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band width (200
μ
s); The other has narrow bandwidth (including 6.7
μ
s). The bandwidth
of ACV path in digital multimeter is 300 kHz. Therefore, frequency components above 300
kHz cannot be measured.
Note that the spectrum sine (
π
fT)/
π
fT of the narrow pulse is clearly beyond the effective
bandwidth of the instrument. Therefore, the accuracy of the final measurement results of
the narrow high-frequency pulse is low.
In contrast, the spectrum of the wide pulses is significantly reduced to the 300kHz
(approximate) bandwidth of the multimeter, so the measurement of such pulses is more
accurate.
Reducing PRF can increase the line density in the Fourier spectrum and increase the
proportion of the input signal spectrum energy in the bandwidth of the multimeter, thus
improving the accuracy.
In summary, when there is a large input signal energy at frequencies above the bandwidth
of the multimeter, the error in the RMS measurement will increase.
Capacitance
As shown in the figure below, the multimeter measures capacitance by applying a known
current to charge the capacitor and then discharging the capacitor by applying a resistor:
