Noty an38f Linear Technology

FilterCAD User’s Manual, Version 1.10

Application Note 38

November 1990

WHAT IS FilterCAD?

FilterCAD is designed to help users without special expertise
in filter design to design good filters with a minimum of
effort. It can also help experienced filter designers achieve
better results by providing the ability to play “what if” with
the values and configuration of various components.

With FilterCAD, you can design any of the four major filter
types (lowpass, highpass, bandpass and notch), with
Butterworth, Chebyshev, Elliptic, or custom-designed
response characteristics. (Bessel filters can be realized
by manually entering pole and Q values, but FilterCAD
cannot synthesize a Bessel response in this version.)
FilterCAD is limited to designs which can be achieved by
cascading state-variable 2nd order sections. FilterCAD
plots amplitude, phase and group-delay graphs, selects
appropriate devices and modes, and calculates resistor
values. Device selection, cascade order and modes can
be edited by the user.

LICENSE AGREEMENT/DISCLAIMER

This copy of FilterCAD is provided as a courtesy to the
customers of Linear Technology Corporation. It is licensed
for use in conjunction with Linear Technology Corporation
products only. The program is not copy protected and you
may make copies of the program as required, provided that
you do not modify the program, and that said copies are
used only with Linear Technology Corporation products.

While we have made every effort to ensure that FilterCAD
operates in the manner described in this manual, we
do not guarantee operation to be error free. Upgrades,
modifications, or repairs to this program will be strictly
at the discretion of Linear Technology Corporation. If you
encounter problems in installing or operating FilterCAD, you
may obtain technical assistance by calling our applications
department at [phone], between 8:00 a.m. and 5:00
p.m. Pacific time, Monday through Friday. Because of the
great variety of operating-system versions, and peripherals

currently in use, we do not guarantee that you will be able
to use FilterCAD successfully on all such systems. If you
are unable to use FilterCAD, Linear Technology Corporation
does guarantee to provide design support for LTC filter
products by whatever means necessary.

Linear Technology Corporation makes no warranty,
either expressed or implied, with respect to the use of
FilterCAD or its documentation. Under no circumstances
will Linear be liable for damages, either direct or conse­quential, arising from the use of this product or from the
inability to use this product, even if we have been informed
in advance of the possibility of such damages.

FilterCAD Download

The FilterCAD tool, although not supported, can be
downloaded at www.linear.com. Locate the downloaded
file on your computer and manually start installation in
that directory.

Your FilterCAD distribution includes the following files. If,
after installing the program, you have difficulty in running
FilterCAD, check to be sure all of the necessary files are
present.

README.DOC (Optional) if present, includes updated

information on FilterCAD not included in
this manual

INSTALL.BAT Automatic installation program—installs

FilterCAD on hard drive

FCAD.EXE Main program file for FilterCAD

FCAD.OVR Overlay file for FilterCAD—used by

FCAD.EXE

FCAD.ENC Encrypted copyright protection file—DO

NOT TOUCH!

FDPF.EXE Device-parameter file editor—used to

update FCAD.DPF (see Appendix 1)

L, LT, LTC, LTM, Linear Technology, the Linear logo, LTspice and FilterCAD are registered
trademarks and QuikEval is a trademark of Linear Technology Corporation. All other trademarks
are the property of their respective owners.

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Application Note 38

FCAD.DPF Device-parameter file—holds data for all

device types supported by FilterCAD

ATT.DRV AT&T graphics adapter driver

CGA.DRV IBM CGA or compatible graphics driver

EGAVGA.DRV EGA and VGA graphics drivers

HERC.DRV Hercules monochrome graphics driver

ID.DRV Identification file for all driver specifica-

tions

Note: Once you have configured FilterCAD and selected
your display type, you can delete unnecessary drivers if
you need to conserve disk space. (Be sure not to delete
any drivers.)

Before You Begin

Please check the FilterCAD program to see if it contains
the README.DOC file. This file, if present, will contain
important information about FilterCAD not included in
this manual. Please read this file before attempting to
install and use FilterCAD. To display the README file on
your screen, type:

TYPE README.DOC [Enter]

Press

The FilterCAD download is at:

http://www.linear.com/designtools/software/#Filter

2. Start the FilterCAD download and open the FilterCAD.zip
to extract the “FilterCADv300.exe.”
“Right Click” on “FilterCAD.exe,”
and select
“Run as an administrator”
then select the following Directory:
C\Program Files\LTC (in a 32-bit system)
or
C\Program Files(86)\LTC (in a 64-bit system).

