term "modern" in the title of any book or article never has set right
with me because it is utterly ambiguous about the era to which "modern"
refers. Sure, it sounds good at the time, but when applied to this 1966
QST article, "modern" should be replaced with "four-decade-old." However,
in this case the content is still relevant even thought it was written
so long ago (or else I would not be reproducing it here). It may well
have been most people's first exposure to elliptical (Cauer)
filters. As you might expect, the rigorous, headache-inducing mathematics
is omitted, but the article does give an example of implementing an
audio frequency bandpass filter by cascading a lowpass filter and a
highpass filter. If you are familiar with filter design, you know that
because of phasing and inband impedance mismatch issues you cannot simply
butt the two together to yield an equivalent bandpass filter. In fact,
the author found it necessary to insert a 13 dB attenuator between
them in order to get acceptable performance.
July 1966 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
An Amateur Application of Modern Filter Design
By Edward E. Wetherhold, * W3NQN
Completed speech filter, less cover, showing the component mounting
boards and front panel with bypass switch and microphone connectors.
The low-pass filter components, marked with the 3-kc. cutoff values,
are mounted on the top phenolic component mounting board. The transistor
amplifier is mounted on the bottom phenolic board, the high-pass
filter with the resistor pad on the middle board. Note the phenolic
washers used to hold the 60- and 88-mH. toroids firmly in place.
of filter design known as "modern network synthesis" leads either to
simpler circuits for a given performance or improved performance for
a given degree of circuit complexity, as compared with the longer-established
design procedures. Here the author uses the system to come up with a
simplified "Filterfier" plus a design for an accompanying high-pass
Low- and High-Pass Audio Filters for Shaping
Over the past several years, there has
been a major revolution in the design of electric wave filters. The
old image-parameter approach developed by Campbell and Zobel1
in the early 1920s with the now-familiar terminology of "characteristic
impedance," "constant-k section," and "m-derived section" has finally
been superseded by a vastly superior filter-design method generally
known as "modern network synthesis." Although this method is not new,
having been first mentioned in 1929 and later expanded during 1940-19502,
it was not practical to apply it to practical filter problems until
the digital computer became available as a design tool. The recent publication
of two texts3,4 with design tables derived by the computer
now makes it possible for the progressive radio amateur to take advantage
of this most recent development in filter design.
The fact that
many radio amateurs apparently are not yet aware of the advantages of
modern filter design techniques is indicated by recent articles5,6
in which the now-passé image-parameter design approach was employed.
The purpose of this article is to illustrate an application of modern
design to a simple filter problem already "solved" by the image-parameter
filter. By comparing the performance and components of the filters that
result from these two different approaches, the degree of superiority
and advantages of the modern filter over the image-parameter filter
should be evident.
The most recent image-parameter design conveniently accessible to QST
readers is the"Filterfier6,"
a low-pass filter designed to be used with s.s.b, phasing-type exciters
to restrict the speech frequency range to that at which the phasing
network performs best (approximately 300-3000 c.p.s.), to reduce the
possibility of generating unwanted side frequencies in excess of 3 kc.
This was accomplished by choosing a cutoff frequency of 2.40 kc. and
designing an m-derived, constant-k image-parameter filter which produced
37 db. of attenuation at 3.0 kc. At higher frequencies, the attenuation
in the stop band was never less than 39 db. The filter required four
readily-available inductors and seven capacitors and was designed to
be terminated in equal resistances of 1106 ohms.
1 - Dual-section elliptical-function lowpass filter.
requirements for the comparative low-pass modern filter design therefore
were as follows:
1) A cutoff frequency of 2.4 kc. to permit
ease of performance comparison with the image-design filter.
2) An attenuation of at least 37 db. at 3.0 kc.
3) A minimum
attenuation in the stop band of approximately 39 db.
source and load resistances of approximately 1000 ohms.
also desirable to utilize the currently-available 88-mH. toroidal inductors
because of their high Q and very low cost.7
Modern Filter Design Applied to the Filterfier
With these thoughts in mind, a filter type classified
by the filter theorists as a "dual-section elliptic function" was chosen
as being most suitable for this particular application. From the many
possible variations available in the computer-derived tables of Geffe's
book'' for the elliptic-function type, one was chosen which best approximated
the desired performance requirements. The tabulated computer-derived
design parameters, all normalized for a cutoff frequency of 1 radian/sec.
and 1-ohm resistance terminations, were scaled to the desired levels
simply by multiplying all normalized values by the proper factors. Normalized
frequencies were scaled by multiplying them by the cutoff frequency
in kilocycles. Component values were scaled by multiplying all capacitances
by 1/Rω and inductances by R/ω, where ω is
The source and load resistances, R, were specifically chosen to assure
that the higher inductance required by the filter would be 88 mH. The
lower inductance came out to be 60.3 mH. The filter values associated
with cutoff frequencies of 2.40 kc. and 3.0 kc. are presented in Table
I. The second cutoff frequency of 3.0 kc. is presented as an alternate
for those who may prefer a wider passband for their particular application.
Note that the same inductance values are required, but the source and
load values are different as are also the capacitance values.
The toroidal inductor used has two separate 22-mH. windings on a toroidal
core. When the windings are connected in series aiding, the total inductance
is 88 mH. with a Q of 45 at 1 kc. One of these inductors is used in
its unmodified form for L2.
A second 88-mH. toroid is modified by removing 62 turns from each 22-mH.
winding so that when the modified coils are connected in series aiding,
the resulting inductance is 60 mH., which is the amount of inductance
required for L4.
2 - Single-section elliptical-function highpass filter.
Mylar capacitors were used because of their small size, low loss
and excellent capacitance stability relative to change of temperature
and time. The capacitances of a large number of Mylar capacitors were
measured with an impedance bridge and the true value marked on each
capacitor case. Appropriate values were then selected and paralleled
to produce the capacitances of C1
through C5 to within ±2
percent of the value specified in Table I. The two toroids and associated
capacitors were mounted on a phenolic board 2-7/16 by 3-5/8 by 1/16
inch thick and wired according to the low-pass filter schematic of Table
I. This completed the filter construction.
Response Evaluation and Performance
filter was subjected to a transmission-loss response evaluation, the
results of which are shown in Fig. 1. Transmission-loss response is
defined as the ratio of the voltage amplitude V1
of the load signal before filter insertion to the value of load signal
V2 at the filter output
terminals after insertion of the filter. This ratio is generally expressed
Fig. 1 - Transmission loss vs. frequency, low-pass
2.4·kc. cutoff using the values shown in Table I. Peak
frequencies are 3.14 kc. (ƒ004)
and 4.51 kc. (ƒ002).
A Heathkit Audio
Generator, Model AG-9 (step-frequency type) was calibrated against a
digital frequency counter to provide known test frequencies to better
than 1-percent accuracy. Input and output voltage amplitudes were measured
with a Heathkit a.c. v.t.v.m., Model AV-2. Resistive terminations, as
specified in Table I, were provided for the filter input and output.
The response curve shows two" ripples" in the filter passband
of less than 1 db., which is sufficiently in accord with the expected
maximum passband attenuation of 0.5 db. The two passband ripples are
typical of the dual-elliptic-type filter. The measured cutoff frequency
occurs at 2.40 kc. where the response curve continues rising above the
level of the maximum passband attenuation. The remainder of the filter
performance is equally in accord with the design specifications.
Fig. 2 - Transmission loss vs. frequency, low-pass
filter with 3.0.kc.
cutoff using the values shown in Table I. Peak
frequencies are 3.92 kc. (ƒ004)
and 5.63 kc. (ƒ002).
Comparing the response curve of the modern filter with that of the image
filter (Fig. 3, page 33, November 1965 QST), no outstanding differences
are noted above 1 kc. However, in comparing the two filter circuits,
the modern filter design requires significantly fewer components only
two inductors and five capacitive elements compared to four inductors
and seven capacitive elements. Another advantage of the modern design
not immediately obvious is the fact that the transmission loss in the
modern filter passband is less than 1 db. whereas the image design used
in the Filterfier has a loss in excess of 6 db. as a result of the 660
(640)-ohm resistor separating the m-derived section from the constant-k
section. If it is desired to install the modern filter in the Filterfier
circuit, it is only necessary to provide the required filter source
and load resistances of 1305 ohms or 1630 ohms, depending on whether
the 2.4-kc. or 3.0-kc. cutoff filter is used.
