NF FRA51602, Инструкция по эксплуатации

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4.7 High-frequency measurements
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FRA 51602
4.7 High-frequency measurements
At frequencies above several hundreds of kilohertz, the effect of signal cables on measurement error tends
to increase with the frequency. The main points of caution for measuring at high frequencies are explained
in this section.
a) Auto-range
In the default settings, CH1 and CH2 are both set to auto-range. This setting simplifies measurement
because the measurement range is switched automatically according to the magnitude of the signal, but
error becomes large at high frequencies. For that reason, small steps (discontinuities) may appear in the
measurement data before and after range switching.
The problem of discontinuities in measurement results can be solved by performing the measurement in
a fixed range but, that greatly reduces the dynamic range of the measurement. Also, if a signal that
exceeds the range is input, correct results cannot be obtained because of excessive signal level. When
measuring with a fixed range, consider these constraints when determining the measurement range.
b) Using the probe
When measuring at high frequencies, the static capacitance between wires of the signal cable may act as
a load on the circuit being measured, greatly affecting the measurement results and the operation of the
circuit being measured. When the cables are long (several percent of the wavelength of the measurement
signal), The influence of signal reflection due to impedance mismatch surfaces and measurement error
increases. In such cases, it is recommended to use a suitable probe to reduce measurement error.
For example, a 10:1 passive probe intended for use with an oscilloscope can be used. Use a probe that
has an impedance of 1 MΩ on the oscilloscope side in the correction range and a maximum capacitance
correction range of at least 25 pF. Adjust the trimmer capacitor of the probe so that the frequency
characteristic is flat before measurement. Specifically, set the analysis mode to the channel the probe is
connected to (CH1 or CH2) and perform a sweep measurement over the range from about 10 Hz to 100
kHz while measuring the OSC output signal with the probe and adjust the trimmer capacitor so that the
difference in gain between the low region and the high region is minimum.
Using a 10:1 passive probe attenuates the signal level to 1/10, but it is possible to compensate for that
approximately by setting a suitable input weighting. For more information on input weighting, refer to
"
5.9.2 Input weighting
".
The equalizing function can be used for more accurate correction of probe errors. For more information
on the equalizing function, refer to "
5.9.3 Equalizing
".