3. Go to

C\Program Files\LTC (in a 32-bit system)
or
C\Program Files(86)\LTC (in a 64-bit system)
open
“OPEN THIS FOLDER TO INSTALL FilterCAD”
and
“Run” SETUP.exe
then FilterCAD is installed in:
C\Program Files\LTC\FILTERCAD (in a 32-bit system)
or
C\Program Files(86)\LTC\FILTERCAD (in a 64-bit system).

END

[Ctrl] S

to pause scrolling. Press any key to resume scrolling. To
print a hard copy of the README file on your printer type:

TYPE README.DOC>PRN [Enter]

Procedure for FilterCAD Installation in Win7 PC

The FilterCAD installation in Win7 downloads reliably to
a target folder.

1. If an LTC program like LTspice® or QuikEval™ has been
installed then the following directory folder exists:

a. C\Program Files\LTC (in a 32-bit system)

or
b. C\Program Files(86)\LTC (in a 64-bit system).

If not then create a directory folder as in a. or b.

HARDWARE REQUIREMENTS

A list of the graphics adapters and modes supported
by FilterCAD will be found in the Configuration section.
FilterCAD is a calculation-intensive program, and should
therefore, be run on the most powerful system available.

WHAT IS A FILTER?

A filter is a circuit that selectively passes a certain range
of the frequencies present at its input to its output, while
blocking (attenuating) other frequencies. Filters are nor­mally described in terms of the frequencies that they pass.

Most filters conform to one of four common types. Lowpass
filters pass all frequencies below a specified frequency
(called the cutoff frequency) and progressively attenuate

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Application Note 38

Figure 1.1. Lowpass Response

Figure 1.3. Bandpass Response

frequencies above the cutoff frequency. Highpass filters
do exactly the opposite; they pass frequencies above the
cutoff frequency while progressively attenuating frequen­cies below the cutoff frequency. Bandpass filters pass a
band of frequencies around a specified center frequency,
attenuating frequencies above and below. Notch or band­stop filters attenuate the frequencies around the center
frequency, passing frequencies above and below. The
four basic filter types are illustrated in Figures 1.1 to 1.4.
There are also allpass filters, which, not surprisingly, pass

1

all of the frequencies present at their input.

In addition, it
is possible to create filters with more complex responses
which are not easily categorized.

Figure 1.2. Highpass Response

Figure 1.4. Notch Response

The range of frequencies that a filter passes is known, logi­cally enough as its “passband.” The range of frequencies
that a filter attenuates is known as its “stopband.” Between
the passband and stopband is the “transition region.” An
ideal filter might be expected to pass all of the frequen­cies in its passband without modification while infinitely
attenuating frequencies in its stopband. Such a response

Note 1: While allpass filters don’t affect the relative amplitudes of signals
with different frequencies, they do selectively affect the phase of different
frequencies. This characteristic can be used to correct for phase shifts
introduced by other devices, including other types of filters. FilterCAD
cannot synthesize allpass filters.

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Application Note 38

is shown in Figure 1.5. Regrettably, real-world filters do
not meet these imaginary specifications. Different types of
filters have different characteristics, less-than-infinite rates
of attenuation versus frequency in the transition region.
In other words, the amplitude response of a given filter
has a characteristic slope. Frequencies in the passband
may also be modified, either in amplitude (“ripple”) or
in phase. Real-world filters all represent compromises:
steepness of slope, ripple, and phase shift (plus, of course,
cost and size).

FilterCAD permits the design of filters with one of three
response characteristics (plus custom responses).
These three response types, which are known as “But­terworth,” “Chebyshev,” and “Elliptic,” represent three
different compromises among the previously described
characteristics. Butterworth filters (Figure 1.6) have the
optimum flatness in the passband, but have a slope that

0

f

C

rolls off more gradually after the cutoff frequency than
the other two types. Chebyshev filters (Figure 1.7) can
have a steeper initial roll off than Butterworths, but at the
expense of more than 0.4dB of ripple in the passband.
Elliptic filters (Figure 1.8) have the steepest initial roll
off of all. But exhibit ripple in both the passband and the
stopband. Elliptic filters have higher Qs, which may (if not
carefully implemented) translate to a noisier filter. These
high Qs have made elliptic filters difficult to implement
with active RC filters because of the increased stability and
center frequency accuracy requirements. Elliptic filters can
be implemented with SCFs due to their inherently better
stabilities and center frequency accuracies when compared
to active RC filters. Chebyshev and elliptic designs can
achieve greater stopband attenuation for a given number
of 2nd order sections than can Butterworths.

GAIN

– ∞

FREQUENCY

Figure 1.5. Ideal Lowpass Response

Figure 1.7. 6th Order Chebyshev Lowpass Response

Figure 1.6. 6th Order Butterworth Lowpass Response

Figure 1.8. 6th Order Elliptic Lowpass Response

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Application Note 38

sωo/ Q

f

2

( )

f

2

Application Note 38

Filters are typically built up from basic building blocks
known as 1st order and 2nd order sections. Each LTC filter
contains circuitry which, together with an external clock
and a few resistors, closely approximates 2nd order filter
functions. These are tabulated in the frequency domain.

1. Bandpass function: available at the bandpass output

pin, refer to Figure 1.9.

G s

= H

H

( )

OBP

OBP

s2+ sωo/ Q

( )

= Gain at ω = ω

2

+ ωo

O

fO = ωO/2π; fO is the center frequency of the com-

plex pole pair. At this frequency, the phase shift
between input and output is –180°.

Q = Quality factor of the complex pole pair. It is the

ration of f

to the –3dB bandwidth of the 2nd

O

order bandpass function. The Q is always mea­sured at the filter BP output.

2. Lowpass function: available at the LP output pin, refer

to Figure 1.10.

2

ω

o

( )

+ ω

2
o

H

G s

= H

( )

OLP

OLP

s2+ s ωo/ Q

= DC gain of the LP output.

3. Highpass function: available only in mode 3 at the HP

output pin, refer to Figure 1.11.

2

G s

= H

( )

H

OHP

OHP

s2+ s ωo/ Q

= gain of the HP output for f →

s

( )

+ ω

2
o

CLK

4. Notch function: available at the N output for several

modes of operation.

2

s2+ ω

n

( )

+ ω

2
o

CLK

H

G s

= H

( )

( )

ON2

s2+ s ωo/ Q

H

= gain of the notch output for f →

ON2

= gain of the notch output for f→0

ON1

fn = ωn/2π; fn is the frequency of the notch

occurrence.

These sections are cascaded (the output of one section
fed to the input of the next) to produce higher-order filters
which have steeper slopes. Filters are described as being
of a certain “order,” which corresponds to the number and
type of cascaded sections they comprise. For example,
an 8th order filter would require four cascaded 2nd order
sections, whereas a 5th order filter would require two
2nd order sections and one 1st order section. (The order
of a filter also corresponds its number of poles, but an
explanation of poles is outside the scope of this manual.)

0.707 H

GAIN(V/V)

BANDPASS OUTPUT

0.707 H

GAIN(V/V)

H

H

H

OBP

OBP

f

fLf

o

H

f(LOG SCALE)

f

o

Q =

; fo= fLf

H

L

2



1





+

1

+



+1









2Q




2



1







+1









2Q




fL= f

f

H

fH– f



–1



o

2Q







= f

o

2Q




OP

OLP

OLP

fc= fo× 1–

fP= fo1–

HOP=H

LOWPASS OUTPUT

fPf

f(LOG SCALE)

1







+ 1–







2







2Q

1

2

2Q

1

×

OLP

1

1

1–

2

Q

4Q

HIGHPASS OUTPUT

H

OP

H

OHP

0.707 H

OHP

GAIN(V/V)












OHP

fCf






×

P

f(LOG SCALE)

1





+ 1–





2





2Q

–1



1



2

2Q



1

1

1

1–

2

Q

4Q

–1



2

1





+1



2





2Q





C

2

1



+1



2



2Q

fC= fo× 1–

fP= fo× 1–

HOP=H

Figure 1.9. 2nd Order Bandpass Section Figure 1.10. 2nd Order Lowpass Section Figure 1.11. 2nd Order Highpass Section

