May 1966 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Wirewound inductors (as most are)
can be mysterious entities even when you are familiar with their
many interdependent physical and electrical properties. Because
of interwinding capacitance and a sometimes
(when a large number of turns are involved)
rather significant series resistance, the equivalent circuit
model gets quite complex - literally in a mathematical sense.
If you have the luxury of staying far away from the self-resonant
frequency (SRF) of the coil, your component will behave very
much like an ideal inductor, that is, XL = 2πfL.
This article delves into what causes inductors to act like parallel
and/or series resonant tank circuits, how to avoid the inconvenience
of unwanted resonances, and how to exploit resonances in your
favor. You'll also learn (if you don't
already know) the distinction between a 'choke' and an
How to Select R.F. Chokes
By Joseph Tartas
There is more to an r.f. choke than some wire wound about
a form. When all its characteristics are known, it can prove
useful as a tuned circuit, transformer, low-or high-pass filter,
as well as an r.f. isolator.
Not so many years ago, the selection of an r.f. choke was
not a difficult problem. You simply asked for a Z-50, if you
were working in the 50-MHz area, or a Z-14 if your circuitry
was in the 10- to 15-MHz range. Chances were, you didn't know
anything about the choke itself, other than taking the manufacturer's
word that it was designed to work in that range.
For very-low-frequency work, the selection of a choke usually
depended more on the inductive reactance and hence was usually
some arbitrarily selected value of high inductance. If you were
really designing, you might make a lot of calculations and measurements,
throw in a few rules of thumb, and come up with the right value.
Today, the picture has changed considerably and instead of
two or three makes of chokes and a dozen or so values to select
from, there are hundreds of sizes and shapes as well as specific
characteristics to meet extreme temperature, moisture, or other
environmental requirements. In a single manufacturer's line,
there are several physical sizes designed to fit requirements
of current-carrying capabilities and space limitations; and
in cases of extremely small volume, there are non-insulated
chokes to reduce the physical size but retain the same electrical
The most important improvement in the RFC field in the past
few years is the information made available to the user through
manufacturers' literature. This information includes inductance;
minimum "Q" (although often misleading-this will be discussed
farther on); approximate resonant frequency, perhaps the most
important item; the approximate distributed capacitance; the
minimum parallel resistance at two widely separated frequencies;
the maximum d.c. resistance; the maximum current rating; and
in the case of one manufacturer, incremental current rating.
Because of extreme environmental requirements in military and
industrial equipment these days, many chokes are also rated
for maximum temperatures ranging from 85° to 500°C and
power dissipations that depend upon body sizes, covering materials
and core materials.
In spite of all this information, it often requires a bit
of reading between the lines before you can make a sensible
selection of the correct RFC to use in your particular circuit.
Before you can do this, however, you must have a thorough understanding
of the various characteristics and circuit requirements that
go into such a selection.
Why a Choke Is Used
Basically, an r.f. choke is nothing more than an inductance
that, because of its own distributed capacitance, forms a parallel-resonant
circuit. This distributed capacitance (Fig. 1) is a multiplicity
of small capacitances existing between adjacent turns and from
anyone turn to all the others, as well as to a shield if one
is used, and to a ground if one exists in the vicinity of the
choke when it is installed in the equipment. For the moment,
consider the self-capacitance, or distributed capacitance itself
(without the choke-to-ground capacitance), as being in parallel
with the pure inductance of the coil and having a resistance
equivalent to the resistive component of the choke. A choke
would then appear schematically as in Fig. 2A, and its impedance
and reactance as in Figs. 2B and 2C, respectively.
Fig. 1 - Besides inductance, on RFC, like
any other coil has some distributed capacitance between turns
and to chassis.
Fig. 2 - (A) How an RFC looks electrically.
(B) and (C) show "Q" and reactance, respectively, for different
values of "Q."
