Filter - Keithley 595 Instruction Manual

OPERATION
3.4.1 Power Line Frequency
The line frequency program is used to set the Model 595
to the available line frequency (either 50 or 6OHz). This pro-
grams is accessed as part of the calibration procedure and
is discussed in Section 7. During normal operation this front
panel program will not appear in the MENU.
3.42 Calibration
An advanced feature of the Model 595 is digital calibration.
The instrument can be calibrated from the front panel or
over the IEEE-488 bus. To use front panel or IEEE-488 bus
calibration, refer to the calibration procedures outlined in
Section Z During normal operation this front panel pro-
gram will not appear in the MENU.
3.4.3 Filter
The Model 595 has three digita filters that can be used to
minimize the effects of noise on individual measurements
and plotted curves. To use the ftiter program, press the
MENU button until "FlLt" appears~ on the display. Next to
"FILt" is a. number. If the number is zero, then the filter
is off, -and the actual unmodified curve will be plotted. To
advance to a higher level filter, press the voltage sourc$ A
ADJUST button. To decrease a filter level, press the~voltage
source V ADJUST button. To exit from the program, press
SIGT and then EXIT.
The suitability of each of these for "curve smoothing"
should be determined by considering the number of
measurements cr samples in the curve; specifically, the fun-
damental portion to be studied (region of interest).
Filter 1 is used when there are 20 or more samples in the
fundamental change area of the curve. This filter provides
minimal smoothing of the curve. Filter 2 should be used
when there are 50 or more samples in the fundamental por-
tion of the curve for maximum curve smoothing. Lastly,
filter 3 is available to smooth slowly changing DC signals
(or curves with more than 200 samples in the fundamen-
tal portion). This filter should normally only be used with
DC signals since it would distort most CV or lV curves.
Figure 3-5, shows an example of proper and improper use
of a filter on a cmve with 60 readings in the region of in-
terest. Notice the distortion of the curve using Filter 3.
An important characteristic of each filter is its "window".
Each filter takes the weighted average of a certain number
of readings. This average is symmetrical about the middle
reading. Filter 0 (when filter is "off'? has a one reading win-
dow, Filter 1 has a three reading window, Filter 2 has a nine
reading window, and Filter 3
has
a 24 reading window.
The weightings (oi coefficients) assigned to each reading
are as follows:
Level 1: 114, 112, 114
Level 2: -1132, l/32,1/8, 15i64, 9i32, W64, I@, 1132, -1132
Level 3: l/32, l/32,1132, l/32, 1132, 1132, 1132, 1132, l/16, 1116,
1116, 1116, l/16,1/16, 1116, 1116, l/32,1/32, 1132, 'JJ32,
1132, l/32, 1132, 1:32.
Figure 3~-6 illustrates~ the coefficients of Filter 2.
3-8