Agilent Technologies 4294A Specifications download pdf (Page 53)

SECTION 4

Measurement error and compensation

4-1. Measurement error

For real-world measurements, we have to assume that the measurement result always contains

some error. Some typical error sources are:

• Instrument inaccuracies (including DC bias inaccuracy and OSC level inaccuracy)

• Residuals in the test fixture and cables

• Noise

The DUT’s parasitics were not listed because the DUT’s parasitics are a part of the DUT and we need

to measure the DUT’s impedance including its parasitics. In the listed error sources, residuals in

the test fixture and test cables can be compensated if they are constant and stable.

4-2. Calibration and compensation

Calibration is to define the “calibration plane”, at which the specified measurement accuracy can be

obtained. To calibrate an instrument, “standard devices” are connected at the calibration plane and

the instrument is adjusted (through computation/data storage) so that it measures within its speci-

fied accuracy. In the case of 4TP configured instruments, the calibration plane is at the UNKNOWN

BNC connectors (Figure 4-1).

Compensation reduces the effects of the error sources existing between the DUT and the instru-

ment’s calibration plane. Compensation, however, can not always completely remove the error, and

the measurement accuracy obtained after compensation is not as good as that obtained at the “cali-

bration plane.” Compensation is not the same as calibration and can not replace calibration, the

measurements required for compensation depend on the calibration accuracy of the instrument, so

compensation must be performed after calibration has been completed. Compensation improves the

effective measurement accuracy of an instrument. The following paragraphs describe three com-

monly used compensation techniques.

Figure 4-1. Calibration plane of 4TP instrument

4-1

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Agilent Technologies 4294A Specifications Page 53