As in all parallel-resonant circuits, the maximum impedance
at resonance will depend upon the "Q" and this, in turn, will
be the deciding factor as to how effective the choke will be
at its self-resonant frequency and over how wide a frequency
range it will still be effective. The impedance depends upon
the amount of self-capacitance that exists in the choke and
the equivalent parallel resistance. A good approximation of
the impedance of a parallel-resonant circuit is Zr
= L/CR as long as the circuit "Q" is at least 10. The value
of R is normally the sum of the series resistances of the inductance
and capacitance, but in a choke the resistance involved in the
distributed capacitance is practically non-existent. This, then,
boils down to the inductance, the distributed capacitance, and
the resistance of the winding (d.c. + r.f. resistance as frequency
goes higher) as affecting the over-all performance of the choke;
that is, the resonant impedance, the self-resonant frequency,
the "Q" and hence the bandwidth of the choke, and its effectiveness
on the r.f. voltages it is being used to suppress. Thus, it
is obvious that the value of inductance of a choke is not as
important as the self-resonant frequency and impedance, although
the inductance does have some bearing on these factors.
Chokes come in many forms, the particular type depending upon
the frequency involved, the inductance necessary to achieve
it, and the distributed capacitance that results from the particular
form factor and winding method. Many are simple single-layer
solenoids with or without powdered-iron cores; others are multi-layer
coils of various configurations; still others are single or
multiple honeycomb windings with one or more sections combined
A single-layer solenoid has the lowest distributed capacitance,
and this capacitance is proportional to the diameter of the
winding and to a smaller degree is affected by the length and
number of turns and, where only a few turns are involved, the
spacing between turns. For a single-layer, close-wound choke,
the distributed capacitance is approximated by Cd
=.75 diameter (in inches), with C in pF.
In a multi-layer coil (Fig. 3A), the capacitance is considerably
greater, not only because of the greater number of turns but
also because of the relative capacitances between layers and
nearby turns. If the layers are wound so that the difference
of potential between any two adjacent turns is small, then the
capacitance is small; if the layers are wound in such a way
as to have adjacent turns non-parallel to each other, as in
the honeycomb or universal winding, the capacitance is at a
minimum between those adjacent turns. The total distributed
capacitance may be further reduced by the use of powdered-iron
or ferrite cores which reduce the number of turns of wire necessary
to obtain the desired inductance value. However, other problems
enter when cores are used, since the core losses, the capacitance
effects due to the dielectric material used in making the cores,
and the useful temperature range and effective permeability
of the core which affect the "Q" must be considered, as well
as a factor that is often ignored in r.f. chokes, the temperature
coefficient of the core, which also affects the temperature
coefficient of the choke.
In multi-section chokes, the individual sections may be alike
or may be designed so that the over-all choke appears as a broadband
filter. In most cases, the choke is not like a single resonant
circuit but appears electrically as a multi-element bandpass
Fig. 3 - Two different methods of winding
multi-turn coils. Note winding arrangement difference between
(A) and (B).
When a large inductance is required and the use of a single-layer
coil is prohibitive because of its size, the multi-layer coil
becomes necessary. This, in turn, means a larger distributed
capacitance which is no longer directly related to the diameter
or coil size. One method of reducing capacitance is to wind
the coil in such a way that the high-numbered turns do not lie
next to low-numbered turns (the condition for greatest capacitance).
Early attempts to reduce capacitance by means of "bank-winding"
(Fig. 3B) resulted in expensive chokes, since this method required
great skill by the winder and it could not be done by machine.
The coil could not be held to the production tolerances then
required (and completely unusable with today's close-tolerance
requirements), and when large inductances were needed, the coil
was so large physically as to be almost impractical.
The development of the universal coil winder provided the
designer with a new and extremely useful tool that allowed him
to repeatedly produce a mechanically stable, relatively low
capacitance coil with a fairly large inductance. This type of
winding, while not having the lowest attainable capacitance,
was better than a "happy medium" between the excessive capacitance
of the layer-wound and the great expense of the bank-wound coil.
The "universal" layer-wound coil, more commonly known as
"pie-wound" because of its resemblance to a pie, is a compromise
between the bank-wound coil of Fig. 3B and the slot-wound coil
of Fig. 4A.
