The "brain" of the analyzer is Atmel 8-bit RISC microcontroller. It controls a AD9958 DDS chip running at 500 MHz and generating two sinusoidal signals. Both signals are low-pass filtered and then put through pulse shapers, producing two rectangular signals with LVDS levels. 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. After the switch, the signal is mixed with the second channel output to produce audio frequency of 2 kHz. A variable attenuator is used to maintain the audio signal level in the desired range. This signal is then filtered and fed through the 16-bit ADC to 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 (AA-600 and AA-1000)	0.1 ... 200 MHz	0.1 ... 200 MHz	DDS1 + 2 kHz	1	/25
2 (AA-600 and AA-1000)	200 ... 600 MHz	67 ... 200 MHz	DDS1 + 667 Hz	3	/3
3 (AA-1000 only)	600 ... 1000 MHz	120 ... 200 MHz	DDS1 + 400 Hz	5	/1

The 320x240 color TFT display and the 6x3 keys keypad are connected directly to the CPU. The analyzer is equipped with 128K of external Flash memory to store measurement results, as well as with 32K of the fast external FRAM. The USB interface chip allows connecting the analyzer to the PC.
 

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.