Section 3: Functions and features
Model 2651A High Power System SourceMeter® Instrument Reference Manual
3-4
2651A-901-01 Rev. A / March 2011
The moving average filter uses a first-in, first-out stack. When the stack (filter count) becomes full, the
measurement conversions are averaged, yielding a reading. For each subsequent conversion placed
into the stack, the oldest conversion is discarded. The stack is averaged again, yielding a new
reading.
The median filter is used to pass the “middle-most” reading from a group of readings that are
arranged according to size. The median filter uses a first-in, first-out stack similar to the moving
average filter. For each subsequent conversion placed into the stack, the oldest conversion is
discarded. The median is then redetermined.
When a moving filter is first enabled, the stack is empty. The first reading conversion is placed in the
stack and is then copied to the other stack locations in order to fill it. Thus, the first filtered reading is
the same as the first reading conversion. The normal moving filter process continues. Note that a true
average or median reading is not yielded until the stack is filled with new reading conversions (no
copies in stack). For example, in the figure below, it takes ten filtered readings to fill the stack with
new reading conversions. The first nine filtered readings are calculated using copied reading
conversions.
Figure 33: Median filter
Conversion
#1
#1
#1
Middle
value
reading
#1
Middle
value
reading
#2
Middle
value
reading
#3
A. Type: Median, readings = 3
Conversion
#2
#1
#1
Conversion
#3
#2
#1
Front panel filter control
Enabling filter
The filter is enabled by pressing the
FILTER
key. The FILT indicator is on while the filter is enabled.
Pressing
FILTER
a second time disables filter.
Response time
The filter parameters have speed and accuracy trade-offs for the time needed to display, store, or
output a filtered reading. These affect the number of reading conversions for speed versus accuracy
and response to input signal changes.
The filter type and count affect the overall reading speed. The moving averaging filter is much faster
than the repeat averaging filter because the unit does not have to refill the filter stack for each
reading. Also, the number of readings averaged affects reading speed; as the number of readings
averaged increases, the reading speed decreases.
Remote filter programming
Configuring the filter
Filter type and count are configured from the filter configuration menu. The same filter configuration is
used for all measurement functions.