The slot-wound coil has a distributed capacitance equal to
a two-plate capacitor, each plate consisting of rings the width
of the slot with one plate having the inner coil diameter A
(see Fig. 4B) and the other plate the outer diameter B. Since
a two-plate capacitor of two greatly different areas depends
to a large extent on the area of the smaller plate, the smaller
the diameter of the inside ring (or the winding form in this
case) the smaller the distributed capacitance, and conversely,
the greater the inductance the pie will have for a given width
and outer diameter.
Fig. 4 - Styles of coil construction. (A)
Method of winding a slot-wound coil. (B) A winding form as used
for slot-wound coils. (C) Self-supporting coil.
The universal-wound pie of Fig. 4C is simply a free-standing,
self-supporting, slot-wound coil with a winding similar to the
bank-wound coil, although restricted because of the narrow cross-section
of the slot.
As the art progressed, powdered-iron cores were developed,
and their use as forms or cores for universal-wound r.f. chokes
allowed a great reduction in over-all size.
Like all new materials, powdered iron had its own limitations.
If the permeability (μ, the ratio of inductance with the
core, to the inductance without the core) was low, and the losses
resulting from the insertion of the core in the coil (eddy losses)
were kept to a minimum (resulting in a reasonably high "Q"),
the size of the coil was not reduced greatly. Increasing the μ
of the core material reduced the choke size, but often it was
found that the "Q" suffered greatly due to core losses so that
the series impedance (QX) was inadequate to warrant the use
of a costly and bulky component.
Eventually, new ferrite materials (ceramic materials with
magnetic qualities) were developed which allowed for great reduction
in the size of an r.f. choke, but as environmental temperature
specifications for equipment were raised above the usual 85°C,
the temperature capabilities of the ferrites fell apart. Either
the temperature coefficient was poor, causing excessive change
in effective inductance with change in environmental temperature,
or the permeability decreased so rapidly above 85°C that
the resulting inductance became too low to be effective long
before the maximum temperature requirements were met.
This led to further improvements in the capabilities of ferrite
core materials, and we now have extremely large inductances
in very tiny packages with excellent temperature and mechanical
stability, and because of the extremely high permeability of
the cores, the distributed capacitance or "self-capacitance"
of the choke remains quite low.
Examples of this size reduction are a miniature 1000-μH
choke with a molded phenolic form having a distributed capacitance
of 5 to 10 pF for several makes, and a ferrite-bodied choke
with the same effective inductance yet having less than I-pF
Chokes having an iron core, for the same inductance, run
somewhere in between, with a maximum capacitance of 7 pF.
In large chokes using non-miniature techniques, the capacitance
is usually not given, and often the self-resonant frequency
is ignored as well. Because these chokes are usually wound with
larger wire to accommodate high currents, the capacitance of
these chokes would be even higher than that of the miniature
The current rating of a choke will often depend upon the
ambient temperature and will have to be derated. Fig. 5 shows
a curve for de-rating one type of miniature choke. If the current
rating is given at the maximum temperature to be encountered,
then only the incremental current is of interest. This is the
current necessary to reduce the effective choke inductance by
5% due to the magnetization of the iron core that effectively
lowers the permeability and hence the inductance.
Fig. 5 - De-rating curve for one type of
miniature r.f. choke.
Chokes are normally used to prevent coupling of r.f. power
(microwatts or megawatts) from one circuit to another in a piece
of equipment. Any wires that lead from stage to stage, circuit
to circuit, or from one sub-unit to another are potential conductors
of undesired r.f.
In r.f. circuitry in particular, there is usually a series
of cascaded amplifier or multiplier stages, and more often than
not, an oscillator or two. Furthermore, mixers, detectors, and
other associated circuits are commonly involved in such circuitry,
and each is a source or terminus for leads that might conduct
unwanted r.f. These leads can be any or all of the following:
"B+," "B-," a.g.c., filament leads, fixed bias leads, leads
to panel controls, and video or audio input and output.
Carefully selected, the proper choke can be an r.f, isolator;
a shunt trap; a tuned circuit either at its self-resonant frequency
or, by means of additional shunt capacitance, at some lower
frequency; or can be combined with other components to make
a low-pass, high-pass, or bandpass filter.
