This invention relates to a novel AC power coil for use in a circuit which is
connected to an RF circuit. The invention also relates to the method of making such
coil and of using it.
The type of power coil with which the invention is concerned is called a choke
or choke coil.
Herein a 'coil' is spoken of as comprising a 'core' and a 'winding'.
To place the invention in context there are a large number of applications, the
most common of which may be CATV distribution networks where the same
conductor carries 60 Hz (50 Hz in some jurisdictions) AC signals and also RF
signals in the 5-1000 MHZ range. The 60 Hz or 50 Hz signals herein are called
"Alternating Current" ('AC') signals herein to distinguish them from the 5-1000 MHZ
signals called herein "Radio Frequency", ('RF') signals.
The invention is also thought to be useful in the broadband network known as
a Hybrid-Fibre-Coax system. Such a system frequently carries both RF digital and
video signals.
A broadband distribution CATV network is common where a conductor for
both AC and RF is connected at a node to a second line carrying AC only and to a
third line carrying RF only. The AC line will carry one or more power coils,
heretofore made with solid ferro-magnetic cores. (Solid ferro-magnetic cores in a
coil may also be used as an alternative power coil where a CATV component using
an air-cored coil might be used).
With the arrangements mentioned in the previous paragraph it is found that
the power coil, due (we believe) to resonances in the cores or winding, provides to
the RF signal of a connected conductor, modulation in the frequency harmonics
60m, 120, 180... Hz (or 50, 100, 150)... Hz, that is the low order harmonics of the AC
signal. This modulation if greater than a predetermined value will cause lines on a
TV screen, and is known as Hum modulation. Hum modulation can also impair
digital signals including Vestigial sideband and Quadrature Amplitude Modulation,
(Q.A.M.). High definition television (HDTV) digital audio and data traffic such as
digital telephony and Internet conductivity can be subject to losses of data from hum
modulation. The Hum modulation tends to increase cumulatively as AC modulated
RF signals travel downstream or upstream through the distribution system.
Where an AC path containing a coil is connected to an RF path and an
AC/RF path at a node, the fact that the results of the presence of an AC coil in the
AC path create Hum Modulation on the RF signal seems to be independent of either
the RF or AC direction at the node.
The presence of hum modulation indicates a lower power capacity of the AC
coil. The strength of Hum modulation must be kept as small as possible. Although
the FCC (U.S.) has set the performance level at -32 dB and the Department of
Communication (DOC) (CA) level is set at -40 dB relative to carrier signal, it may be
that the permissible limit will be set at (for example) -70 dB at 10 amperes relative to
the carrier signal.
At the present time, for testing, the Hum modulation is measured at 8
separate frequencies between 5 and 1000 MHZ.
In accord with the invention, in the power coil, the longitudinally extending
core is located inside the energizing winding which creates a flux path extending
longitudinally therealong. The core is made up of segments of ferro-magnetic
material preferably spaced by non magnetic material (such as an air gap or
insulation layer) extending across the flux path.
This arrangement is chosen to provide a number and spacing of segments
which acts to lower the Hum modulation. Lowering the hum modulation indicates a
higher capacity of the coil to carry AC current.
Preferably the core is substantially straight, defining a longitudinal axis, and
said segments are shaped and arranged to define a constant section when taken
perpendicular to said axis.
Preferably the core segments as described in the previous paragraph are of
constant cross-section ('section' hereafter) in directions perpendicular to the
longitudinal axis and the faces perpendicular to said axis are substantially planar.
Preferably the AC power winding is thick enough to be structurally self
sustaining and includes means for mounting adjacent segments in position and
spaced from each other and preferably said mounting means are non-magnetic.
Preferably the segment spacing means is adapted to maintaining an air space
between said segments.
In a preferred form of the invention the winding usually includes at least one
bleeder resistor connected between spaced winding turns.
In the alternative described previously each of: the bleeder resistor locations
and turn connections, the segment spacing; and the spacing between the winding
and the ferro-magnetic core segments are selected so they do not unduly decrease
return loss at low RF frequencies.
In one aspect of the invention the method of preparation of an inventive,
power coil comprises: preparing a plurality of ferro-magnetic segments for forming a
core in combination with non-magnetic spacer elements, shaping said spacers and
said core segments to form said core, and providing a conducting winding for
surrounding said core. The coil is preferably manufactured by: preparing a winding
for an AC power coil which is self-supporting for surrounding a core to define a flux
axis therein, preparing a core formed of ferro-magnetic segments spaced by non-magnetic
material and inserting said core in said winding.
The invention also comprises: an electric conductor at a node wherein a first
conductor carrying first, primarily AC signals meets a second conductor carrying
mixed AC and RF signals, wherein said first conducting circuit includes a coil having
a longitudinally extending core, and a winding arranged to create a flux path in said
core, along a longitudinal axis, wherein said core is divided into spaced segments,
preferably spaced by spacing means which modifies said flux patterns.
The circuit described in the previous paragraphs usually includes: a third
conductor connected to said node, carrying primarily RF signals.
The invention also extends to a method of reducing Hum modulation in an RF
circuit connected to an AC circuit containing a power coil, including the steps of
installing a power coil core divided into segments spaced along the flux axis.
In drawings which illustrate preferred embodiments of the invention:
Figures 1-4 show examples of circuits wherein one or more nodes represent
the meeting of a conductor having a power coil therein and connected to carry AC,
connected to a conductor carrying RF signals and a conductor carrying mixed AC
and RF signals, Figure 5 shows a method of assembling a power coil core, Figure 6 shows the installed arrangement of the core of Figure 5 in a power
coil, Figure 7 shows a graph and table indicating a relationship between number of
core segments and the Hum modulation. Figure 8 shows a graph and table indicating a relationship between segment
spacing and Hum modulation.
Figures 1-4 illustrate circuitry providing alternative examples where a
conductor A carrying mixed AC and RF signals meets at a node N with a conductor
D carrying RF signals and conductor E carrying AC signals. It is not found that the
results of use of the invention are materially affected by the direction of the AC or the
RF signals at any of the nodes.
Thus the design of the power coils L is designed to reduce Hum modulation in
a line A or a line D. The capacitors C1 substantially prevent the passage on
conductor D of AC signals of the fundamental frequency, e.g. 60 Hz.
The power coils L effectively prevent the passage of RF. Figure 1 shows a
preferred configuration for a power inserter which supports RF/AC at ports P1 and
P2 and AC (insertion) at port P3. The shunting capacitors C2 prevent material
amounts of RF being carried or picked up on the AC path. The fuses F act as
means of breaking the AC path and providing current limiting protection. Most
applications where the power coil will be used will allow AC current blocking capacity
either through fuses or a simple jumper that can be removed to break the AC path.
Figure 2 shows a single secondary port P6 supporting both AC and RF, as do
ports P4 and P5.
Figure 3 shows circuitry having secondary RF at ports P8 and secondary AC
circuitry at ports P10 where, as in the other circuits, capacitors C2 act as RF
grounds for the AC circuits. Ports P8 may be secondary tap ports, and, in accord
with CATV will commonly total 2, 4 or 8.
Figure 4 shows a circuit where a single power coil L is located in the AC
circuit and blocking capacitors C1, prevent material amounts of AC in the RF circuit.
Figure 7 shows the result of measurement of Hum modulation at arbitrarily
selected frequency intervals in circuits such as those in Figures 1-4 as indicated. In
Figure 7 the segments in samples 152462 and 152463 were spaced at about .008"
by paper. At this stage the spacing amount was arbitrary.
Considering that -70 dB (at 10 AC amps) relative to RF signal peak is
considered a suitable level, Figure 7 indicates that 6 segments give better results
than 3 or 1 (1 being unsegmented). Figure 7 also indicates that the level of Hum
Modulation is not suitable at 5 MHZ and at 1000 MHZ for 6 segments even though,
the six segments core gives generally good results.
Figure 8 indicates testing of various spacings using 6 segments. From this it
may be determined that at lower frequencies Hum modulation is worst at small
spacing while over most of the range, the results are better with .008" spacing.
Figure 8 shows that when the spacing is further increased, further increased
improvements in hum mod occurred.
The amount of the spacing cannot be increased indefinitely however since, at
a certain spacing, deterioration might be expected because of the effects of higher
reluctance, decreasing the inductance of the core so that there may, at some
frequencies, may be an unacceptable reflection for the RF signal at a node, causing
undesirable return losses.
The following Table is the return loss performance reduction in the 5 to 10
MHZ region related to the changes on the 6 segment power coil for changing
spacing after averaging the results for various frequencies across the band.
Spacing Inches | Return Loss dB |
Solid | 17.9 |
0.000" | 16.5 |
0.004 | 15.6 |
0.008 | 14.2 |
0.020 | 10.7 |
0.030 | 9.1 |
As can be seen, the return loss suffers greatly as the spacing increases. Care must
be taken in trading off the hum modulation performance and the bottom (low
frequency) end return loss performance.
