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(11) | EP 0 848 391 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Power coil |
| (57) A coil having a segmented core where segment spacing is much smaller than segment
length may be adjusted to keep hum modulation lower than otherwise when a line containing
such coil is connected to a line carrying RF. |
[0001] 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.
[0002] The type of power coil with which the invention is concerned is called a choke or choke coil.
[0004] 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.
[0005] 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.
[0006] 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).
[0007] 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.
[0008] 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.
[0009] 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.
[0010] At the present time, for testing, the Hum modulation is measured at 8 separate frequencies between 5 and 1000 MHZ.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] Preferably the segment spacing means is adapted to maintaining an air space between said segments.
[0017] In a preferred form of the invention the winding usually includes at least one bleeder resistor connected between spaced winding turns.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] The circuit described in the previous paragraphs usually includes: a third conductor connected to said node, carrying primarily RF signals.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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 |
[0035] 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.
[0036] 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.
[0037] The core may be constructed in any manner which will satisfy the criteria discussed. The method currently proposed is discussed.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
1. In a network where a conductor for both AC and RF is connected at a node to a second
line carrying AC only,
at least one power coil located in said second line, said power coil comprising an energizing winding surrounding a longitudinally extending ferro-magnetic core,
said core being made up of segments of ferro-magnetic material spaced by non-magnetic material.
2. In a network as claimed in Claim 1 wherein said core defines a substantially straight
axis and said segments are similar in section perpendicular to said axis.
3. In a network as claimed in Claim 2 wherein said segments terminate in opposed faces,
generally perpendicular to the axis.
4. In a network as claimed in any of the preceding claims where means maintains air spacing
between said segments.
5. In network as claimed in any of the preceding claims wherein said spacing as limited
so that it does not unduly decrease return losses at low RF frequencies.
6. In a network as claimed in any of the preceding wherein said spacing means are limited
so that said core means is long relative to said spacing means.
7. In a network as claimed in any of the preceding claims wherein said spacing means
are limited so that said spacer is less than 10% of its average segment length.
8. A power coil comprising a longitudinally extending ferro-magnetic core, said core
being made up of segments of ferro-magnetic material spaced spacers whose thickness
measured in the longitudinal direction is less than that of one of said segments.
9. A power coil as claimed in claim 8 wherein said core defines a substantially straight
axis and said segments are similar in section perpendicular to said axis.
10. In a power coil as claimed in claim 9 wherein said segments terminate in opposed faces
generally perpendicular to the axis.
11. In a power coil as claimed in any of claims 8 to 10 including means to maintain air
spacing between said segments.
12. In a power coil as claimed in any of claims 8-11 wherein said spacing is limited so
that it does not unduly decrease return losses at low RF frequencies.
13. In a power coil as claimed in any of claims 8-12 wherein along the flux axis said
segments are long relative to said spaces.
14. In a power coil as claimed in any of claim 8-13 wherein along the flux axis, said
spaces are not more than 10% of said segments average length.
15. In a network as claimed in any of claims 1-7 wherein said winding has tightly wound
turns at each end and middle turns loosely wound.
16. In a network as claimed in claim 15 having a substantially equal number of turns at
each end with the middle turns spread apart roughly the space of the wire diameter.
17. A power coil as claimed in any of claims 8-14 wherein said winding has tightly wound
turns at each end and middle turns loosely wound.
18. A power coil as claimed in claims 8-14 wherein said winding has tightly wound turns
at each end and middle turns loosely wound.