A recently introduced innovation is a series-resonant trap
that is in appearance an r.f. choke. Utilizing the capacitance
between turns as a series-coupling element, it can be made in
a wide range of series self-resonant frequencies. Like the parallel-resonant
choke, the series trap will probably find applications that
formerly required an external capacitor in conjunction with
an RFC to make a series-resonant trap.
How to Use Chokes
Because a choke is composed largely of distributed inductance
and capacitance rather than lumped inductance and capacitance,
it appears electrically as a transmission line rather than as
a tuned circuit. Depending upon the values of these constants
and the terminating impedance of this transmission line, a choke
can act as an impedance varying from extremely high to almost
a perfect short.
At the fundamental (self-resonant) frequency and at all odd
multiples of this frequency, a choke appears as a parallel-resonant
circuit. At even multiples, the same choke acts as a series-resonant
circuit with the lowest impedance limited only by the series-resonant
resistance, or essentially the resistance of the coil itself.
This phenomenon occurs because a choke does act as a transmission
line and is not terminated in its characteristic impedance.
If it were so terminated, a choke would be little more than
a straight conductor or loss less transmission line for the
Because of the distributed elements, a choke may be thought
of as a series circuit composed of a number of elements, each
element consisting of several small inductances and capacitances
in parallel; as two parallel branches, with one leg a series
of small coils and the other leg a series of small capacitors;
or even as a combination of the two, which would make it the
equivalent of multi-element filter circuits. Hence, at any given
frequency, a choke can appear as almost anything from a high
parallel-resonant impedance to a very low series-resonant impedance.
It is customary, however, to select a choke having its highest
parallel-resonant impedance when it is to be used as an r.f.
suppression element in a supply line (in series with a "B+"
line, for example), as shown in Fig. 6A. If a choke is selected
having the same self-resonant frequency (SRF) as the frequency
it is desired to suppress, then the choke must have the highest
series impedance at that frequency. And because a choke is a
completely self-contained high-impedance element, its SRF is
not affected by external elements connected from either end
to ground, and further attenuation is achieved by bypassing
either or both ends of the choke. It is, however, affected by
any parallel elements, and hence its SRF, "Q," or parallel-resonant
impedance can be lowered.
Because it can be altered, such a choke may be utilized as
a tank coil and tuned by a fixed or variable capacitor connected
across the choke (Figs. 6B, 6C, and 6D) and have its loaded
"Q" adjusted by adding a parallel resistor (or tube or transistor
circuit). Two chokes in series, with the combination shunted
by a capacitor, provides a tuned circuit with a tap (or an autotransformer),
as shown in Fig. 6E. The tap can be adjusted by the proper selection
of the inductance ratio or impedance ratio desired. Because
of the extremely wide range of available chokes (0.10 μH
to many millihenrys) in small increments, almost any ratio is
Fig. 6 - Various uses for r.f. chokes. (A)
R.f. suppressor. (B), (C), (D)Used as a tank coil. (E) Tuned
circuit with tap. As a transformer, an RFC can be used with
(F) mutual, (G) top C, (H) top L, (I) bottom C, (J) bottom L
It is also possible to use two chokes as primary and secondary
of a transformer (Fig. 6F). Again, the wide range of values
can be utilized for almost any transformation ratio. Depending
upon the size and type selected (magnetically shielded types
would not be suitable), the degree of coupling can be varied
over a considerable range by adjusting the relative separation
between the two choke bodies. If greater coupling is desired
than can be obtained by mutual coupling, the usual methods of
top C, top L, bottom C, or bottom L, as shown in Figs. 6F through
6J, can be utilized. However, because of the mutual inductance
that exists between two chokes in close proximity to each other,
additional coupling through a physical capacitance may actually
reduce the effective coupling because one effectively tunes
out the other (Figs. 7A and 7B). To prevent this from occurring,
sufficient separation to minimize mutual coupling will give
better control through the coupling elements, as indicated by
Fig. 7 - (A) & (B) Coupling through a
capacitor may reduce effective coupling as one RFC may tune
out other. (C) Coil separation produces controllable coupling.
(D) How RFC's are used in a typical high-pass filter unit.
