China Electronics Technology Instruments AV3672 Series, User Manual

Page: 220 / 424

7 Calibration
7.5 Measurement Errors
216
7.5 Measurement Errors
Understanding of the source of measurement error and the way of error correction can help to improve the
measurement accuracy, as a certain degree of uncertainty still exits no matter how carefully the measurement is
done. The following kinds of errors may be produced in measurement with the analyzer.
7.5.1 Drift Errors
➢
Drift errors are produced by performance changes of the instrument or test system after calibration.
➢
The thermal expansion of the interconnected cables in the instrument and the performance changes of
microwave mixer are main causes of the drift error. The drift errors can be eliminated by recalibration.
➢
The duration of accurate calibration depends on the test environment. The stable environment can help to
minimize the drift error.
7.5.2 Random Errors
The random error cannot be predicted or eliminated by means of calibration. The influence on measurement results
can be reduced by some methods. Random errors are mainly divided into three types:
1) Random error of instrument
a) Electrical disturbance of internal components of the analyzer may result in random noise, mainly including:
➢
Low-level noise arising from the base noise within the receiver broadband;
➢
High-level noise or data trace jittering, which is mainly caused by the base noise inside the instrument and
phase noise of the LO source.
b) The random noise error can be reduced in the following methods:
➢
Increase the source power input into the DUT;
➢
Reduce the intermediate frequency bandwidth;
➢
Apply the function of sweep averaging.
2) Switch repeatability error
The switch is used to change attenuator settings of the source. If the switch acts, contact closing may be different
from previous closing sometimes. In this case, the measurement accuracy will be affected seriously. Therefore, the
attenuator settings should not be changed in high-accuracy measurement so as to reduce the switch repeatability
error.
3) Connector repeatability error
Connector wear may result in electrical performance changes. Proper connection maintenance can help to reduce
the connector repeatability error.
7.5.3 System Errors
System errors are caused by non-ideal hardware properties and may be repeatable (therefore, it can be predicted). It
is assumed that system errors do not change over time. System errors can be determined by calibration and
eliminated by mathematical calculation in measurement.
System errors cannot be fully eliminated. Due to limitations of the calibration process, some residual errors may
exist after calibration, mainly resulting from:
➢
Non-ideal calibration standard;
➢
Connector connection;
➢
Interconnected cable;
➢
Instrument.
All measurements are affected by the dynamic accuracy and frequency errors. For reflection measurement, relevant
residual errors include: