1. Field of the Invention
This invention pertains principally to signal transmission systems. More particularly, this invention pertains to a power takeoff inductor for such a system. Further, this invention pertains to a novel inductor design.
2. Background of the Invention
In signal transmission systems (such as CATV systems or hybrid fiber coax telephony systems), a signal is transmitted over a wide range of frequencies. For example, the signal may be carried over a radio frequency spectrum from below 5 Megahertz to above 1 Gigahertz. Commonly, such signals are carried over coax cables having a signal conductor surrounded by a grounded shield. In addition to carrying the signal transmission, the signal conductor will also carry a power transmission. In a typical application, power is transmitted over the signal conductor at about 60 Hertz and at a voltage between 30 and 90 volts RMS. The signal transmission is typically carried at less than 1 volt RMS.
In field applications of such signal transmission systems, it is necessary to extract the 60 Hertz power transmission without degradation of the radio frequency signals. Devices used to extract the power transmission must present a low impedance to the 60 Hertz power transmission while presenting a high impedance to the radio frequency (RF) signals. This is normally performed by an inductor (alternatively referred to as a choke) shunted directly across the incoming coaxial cable. For typical applications as referenced above, the inductor must be capable of passing in excess of 15 amperes of 60 Hertz power transmission. Also, power may be reinserted in the field for distribution to subsequent field locations in the transmission system.
A difficulty frequently encountered in power takeoff in signal transmission systems is that the inductor must present a high impedance across the entire radio frequency spectrum at which the signal is being transmitted (i.e., in the example given the inductor must present a high impedance from about 5 Megahertz to 1 Gigahertz) to avoid partially shorting the desired signal. Inductors for drawing off the power signal are available in a wide range of sizes, geometries and physical attributes. For example, such an inductor may be a coil which is air-wound (i.e., has no magnetic loading by reason of a magnetically permeable core disposed within the winding). For the applications thus described, an air-wound inductor would be prohibitively large. Also, such an inductor would require a very large number of windings to attain suitable inductance values. Moreover, the distributed capacitance between the windings of such an inductor would result in resonant circuits.
It is well known to magnetically load an inductor through use of a magnetic permeable core placed within the winding of the inductor. A magnetic permeable core affects the magnetic field of the inductor by compressing the magnetic flux lines of the magnetic field.
The use of a magnetically permeable core raises the inductance of the inductor. As a result, a physically smaller inductor with fewer windings can be used to attain the same inductance and impedance of a much larger air-wound inductor. Also, due to the fact there are fewer windings, there is less opportunity for resonance resulting from a capacitance effect between opposing surfaces of the windings. Such resonances can reduce the impedance at their natural frequencies and can degrade RF signals at these frequencies. Also, in CATV applications, a smaller length of the inductor reduces the total low frequency resistance of the inductor. This reduces the loss of the power signal which would otherwise be caused by heating of the inductor due to high currents of the power signal.
For effective loading at the lower end (i.e., 5 Megahertz in the above example) of the frequency range, high magnetic permeability material is required. Unfortunately, such materials typically present high circuit losses at high frequencies. Conversely, materials that offer good high frequency loss characteristics have lower permeability and are less effective at the low end of the frequency. Commonly, in designing power takeoffs, a compromised design is selected where a compromised material of intermediate permeability is used that has a reasonable permeability at low frequency but reasonable losses at high frequencies. Nevertheless, the design is compromised resulting in losses at high frequencies.
An alternative to a compromised inductor design is a so-called Pi-wound inductor. A Pi-wound inductor has a common core with a gap placed in the winding to move resonances out of the frequency bands. Another option is to place two inductors of different inductive values in series. One of the inductors is tuned to the low frequency (i.e., 5 Megahertz in the above example). The other is tuned to the higher frequency (i.e., 1 Gigahertz). However, it is believed these options still present a compromised design with respect to intermediate frequencies in the frequency range.