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Application Note 38

STEP ONE, THE BASIC DESIGN

The first item on FilterCAD’s MAIN MENU is “DESIGN
Filter.” To access the DESIGN Filter screen, press

1

On the DESIGN Filter screen, you make several basic deci­sions about the type of filter you’re going to design. First,
you must select your basic filter type (lowpass, highpass,
bandpass, or notch). Press the

Spacebar

to step through the options. When the filter type that you
want is displayed, press

[Enter]

0dB

A

MAX

A

GAIN

MIN

Next, you must select the type of response characteristic
you want (Butterworth, Chebyshev, Elliptic, or Custom).
Again, use the

Spacebar

to step through the options and press

[Enter]

when the response type you want is displayed.

Next, you will enter the most important parameters for
your filter. Exactly what these parameters will be depends
on the type of filter you have chosen. If you have selected
lowpass or highpass, you must enter the maximum pass­band ripple, in dB (must be greater than zero, or, in the

0dB

A

MAX

A

GAIN

MIN

f

C

FREQUENCY

Figure 2.1. Lowpass Design Parameters: A

f

S

= Maximum

MAX

Passband Ripple, fC = Corner Frequency, fS = Stopband
Frequency, A

0dB

A

MAX

Figure 2.3. Bandpass Design Parameters: A

= Stopband Attenuation

MIN

GAIN

SBW

PBW

f

C

FREQUENCY

A

MIN

= Maximum

MAX

Passband Ripple, fC = Center Frequency, PBW = Pass
Bandwidth, SBW = Stop Bandwidth, A

= Stopband

MIN

Attenuation

f

S

FREQUENCY

Figure 2.2. Highpass Design Parameters: A

f

C

= Maximum

MAX

Passband Ripple, fC = Corner Frequency, fS = Stopband
Frequency, A

0dB

A

MAX

Figure 2.4. Notch Design Parameters: A

= Stopband Attenuation

MIN

GAIN

FREQUENCY

PBW

SBW

A

MIN

f

C

= Maximum

MAX

Passband Ripple, fC = Center Frequency, PBW = Pass
Bandwidth, SBW = Stop Bandwidth, A

= Stopband

MIN

Attenuation

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Application Note 38

case of Butterworth response, must be 3dB); the stopband
attenuation, in dB; the corner frequency (also known as
the cutoff frequency), in Hz; and the stopband frequency.
If you chose a bandpass or notch filter, you must enter the
maximum passband ripple and the stopband attenuation,
followed by the center frequency, in Hz; the pass bandwidth,
in Hz; and the stop bandwidth, in Hz. (The meanings of
these various parameters in the different design contexts
are illustrated in Figures 2.1 to 2.4.) If you chose a custom
response, you’re in an entirely different ball game, which
will be described later.

Type in the parameters for your chosen filter, pressing

[Enter]

after each parameter. If you want to go back and alter one
of the parameters you have previously entered, use the

Up-Arrow

and

Down-Arrow

keys to move to the appropriate location and retype the
parameter. When you have entered all of the correct pa­rameters, move the cursor to the last parameter in the
list and press

[Enter]

FilterCAD will now calculate and display additional param­eters of the filter you have designed, including its order,
actual stopband attenuation, and gain, and will display
a list of the 2nd order and 1st order sections needed to
realize the design, along with their f

, Q, and fn values

O

(as appropriate). These numbers will be used later to
implement your filter design and calculate resistor values.

FilterCad will, in many cases, prevent you from entering
inappropriate values. For instance, in the case of a lowpass
filter, the program will not permit you to enter a stopband
frequency that is lower than the corner frequency. Similarly,
in the case of a highpass filter, the stopband frequency
must be lower than the corner frequency. In addition, you
cannot enter a maximum passband ripple value that is
greater than the stopband attenuation, nor can you enter
a set of values that will lead to a filter of an order greater
than 28.

