Operator`s manual

35 Signametrics
Consideration must be given to the selected Aperture. This is particularly important at signal frequencies
lower than 100Hz. Two error sources are suppresssed using the right Aperture, the RMS converter low
frequency cutoff and signal aliasing with the Aperture. At these lower frequencies make sure to set the
Aperture to a value that is at least ten (10) times the period of the measured signal.
4.1.3 AC Peak-to-Peak and Crest Factor (SMU2064)
Measurement of Peak-to-Peak, Crest Factor and AC Median values requires a repetitive waveform
between 30 Hz and 100 kHz. The DMM must be in AC voltage measurement mode, with the appropriate
range selected. Knowing the Peak-to-Peak value of the waveform is useful for setting the Threshold
DAC (described below). This latter function is a composite function, and may take over 10 seconds to
perform.
4.1.4 AC Median Value Measurement (SMU2064)
To better understand the usage of this function, you should note that the DMM makes all AC voltage
measurements through an internal DC blocking capacitor. The voltage is thus “AC coupled” to the
DMM. The measurement of the Median value of the AC voltage is a DC measurement performed on the
AC coupled input signal. This measurement returns the mid-point between the positive and negative peak
of the waveform. The Median value is used for setting the comparator threshold level for best counter
sensitivity and noise immunity. (It is difficult to measure the frequency of a low duty cycle, low
amplitude AC signals since there is DC shift at the comparator input due to the internal AC coupling. The
SMU2064 overcome this problem by allowing you to set the comparator threshold level). For further
information on the usage of AC Median value and Peak-to-Peak measurements, and the Threshold DAC,
see the “Frequency and Timing Measurements” section below.
This function requires a repetitive signal. The DMM must be in AC voltage measurement mode, with the
appropriate range selected.
4.1.5 Average AC Voltage Measurement (2064)
To make average AC voltage measurement, the 2064 model DMM should be set to DC voltage
measurement, and the appropriate range be selected. This is followed by executing the
DMMGetAverageVAC() command, which returns the average value of the input voltage.
Average AC voltage is the mean of the rectified voltage over one period of the waveform. For a
sinusoidal waveform V
RMS
= 0.707Vpk and V
AVG
= 0.637Vpk. It is necessary to enter the frequency of the
signal while using DMMGetAverageVAC(). If it is not known, use the DMM’s frequency counter to
measure it prior to performing this measurement. The frequency range of this measurement is from 0.5Hz
to 1kHz. It is important to select the appropriate DC voltage range. For instance, a sinewave with 2V
RMS value has a peak voltage of 2.828V, and therefore the 24V range mus be selected.
The more abrupt the signal, the less stable the measurement will be. Therefore a measurement of a square
wave will be noisier than that of a sine.
This function requires a repetitive signal. Connect the input signal between the V+ and the V- terminals.
4.1.6 Low frequency RMS Voltage Measurement (2064)
A special function is provided to make RMS voltage measurements at low frequencies. To use it, set the
2064 model DMM to DC voltage measurement, and select the appropriate range for the intended input
voltage. Follow this with the executing the DMMGetLowFreqVRMS() command, which returns the
RMS value of the input voltage.
It is necessary to enter the frequency of the signal while using DMMGetLowFreqVRMS (). If it is not
known, use the DMM’s frequency counter to measure it. The frequency range of this measurement is
from 0.1Hz to 66Hz. It is important to select the appropriate DC voltage range. For instance, a sinewave
with 2V RMS value has a peak value of 2.828V, and therefore the 24V range is required.