54 Antenna Analyzer is located below:

The “brain” of the analyzer is Atmel 8-bit RISC microcontroller. It controls two AD9833 DDS chips running at 25 MHz and generating two sinusoidal signals. Both signals are low-pass filtered and then put through pulse shapers, producing two square signals with CMOS levels. Resistive bridge was chosen to measure parameters of a load because of its simplicity and good frequency response. The switch commutates two outputs of the bridge. After the switch, the signal is mixed with the second channel output to produce audio frequency of 1.7 kHz. A variable attenuator is used to maintain the audio signal level in the desired range. This signal is then filtered and fed into the 10-bit ADC of the microcontroller. There is a simple relationship between the measurement frequency, DDS frequencies and attenuator division ratio:

Subband Measurement
frequency
DDS1
frequency
DDS2
frequency
Harmonic
number
Attenuator
ratio
1 0.1 … 10 MHz 0.1 … 10 MHz DDS1 + 1.744 kHz 1 /25
2 10 … 30 MHz 3.333 … 10 MHz DDS1 + 581 Hz 3 /3
3 (AA-54 only) 30 … 54 MHz 6 … 10.8 MHz DDS1 + 349 Hz 5 /1

The 128×64 graphical LCD and the 6×3 keys keypad are connected directly to the CPU. Also, the AA-54 is equipped with 128K of external Flash memory to store measurement results. The USB interface chip allows connecting the analyzer to the PC.

Close look at the bridge

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

The switch which is controlled by the CPU feeds the signal from the certain side of the bridge to the mixer 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.

See also: Data exchange with RigExpert antenna analyzers