Model 2601B-PULSE System SourceMeter Instrument Reference Manual Section 7: Theory of operation
2601B-PULSE-901-01A April 2020 7-5
Optimizing apertures and filters for the 1 MS/s sample rate
When the pulser is enabled, a measurement request or measurement trigger generates a burst of
readings. Each burst is equal to the measure count times the filter count. A burst is limited to a
maximum of 3500 readings. The maximum is determined by the size of the hardware buffer that
stores data before it is processed and stored in the reading buffer.
Once the burst is acquired, data needs to move from the hardware buffer into the reading buffer.
When the instrument generates fast pulses or fast measurements, it can get into a situation where it
cannot keep up with both the pulses and the measurements. If the instrument cannot maintain the
pulse rate, an error is generated that indicates that the pulse data rate was exceeded. If
measurements cannot be moved out of the hardware buffer before they fill up, an error is generated
that indicates that the measurement rate was exceeded. If you are trying to generate fast pulses at a
high measurement rate, either of these errors can occur.
Apertures and filters
To make a measurement with a given aperture, multiple conversions are averaged together to make
one reading.
The filter for the pulser is a repeating-average filter and is applied when the reading moves from the
hardware buffer to the reading buffer. The instrument waits to process the final reading until the
number of readings specified by the filter count are made.
For example, if the aperture is 10 µs and the filter count is five, 10 conversions from the 1 MS/s ADC
are averaged and sent to the hardware buffer as one ADC reading. This collection of 10 conversions
per aperture happens five times. When five readings have occurred, they are averaged by the filter
and one averaged measurement is stored in the reading buffer as a final measurement. In this
example, the final measurement contains 50 µs of averaged measurement data.
You can achieve the same results if you set the aperture to 50 µs and set the filter count to 1, or if you
set the aperture to 1 µs with a filter count of 50. The only difference in the three settings is the number
of entries in the burst and how much of the hardware buffer space is used:
• Aperture = 1, filter = 50: The burst count is 50
• Aperture = 10, filter = 5: The burst count is 5
• Aperture = 50, filter = 1: the burst count is 1
Any one of these puts one reading into the reading buffer. Using the aperture setting instead of the
filter setting can reduce the amount of space in the hardware buffers needed for a burst.
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