Agilent Technologies 35670-90066 manual DC-Offset Tables and Frequency Correction Curves

Internal Test Descriptions	Agilent 35670A
Calibration Routine Description

DC-Offset Tables and Frequency Correction Curves

The dc-offset calibration builds 5 dc-offset tables — one for each anti-alias filter and one for each channel when the anti-alias filters are bypassed. The values in the dc-offset tables are sent to the channel dc-offset DACs to compensate for dc offsets introduced by analog input circuits. Forty values are entered in each table — a value for each range setting from –51 dB to +27 dB, in 2 dB increments. For each range, the table value is derived by changing the offset values of the dc offset DAC until the best possible offset compensation is found. In all instrument modes, the analyzer corrects for dc offsets by setting each channel’s dc-offset DAC to the value from the table that matches the current anti-alias filter and range setting.

The frequency calibration generates correction curves in the frequency domain to compensate for unflatness in the analog input circuits. A precise signal is connected from the source to the input channels via the calibration path (CALP). Correction curves are then produced for each range setting by taking the difference between the source output and the measured response. In FFT analysis, correlation analysis, and swept sine instrument modes, the analyzer multiplies the measured result with the value from the frequency correction curve that matches the current range and span setting. In this way, errors introduced by circuits in the analyzer are removed before the measurement is displayed.

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