Q ENHANCEMENT
4A) ENTER FILTER FREQUENCY
When the fO • Q product required for a pole-pair section is
above ≈100kHz at frequencies above ≈3kHz, op amp gain-
bandwidth limitations can cause Q errors and gain peaking.
To mitigate this effect, the program automatically compen-
sates for the expected error by decreasing the design-Q
according to a Q-compensation algorithm(1). When this
occurs, the value under the Q heading on the UAF42 Filter
Component Values print-out will be marked with an asterisk
indicating that it is the theoretical Q, not the actual design
Q. The actual design Q will be shown under an added
Move the cursor to the Filter Frequency line in the Param-
eters section.
Low-pass/high-pass filter: enter the f–3dB or cutoff frequency.
Band-pass filter: enter the center frequency, fCENTER
.
Band-reject (notch) filter: enter the notch frequency, fNOTCH
.
If your filter is low-pass or high-pass, go to step 5.
4B) ENTER FILTER BANDWIDTH
heading labeled QCOMP
.
If the filter is a band-pass or band-reject (notch), move the
cursor to the bandwidth line and enter bandwidth.
USING THE FilterPro™ PROGRAM
If you press <ENTER> with no entry on the bandwidth line,
you can enter fL and fH instead of bandwidth. fL and fH are
the f–3dB points with regard to the center frequency for
Butterworth and Bessel filters. They are the end of the
ripple-band for Chebyshev types. This method of entry may
force a change in center frequency or notch frequency.
With each data entry, the program automatically calculates
filter performance. This allows you to use a “what if”
spreadsheet-type design approach. For example; you can
quickly determine, by trial and error, how many poles are
needed for a desired roll-off.
5) PRINT-OUT COMPONENT VALUES
GETTING STARTED
Press function key <F4> to print-out Filter Component
Values and a Filter Block Diagram. Follow the instructions
in the filter implementation section of this bulletin to as-
semble a working filter.
The first time you use the program, you may want to follow
these suggested steps.
Type FILTER42 <ENTER> to start the program.
Use the arrow keys to move the cursor to the Filter
Response section.
USING THE PLOT FEATURE
A Plot feature allows you to view graphical results of filter
gain and phase vs frequency. This feature is useful for
comparing filter types.
1) SELECT FILTER RESPONSE
Press <ENTER> to toggle through four response choices:
Low-pass
High-pass
To view a plot of the current filter design, press <F2>.
Band-pass
GRAPHIC DISPLAY COMMANDS
Notch (band-reject)
While viewing the graphic display, several commands can
be used to compare filter responses:
When the desired response appears, move the cursor to the
Filter Type section.
<F1> or S—Saves the plot of the current design for future
recall.
<F2> or R—Recalls the Saved plot and plots it along with
the current design.
2) SELECT FILTER TYPE
Move the cursor to the desired filter type and press
<ENTER>. The selected filter type is highlighted and marked
with an asterisk. There are four filter-type choices:
<F3> or Z—Plots a Zero dB reference line.
GRAPHIC DISPLAY CURSOR CONTROL
Butterworth
Chebyshev
Bessel
Inverse Chebyshev
While viewing the graphics display you can also use the
arrow keys to move a cursor and view gain and phase for
plotted filter responses.
If you choose Chebyshev, you must also enter ripple (i.e.
pass-band ripple—see Chebyshev filter description).
If you choose Inverse Chebyshev, you must also enter AMIN
(i.e. min attenuation or max gain in stop-band—see Inverse
Chebyshev filter description).
RESISTOR VALUES
With each data entry, the program automatically calculates
resistor values. If external capacitors are needed, the pro-
gram selects standard capacitor values and calculates exact
resistor values for the filter you have selected. The 1%
Resistors option in the Display menu can be used to calcu-
late the closest standard 1% resistor values instead of exact
resistor values. To use this feature, move the cursor to the
resistors line in the Filter Response section and press
3) ENTER FILTER ORDER
Move the cursor to the Filter Order line in the Parameters
section. Enter filter order n (from 2 to 10).
(1) L.P. Huelsman and P. E. Allen, Theory and Design of Active
Filters, p. 241.
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