December 1953 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Designing, building, and tuning low frequency filters is
much easier for the person without a professional grade suite of software, fabrication, and test
equipment than is RF / microwave frequency filters. Most of my design and integration work has been
with system level transmit and receive racks for radar and satellite earth station installations,
and typically for prototyping and/or very low quantity production. Accordingly, I often used
connectorized components cascaded together where each functional block was predefined and tested. I
would be handed a system input/output document that specified parameters for gain, phase noise,
intercept points, noise figure, group delay, bandwidth(s), power levels, switching and settling
times, current consumption, volume, weight, cost constraints, etc. That explains why my software
offerings like RF Stencils for
Visio and RF Cascade
Workbook all deal with system level design. On occasion, I needed to do some filtering at
baseband, right in front of an A/D converter in order to whack interference getting into the signal
path from ambient sources that ranged from non-compliant, unintentional RF radiators to noise spikes
on power lines cause by nearby equipment switching.
Filter Building Made Easy
Inexpensive Construction with Good Performance
By Charles L. Hansen,* W0ASO
The availability of ferrite-slug inductances offers the opportunity to make audio-frequency filters
of good performance. Here is a practical method of constructing low-pass configurations such as might
be used in low-level speech clippers.
Fig. 1 - Circuit diagram of the filter discussed in the text. See Table I for sets
of values for several cut-off frequencies in the audio range.
The experimenter often needs a good low-pass filter that will pass frequencies in the audio range
up to a required cut-off point and provide 50 to 60 db. attenuation beyond cut-off. Commercially-designed
units for carrier telephone application can be obtained, but usually cost thirty-five to one hundred
and fifty dollars. Special filters designed to cut off to the purchaser's specifications, as well as
commercially available filters, are priced beyond the reach of the average experimenter. The compromise
method of making a filter out of power-supply chokes is frequently taken, but at the expense of performance
in the final equipment. This is not very rewarding, to say the least.
The purpose of this article is to describe a method of designing and building a sharp cut-off low-pass
filter with components that are available from any well-stocked radio parts supply house. The passband
and the sharp attenuation at the cut-off point of this home-built filter are comparable with and in
many cases equal to commercial low-pass filters costing more than ten times the price of components
used for this filter.
Table I - Filter component values
A - Grayburne type V-25 variable coil, 5-43
B - Grayburne type V-6 variable coil, 0.65-6 mh.
Dimensions refer to length of slug inserted
in coil as a preliminary setting before tuning adjustments.
The capacitors should be good-quality
Most articles on "how to design filters" deal with the mathematical derivation of the sections or
meshes that make up the filter proper. After the filter has been designed mathematically and diagrammed,
many experimenters have been disappointed in the actual performance of the completed filter. Because
practical components fall short of the ideal reactances on which the filter formulas are based, as well
as the difficulty of obtaining exact values, there is no substitute for practical experimentation with
any filter, and the final values of capacitance and inductance may differ quite a bit from the calculated
values. Also, most of us do not have the equipment, time, or inclination to design, build and adjust
filters from theoretical information.
With the availability of recently developed variable ferrite slug-tuned inductors1 good
filters are now within the budget of everyone. Not only do we have an inductance that can be varied
but we have the added advantage of a slug made of ferrite, which increases the Q by reducing the resistance
per unit inductance. These inductances possess all of the qualities necessary for a good reactance which
in turn results in the building of a good practical filter.
A Practical Filter Design
The configuration chosen for the filter described here and shown in Fig. 1 provides for a minimum
of inductances. The filter contains two shunt m-derived half sections, one on each end, to provide a
good impedance match from and to the flanking circuit units. An m-derived full section and a constant-k
section make up the other two meshes. The m-derived section sharpens the cut-off characteristic and
the constant-k section assures that attenuation of the unwanted frequencies beyond the passband will
remain high. The impedance this filter must work into and out of is 500 to 600 ohms. Insertion of a
filter into a circuit whose load impedance varies with frequency will result in erratic operation.
Fig. 2 - Test set-ups for adjusting
filters. If equipment having output and input impedances matching the filter is available the simple
arrangement shown above may be used; otherwise the use of isolating pads as shown in the lower drawing
is recommended. Another alternative, when using an a.c. v.t.v.m., is to terminate the filter in its
characteristic impedance, and adjust the input voltage to a fixed value for each frequency before making
output measurements across the load resistance.
Fig. 3 - Measurements made by the author on a 5-kc. cut-off filter constructed from
the data in Table I.
Good practice in an extreme case of changing load impedances dictates the use of T pads connected
before and after the filter. The use of these pads reduces the effects of source and load impedance
variations with frequency.
Table I gives practical component values for several frequency ranges. For example, if a low-pass
filter having a cut-off of about 5000 cycles is needed, a set of values will be found in the second
row. Wire the condensers and inductances as shown in Fig. 1 and adjust the ferrite slugs to the specified
distances. Connect the completed filter to an oscillator and a measuring set having 500- to 600-ohm
impedance. If an oscillator and measuring set with this impedance value are not available use a pad
set-up as shown in the lower drawing of Fig. 2.
Manually sweep the oscillator through the passband and check the uniformity of response with the
output meter or measuring set. Also check for the correct cut-off frequency of 5000 cycles. Adjust the
slugs on L2 and L3 alternately to place the cut-off frequency at the desired frequency.
Manually sweep through the passband again and adjust the slugs on L1 and L4 for
improvement in the smoothness of the passband. These operations should be repeated until the passband
response is reasonably flat (within 0.2 db.). The frequencies beyond 5700 cycles should be attenuated
60 db. or more, with an attenuation peak of 70 db. or so at 6500 cycles.
After adjustment the filter is ready for use. It may be enclosed in a metal box taking up no more
room than an average 20-watt output transformer. The individual sections may be shielded from each other
Among the many applications that can be thought of for such filters are (1) speech filters in communication
work; and (2) audio use in recording and high-quality home systems (see Audio Engineering for several
discussions). For example, a 5000-cycle filter can be used for sharply cutting off the hiss and scratch
from old records.
1 The ones used by the author are made by the Grayburne Corp., 4-6 Radford
Place, Yonkers, N. Y.
Posted December 1, 2016