July 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
The 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
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.
An Amateur Application of Modern Filter Design
By Edward E. Wetherhold, * W3NQN
The method 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
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.
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has finally been superseded by a vastly superior filter-design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
Table 1 - Dual-section elliptical-function
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.
Desired performance 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
4) Equal source and load resistances of approximately
It was also desirable to utilize the currently-available
88-mH. toroidal inductors because of their high Q and very
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.
Table 2 - Single-section elliptical-function
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.
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.
Filter Response Evaluation and Performance
The completed 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 in decibels.
Fig. 1 - Transmission loss vs. frequency,
low-pass filter with 2.4·kc. cutoff using the values shown
in Table I. Peak rejection 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
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 rejection frequencies are 3.92 kc. (ƒ004)
and 5.63 kc. (ƒ002).
Another view of the filter, showing the
back side of the front panel. The input transformer is clearly
visible in this view.
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.
Bandpass Speech Filter Using Modern Filter Design
Because the 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 of equipment.
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 connector.
Cascading the Low-Pass and High-Pass
Transistor Amplifier Design
To overcome 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.
Several combinations 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.
- Circuit diagram of the bandpass speech filter. Resistances
are in ohms, K = 1000; resistors are 1/2·watt, 5 percent
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.
Performance 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 respect.
The 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
* Dept. 2N, Electro International, Inc., Box 391, Annapolis,
1 Zobel, "Theory & Design of Electric
Wave Filters," The Bell System Technical Journal, January.
2 Zverev, "Introduction to Filters,"
3 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,
Genaille, "Low-Pass Audio Filters for Increased Talk Power,"
Electronics World, September, 1963.
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,
Posted February 12, 2013