It is just this capability of being coupled to a coil or
another choke that often requires that a choke be connected
so that such coupling is at a minimum. Contrary to the usual
v.h.f. and u.h.f. wiring practices, it is often necessary to
use maximum lead lengths on chokes so that they may be placed
away from tuned circuits or supply lines containing similar
or different chokes. And because of the peculiarities of the
distributed constants that make the choke what it is, the extra
lead length has little effect on its performance. Furthermore,
placing a choke near the chassis, although increasing the effective
distributed capacitance somewhat, will provide a certain degree
If the choke is magnetically shielded, so much the better,
for such a choke will not only have little or no coupling to
circuits but will also not radiate any r.f. to surrounding circuits
as is possible with unshielded chokes. However, a note of caution
is necessary. If a magnetically shielded coil is placed too
close to a tuned circuit, it can effectively load, or lower,
the "Q" of that circuit considerably because the closed ferrite
body of the choke acts as if a shorted turn were coupled to
As a series element, a choke may also be used as a trap,
either at its SRF or at some other frequency, by tuning it as
a parallel circuit with additional capacitance. As such, it
may be coupled to a circuit either magnetically or capacitively.
As a series-tuned trap, the tuning capacitance should be large
relative to the distributed capacitance of the choke so that
the effect of the distributed capacitance is minimized. If the
choke inductance is small, its effective inductance may be adjusted
by the length of its leads, as much as 10%.
Chokes may also be used, because of the small physical sizes
and wide range of inductances available, in the construction
of miniature filters. A typical high-pass filter is shown in
Fig. 7D. In a majority of cases, the capacitance required is
large, the inductance values small, and the mutual coupling
must be at a minimum. Magnetically shielded microminiature chokes,
combined with miniature ceramic capacitors, can produce a high-pass,
low-pass, or bandpass filter of incredibly small size yet with
extremely sharp bandpass characteristics and large off-frequency
R.f. chokes, like all other components, must be selected
with a bit of wisdom and a lot of understanding. In general,
most standard makes of chokes will give satisfactory results
even beyond their rated capabilities, although this is not a
recommendation to use them in that way.
As long as chokes are operated within their intended ratings,
and their specifications and intended applications are considered
wisely, there should be no problems either in experimental use
or in production.
Recent impartial tests indicate that potted, cement-coated,
or otherwise encapsulated coils tend to have a higher rate of
failure as wire diameter is decreased.
Encapsulation tends to restrict the thermal expansion of
the windings due to the different expansion rate of the encapsulating
material. These tests indicate that such a restriction causes
the wires to bite through the insulating coating and short adjacent
turns together. However, these tests also show that the failure
rate only increases rapidly above #34 AWG and at temperatures
in excess of 150°C. Furthermore, most encapsulated chokes
use materials that do not hinder expansion of the windings since
the materials are, to a wide degree, either compatible in expansion
coefficients or are thermosetting and tend to soften slightly
with elevated heat.
How to Measure
Any of several instruments may be used to determine the various
characteristics of r.f. chokes. The necessity for doing so,
however, will depend upon what kind of information is made available
by the manufacturer and just what interpretation can be made
of this information.
The approximate SRF (self-resonant frequency) may be found
quite simply with a grid-dip oscillator, provided the choke
is not completely magnetically shielded. If more than one dip
is observed, the lowest frequency indicated will be the fundamental
SRF. With this set-up, you can then alter this frequency with
additional shunt capacitance, meanwhile monitoring the change
until the desired frequency has been reached.
For any type of choke, shielded or not, a "Q" meter can be
used to determine the SRF and, simultaneously, the distributed
capacitance and the "Q" at the SRF. This last point is often
overlooked in the application of a choke and the use of the
available data. A careless reading will lead the user to believe
that the "Q" shown is at the SRF, when it is actually the "Q"
that is obtained at the frequency of measurement at which the
inductance measurement is made on the "Q" meter. Only at the
given test frequency does the "Inductance" dial read correctly,
but the test frequencies for the various inductance ranges differ
from the SRF by a factor of 2 to 20 in the normal range of miniature
choke values. The differences for one make of choke are shown
in Table 1. It is obvious from this table and from the vast
differences in "Q" for the same coil at various frequencies
that one measurement cannot substitute for the other. Of course,
the "Q" may not be important in many applications, but where
it plays a significant part, it must be determined for the SRF
or the frequency at which it is to be used.