Custom Filters

The custom-response option on the DESIGN screen can
be used in two ways. It can be used to modify filter de­signs created by the method previously described, or it
can be used to create filters with custom responses from
scratch, by specifying a normalization value and manually
entering the desired f

, Q, and fn values for the necessary

O

2nd order and 1st order sections. To edit the response of
a filter that has already been designed, press

ESC

to exit the DESIGN screen, then press

1

to re-enter the DESIGN screen. Move the cursor to “FILTER
RESPONSE” and use the

Spacebar

to select “CUSTOM.” You can now edit the fO, Q, and fn
values for the 2nd order and 1st order sections in the
window at the bottom of the screen. When you custom­ize an existing design, the normalization frequency is
automatically set to the previously-specified corner/center
frequency. It can, however, be edited by the user.

To design a custom filter from scratch, simply select
“CUSTOM” as your response type upon first entering the
DESIGN screen, then type in the appropriate f

, Q, and fn

O

values for the desired response. By default, the normaliza­tion value for custom filters is 1Hz. Once you have entered
your values, you can change the normalization frequency
to any desired value and FilterCAD will scale the f

frequencies accordingly. To change the normalization

f

n

, and

O

frequency, press

N

type in the new value, and press

[Enter]

By alternately graphing the resulting response and modify­ing the f

, Q, and fn values, almost any kind of response

O

shape can be achieved by successive approximations.

It should be understood that true custom filter design is
the province of a small number of experts. If you have

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Application Note 38

a “feel” for the type of pole and Q values that produce a
particular response, then FilterCAD will allow you to design
by this “seat of the pants” method. If you lack such erudi­tion, however, it is beyond the scope of the program or
this manual to supply it. Nevertheless, novice designers
can make productive use of FilterCAD’s custom-response
feature by entering design parameters from published
tables. An example of this technique will be found in the
following section.

STEP TWO, GRAPHING FILTER RESPONSE

After you have designed your filter, the next step is item
two on the MAIN MENU, GRAPH Filter Response. You
can graph the amplitude, phase, and/or group-delay
characteristics of your filter, and you can plot your graph
on either a linear or logarithmic scale. The graph also
highlights the 3dB-down point(s) (for Butterworth filters
only) and the point(s) where the calculated attenuation is
achieved. An additional option on the Graph Menu is called
“Reduced View.” This option displays a reduced view of
your graph in a window in the upper right-hand corner of
the full-sized graph. This feature is useful in conjunction
with the “Zoom” option.

Use the

Up-Arrow

and

Down-Arrow

keys to move through the list of graph parameters and
press the

Spacebar

to step through the selections for each parameter that you
wish to modify. When all of the graph parameters have
been set correctly, press

[Enter]

to begin plotting the graph.

Plotting to the Screen

If you chose the screen as your output device, FilterCAD
will immediately begin plotting the graph. Generating the
graph is a calculation-intensive process. It is here that the
speed and power of your CPU and the presence or absence

of a math coprocessor will become evident. The graph will
be generated in a matter of seconds. Note, however, that
the speed of calculation and plotting can be increased by
reducing the number of data points to be plotted. To modify
this parameter, use the “Change GRAPH Window” option,
found under item6, “Configure DISPLAY Parameters,” on
the Configuration Menu. The number of points can range
from 50 to 500. Of course, choosing a smaller number of
points will result in a courser graph, but this may be an
acceptable trade-off for quicker plotting.

The Zoom Feature

When you display the graph on the screen, you have the
additional option of magnifying or “zooming in” on areas
of the graph that are of particular interest. (Before using
the zoom feature, it is a good idea to enable the “Reduced
View” option on the graph menu. When you zoom in, the
area of the zoom will be indicated by a box on the reduce
view of the full-sized graph.) Note the arrow in the lower
right-hand corner of the graph. This arrow can be used to
select the region of the graph to zoom in on. It can also
be used to pinpoint the frequency and gain values of any
given point on the graph. (These values are displayed at
the upper right-hand corner of the screen.) The location
of the arrow is controlled by the arrow keys (the cursor
control keys) on the numeric keypad. The arrow can move
in either fine or coarse increments. To select course move­ment, press the

+

key. To select fine movement, press the

–

key. Move the arrow to one corner of the area that you
wish to magnify, and press

[Enter]

Next, move the arrow to the opposite corner of the area
of interest. As you move the arrow, you will see a box
expand to enclose the area to be magnified. If you want
to relocate the box, you must press

ESC

and restart the selection process. When the box encloses
the desired area, press

[Enter]

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