Operator`s manual

35 Signametrics
4.11.2 Analog Threshold Trigger
sets a threshold value and arms the
and captures up to 64 readings,
There are several software trigger functions. They can commend the to make a predefined number of readings, with
a specified number of settling readings. These include DMMSetBuffTrigRead, DMMSetTrigRead,
DMMTrigger, DMMBurstRead and DMMBurstBuffRead. Read about these functions in the Windows
Command Language section (5.6) for details.
4.12 Frequency and Timing Measurements (SM2042, SM2044)
While the maximum RMS reading is limited to the set range, you can use most of the timing functions even if the
RMS voltage reading indicates over range. This is true as long as the input peak-to-peak value does not exceed 5.75
times the selected range (5.75 x 330 mV = 1.9 V p-p with the 330 mV range).
4.12.1 Threshold DAC
All timing measurements utilize the AC Voltage path, which is AC coupled. You need to select the appropriate
ACV range prior to using the various frequency and timing measurement functions. The SM2042 and SM2044 have
a novel feature to accurately make these measurements for all waveforms. Unlike symmetrical waveforms such as a
sine wave and square wave, non-symmetrical waves may produce a non-zero DC bias at the frequency counter’s
comparator input. Other DMMs have the comparator hard-wired to the zero crossing, and therefore cannot handle
asymmetrical wave such as a very low duty cycle signal. The SM2042 and SM2044 have a bipolar, variable
Threshold DAC that enables these DMMs to performance of these measurements. Functions affected by the
cle and the Totalizer/Event Counter.
he Threshold DAC has 12 bits of resolution. Depending on the selected ACV range, this bipolar DAC can be set
from tions sections for the limits of AC
bed
e
This mode triggers the DMM at a specific input level. A command to the DMM
MM Analog trigger. The DMM’s local controller waits for the level crossing
D
which are saved on board at the current DMM measurement function, range and rate. The reading rate must be set to
10 rps or higher. You can abort this mode by sending the DMM a Disarm command to the Analog Trigger.
4.11.3 Software Issued Triggered Operations
Threshold DAC include frequency, period, pulse-width, duty-cy
T
a few mV to several hundred volts, positive or negative. See the Specifica
Median Value measurements and Threshold DAC settings.
The best setting of the Threshold DAC is based on the AC Median Value and Peak-to-Peak measurement descri
earlier. For example 5 V logic signal with 10% duty cycle. This input has a median value of 2 V. A 90% duty cycl
signal will have a –2 V median value. Setting the Threshold DAC to the appropriate median value will result in
reliable and accurate timing measurements in each case.