Another view of the filter, showing the back side of the front panel.
The input transformer is clearly visible in this view.
Speech Filter Using Modern Filter Design
results of applying modern filter design techniques to the low-pass
filter application were so successful, it was decided also to design
and construct a high-pass filter so that, in combination, the two filters
would provide a bandpass of 300-3000 c.p.s. The bandpass filter is intended
to be used with an active device that will be inserted between a microphone
and speech input amplifier so as to provide approximately unity gain.
The component values and other associated information for the 3.0-kc.
low-pass filter are presented in Table I. The transmission-loss response
curve is shown in Fig. 2.
Considerations for the design of the
high-pass filter were that, for simplicity, only one toroid be required,
the minimum attenuation in the stop band be 20 db., and that maximum
pass-band ripple be 0.5 db. The most suitable compromise appeared to
be a design which required two 0.1-µf. capacitors, one 0.235-µf.
capacitor and one 3.11-henry toroid for source and load impedances of
4260 ohms. See Table II for filter parameters and schematic. With these
component values, the cutoff frequency was 294 c.p.s. and the resonant
frequency of the series-tuned circuit was 186 c.p.s. The cutoff frequency
and impedance level were deliberately juggled to make C1
and C3 come out to a nice
even 0.10 µf. The 3.11-henry toroid uses a core of permalloy and
has a Q of 50 at 1 kc., or approximately 15 at the ƒco
of 294 c.p.s. The filter was assembled, evaluated and found to perform
satisfactorily in every respect. The next step was to cascade the low-pass
and high-pass filters to form the desired bandpass filter.
Fig. 3 - Relative attenuation vs. frequency, cascaded
low-pass and high-pass filters.
Insertion loss due to matching pad
is 13 db. Arrows indicate attenuation
in excess of measurement capability
Cascading the Low-Pass and High-Pass Filters
A 13-db. pad was installed between the high-and low-pass filters
to provide impedance matching and also some degree of isolation. The
cascaded filters and pad were then evaluated for relative attenuation
vs. frequency, using the test circuit shown in Fig. 3. The response
curve is also presented in Fig. 3. The high-pass filter was purposely
placed after the low-pass filter so as to attenuate any 60-cycle hum
that might be picked up by the low-pass filter. The output of the high-pass
filter is terminated in its specified load impedance of 4300 ohms. Since
the filter is designed to work into the input resistor of a microphone
preamplifier, which is generally in excess of 1 megohm, the filter load
termination of 4300 ohms will be relatively unaffected by connection
to the speech preamplifier. In fact, if a volume control is desired
a 5000-ohm potentiometer shunted by 30,000 ohms could be used as the
high-pass filter load with the potentiometer arm wired to the output
Cascading the Low-Pass and High-Pass Filters
Transistor Amplifier Design
the losses in the resistive filter matching pad and input matching transformer,
an amplifier voltage gain of approximately 40 db. was required. Also,
a low-impedance source was required to drive the filter input for best
results. The required gain was obtained from a common-emitter transistor
stage with a voltage gain of between 100 and 150. Using the input transformer
specified in Fig. 4, an input impedance of about 300,000 ohms is anticipated,
which should be sufficient to assure a flat response down to 300 c.p.s.
even if a crystal microphone is used. The low-impedance signal source
for the filter is provided by a common-collector stage which is direct-coupled
from the common-emitter amplifier stage, thus eliminating the necessity
for a coupling capacitor and bias resistors. The output impedance of
the common-collector stage is approximately 40 ohms. Placing a 1600-ohm
5 percent resistor between the emitter follower and low-pass filter
very nicely solves the matching problem.
The transistors, manufactured
by General Electric and available from Allied Radio Corp. for about
80 cents each, are n-p-n silicon planar passivated types specifically
designed for low-level audio applications. The input transistor, a 2N3391A,
has a controlled noise figure and high beta and so is very well suited
to its application in this design. The 2N3392 is similar but has a lower
beta and no specification regarding noise figure.
of parallel resistors were installed for R1 until a Q1
emitter current of 1.3 ma. was obtained. In this particular case, the
required resistance for R1 was 44,000 ohms. Switch S1,
which simultaneously bypasses the entire circuit and also switches out
the battery, was provided as a convenient means to permit comparison
of the modulated transmitter output with and without the bandpass speech
filter. With an operating duty cycle of 2 hours per day, the useful
life of the 15-volt battery may be expected to be in excess of one month.
If it is desired to omit the resistive pad and the high-pass filter,
simply terminate the low-pass filter with a 1600-ohm resistor, change
R4 to 1300 ohms, and if R2 is made 220 ohms unity
gain should be approximated.
Fig.4 - Circuit diagram of the bandpass speech
filter. Resistances are in ohms,
K = 1000; resistors are 1/2·watt,
5 percent tolerance.
The desired filter performance
will be assured if reactors with a ±2 percent tolerance, resistive terminations
with a ±5 percent tolerance, and inductors with as high a Q as practical
are used. There will be relatively little difficulty and expense in
obtaining the 88- and 60-mH. inductors. However, obtaining the 2 percent
capacitors will require some extra effort. Also, the 3. 1-henry toroid
may prove to be more expensive than anticipated. This toroid is available
from the Allen Organ Co. (3.11 henry, ±2 percent, Q = 50 at 1 kc.) at
a cost of $1.43 each with a minimum billing charge of $20. An alternate
source is Newark Electronics Corp. (Stock No. 39F2806, Collins toroid
type MP-930-37B, 3.0 henry, ±1 percent, Q = 58 at 1.5 kc.) at a cost
of $7.23. The author employed the following procedure: Mylar capacitor
and Allen toroid data sheets were requested from the Components Division
of Allen Organ Co., Macungie, Pa., and $20 worth of mylar capacitors
and permalloy toroids was selected and ordered. The capacitor cost ranged
from 13 cents for 0.007 µf. to 17 cents for 0.10 µf., and
about fifty capacitors of mixed values were obtained for $8. The remainder
of the $20 was invested in toroids, one of which was the 3.11-henry
value. An impedance bridge was borrowed and all the capacitors were
measured to an accuracy of 2 percent or better and the values marked
on the capacitor cases. Appropriate values were then selected and paralleled
to produce the desired filter capacitance values.
of the Completed Unit
When first tested, the
gain of the bandpass speech filter was found to be greater than unity
by 4.5 db. R2 was added to the circuit and adjusted until
the desired unity gain was achieved. The 3.1-henry toroid in the high-pass
filter was found to be sensitive to hum pickup, and therefore the filter
should not be placed in the immediate vicinity of power transformers.
The overall frequency response of the entire unit was found to he essentially
the same as that of Fig. 3 except that the attenuation was greater than
indicated by the response curve at frequencies below 100 c.p.s., because
of the roll-off effects of C3 and possibly T1.
An operational check of the filter on the air was satisfactory in every
author wishes to thank John Brennan, Jr. for providing the photographs,
Tom Miller, W7QWH/3, for performing the operational checkout, and Millicent
Schaffer for typing the manuscript.
* Dept. 2N,
Electro International, Inc., Box 391, Annapolis, Md. 21404
1 Zobel, "Theory & Design of Electric Wave Filters,"
The Bell System Technical Journal, January. 1923.
"Introduction to Filters," Electro-Technology, June,1964.
Geffe, Simplified Modern Filter Desiqn, John F. Rider Publisher, Inc.,
New York City, 1963.
4 A Handbook on Electrical Filters,
published by White Electromagnetics, Inc., Rockville, Maryland, 1963.
5 Genaille, "Low-Pass
Audio Filters for Increased Talk Power," Electronics World, September,
6 MacCluer and
Thompson, Jr., "The Filterfier," QST, November, 1965.
7 For example, 88- and 44-mH.
toroids are available 5 for $1.75, postpaid, from Buchanan & Associates,
1067 Mandana Blvd., Oakland, California 94610.