The minimum spacing of a winding from the core is usually the thickness of
the insulation layer on the winding. The widening of the spacing is believed to lower
the amount of Hum modulation. However the spacing will also reduce the coupling
between the winding and the core. Such loss of coupling will again reduce the
inductance as encountered by the RF signals and again tend to cause undesired
reduction of return losses in RF signals at the node.
In order to develop a power coil 15 it is desired to provide a design which will
use a core with spaced segments and preferably provide spacing between the
winding and the core. As will be seen, the winding is preferably self sustaining and
will support the core therein on a printed circuit board 11.
The core may be constructed in any manner which will satisfy the criteria
discussed. The method currently proposed is discussed.
The ferrite may be molded into desired segment shapes and fired at high
temperature.
The ferro-magnetic core material, preferably ferrite may also be obtained in a
constant section when viewed in the longitudinal direction, which corresponds to the
direction of the magnetic flux.
The preferred core material may then be sawn into parallel segments of the
desired length, with faces perpendicular to the axis of the core material which
preferably coincides with the prepared axis of the core. The length of the core
segments is selected having regard to the desired number of core segments and the
desired spacing between them. In the invention as shown in Figure 5 and 6, seven
segments 13 of ferrite were selected and lengths and spacing chosen so that the
Hum modulation is as small as possible.
The spacing of the segments 13 could be provided in a number of ways,
easily available to those skilled in the art. However we prefer to supply a cylindrical
plastic holder 15 for the segments formed of two semi-cylindrical axially extending
halves. Ridges 17 inside the holder provide the desired spacing between the
segments. The two semi-cylindrical halves may be thinly joined, separate or hinged.
With the segments 'loaded' in one of the semi-cylinders the other semi-cylinder may
be closed on the first. If they are separate, or break during co-assembly, they may
still be held in assembled form for axial insertion in the winding L whose inside
diameter may be brought closely enough to the outside diameter of the core that the
latter is frictionally retained in place. Thus the plastic shell thickness TH sets the
spacing of the winding from the core. A bleeder resistor 19, as well known to those
skilled in the art is connected between spaced turns T [as] and tends to prevent the
build-up of capacitance and inductor resonances which would obviously cause a
loss of inductance and performance.
A limited increase of the spacing of the coil and core was found to reduce the
number of bleeder resistors required. However this increased spacing is subject to
limitation because too much spacing decreases the coupling and which tends to
reduce the impedance match causing reduction in return losses. Reduction in return
losses causes increased Hum modulation so that a balance must be found between
the shell thickness, and the reduction of the number of bleeder resistance circuits
without undue reduction in return losses.
Parameters: The plastic shell need not be used but may be replaced by
anything which will maintain the segment spacing, the segments in position and the
segment spacing form the coil. Thus the replacement or equivalent may be any
element which will accomplish these things in combination with the coils (as here) or
otherwise.
The ferrite of the segments may be replaced by another suitable ferro-magnetic
material.
The segment spacing, here performed by air may be provided by any non-magnetic
material, e.g. paper, plastic, ceramic, etc. or a combination thereof.
Coil inductance drops as gaps in the coil are introduced. For good
performance the gaps must be less than 10% of the average core segment length.
Inductance may then be achieved by a core length of about 1.5". The optimum
number of segments is believed about 5. (The 1.5" core length recovers some of the
inductance lost when a coil of originally 1" long is segmented.
The winding spacing from the core here provided by a plastic shell may be
any material, e.g. air, rubber, which does not interfere with performance.
Care must be taken to determine in replacing one spacing material with
another what changes must be made in the spacing dimension. Small segment
spacing may be achieved by coating individual segments polysol 25 or other suitable
non-magnetic coating preferably of 0.001" to 0.002" giving a spacing of 0.002" to
0.004" between segments, which is presently believed to give the best result.
The winding should have the end turns ET at each end tightly wound and the
middle turns MT are loosely wound. Preferably there are three tightly wound turns
(collectively ET) at each end with the middle turns MT being spread apart roughly
the space of the wire diameter. This winding configuration is believed to create
parallel resonances near the upper frequency of the bandwidth utilizing inter winding
capacitance from close wound turns. These parallel resonant circuits at each end of
the coil have a high impedance which helps decouple the lead leakage inductance
from the body capacitance thus extending the upper bandwidth limit. The damping
resistor is typically 470 ohm.