Table 1 - Self-resonant frequency is higher
than test frequency.
The SRF is measured on the "Q" meter by the following procedure.
Connect the test leads to the "Cap" terminals. With the choke
disconnected and the leads in place, connect to the "Coil" terminals
a relatively high "Q" work coil that will resonate in the region
of the expected SRF of the choke. The choke is then connected
to the test leads on the "Cap" terminals and the "Capacitance"
dial is tuned to re-resonate the combination. If no change is
necessary, then that frequency is the SRF of the choke. If the
"Capacitance" dial must be shifted to re-resonate, then the
entire procedure must be repeated, but at a higher or lower
frequency until the SRF is found. As a guide, Table 2 shows
the relationships among the various indications.
Table 2 - The relationship between choke
and "Q" meter tuning.
The effective parallel resistance Rp and effective
parallel reactance Xp at any frequency may be found
by a similar method, except that the process used to pinpoint
the SRF is not necessary.
Adjust the "Q" meter for resonance at the test frequency
F and record the indicated "Q" as Q1 and capacitance as C1.
Remove the test leads from the "Cap" terminals and record the
increase in capacitance needed to re-resonate the "Q" meter
as C2. Reconnect the leads and the choke to be measured, and
re-resonate at F. Call the new reading Q2.
The effective parallel resistance is found from Rp
= [(1.59 x 105) (Q1Q2)] / [f (C1 + CL) (Q1 - Q2)]
and the effective parallel reactance is found from Xp
= (1.5 x 105) / f (C2 - C1) where f is in MHz and
C is in pF. The impedance is
at any frequency, and Z = Rp at the SRF since Xp
= 0 and
Under some conditions (C2 - C1) may be negative, indicating
a capacitive reactance for the choke at that frequency.
The "Q" is found from "Q" = [ (C2 - C1) (Q1Q2)] / [C1 (Q1-Q2)]
and is the "Q" of the unknown impedance, or in this case the
choke being tested. This is obtained from the relation "Q" =
The distributed capacitance may be measured with the "Q"
meter by the two-measurement method when the distributed capacitance,
Cd, is 10 pF or less, as follows.
Connect the choke to the "Coil" terminals and resonate the
choke by varying the "Frequency" dial, with the capacitance
set at a low value (30 to 50 pF). Note the frequency and call
this capacitance C1. Change the frequency setting to one-half
the previous frequency setting and re-resonate the coil by varying
the "Capacitance" dial. The new value is called C2. The distributed
capacitance, within 20%, is found by Cd = (C2-4C1)
A much simpler method of determining the approximate value
of Cd is from a resonance chart, where frequency,
inductance, and capacitance are shown. As long as Cd
is small, it has a negligible effect on all but very small values
of inductance. For this reason, the apparent inductance will
be close to the true inductance, and the chart would probably
show the required capacitance to resonate the apparent inductance
with as good an accuracy as the "Q" meter.
For example, a 10-μH ±10% choke measured by the above
method had an apparent inductance of 10.7 μH and a distributed
capacitance of 0.67 pF. The "Q" at SRF was not measured because
the SRF was above the range of the "Q" meter.
From a resonance chart, the apparent inductance given in
the manufacturer's data sheet gives a capacitance value slightly
The actual inductance that would be measured if the choke
had no distributed capacitance at all is found from the value
obtained in the Cd measurement.
The true inductance is LT = LA [1[C1
/ (C1 + Cd)]. In the case of the 10-μH choke just
mentioned, LT = 10.7 [38/(38+.67)] = 9.83 μH where
C1 is the capacitance required to measure the apparent
inductance, 38 pF in this case; and LA is the apparent
Thus, the SRF can be determined from the values of LT
and Cd, or the approximate Cd can be obtained
from the approximate SRF and a resonance chart. Because of the
nature of r.f. chokes and their various circuit requirements,
these approximations will quite often be adequate.
Posted November 27, 2014