JP4947382B2 - Signal splitter circuit and cable TV network - Google Patents

Description

FIELD OF THE INVENTION This invention relates to signal splitter circuits for reducing noise influx and cable television networks incorporating such splitters.

BACKGROUND OF THE INVENTION Cable television networks are no longer purely the distribution networks used for television and radio distribution, but now provide customers with access to the network. Thus, television and radio signals are delivered from the local center or optical node by a signal splitter whose output is connected to each customer. Return traffic from each customer is returned through the splitter to the local center or optical node and from there back to the rest of the network. Such return traffic may include a pay-per-view television program request.

  Typically, traffic from a customer to a local center or optical node is referred to as “return path traffic” or “upstream signal”. The upstream signal is carried using a different frequency range than the distribution signal originating from the network provider (usually referred to as the “downstream signal”). Current cable television networks typically use 5 MHz to 65 MHz for upstream signals and 85 MHz to 862 MHz for downstream signals, although other frequency ranges are also used.

  All upstream signals are conveyed to the local center or optical node, regardless of how they occur. Therefore, unwanted noise in the upstream signal will also be injected into the network. Undesirable signals come from a variety of sources, but the main part is due to the radiation of the external transmitter within the upstream frequency range used. The combination of these undesirable signals is known as “infusion”. Most of the infusion originates from customer premises equipment and is therefore injected into the network at the customer access point. This infusion is a major problem in the network because all these undesired signals are combined and will limit the signal-to-noise ratio (and hence capacity) of the upstream signal.

  The intent of the present invention is to provide a signal splitter circuit that reduces noise ingress into the cable television distribution network.

SUMMARY OF THE INVENTION In accordance with the present invention, a signal splitter circuit comprising a signal splitter having an input and a plurality of outputs is provided, wherein alternating outputs at least partially reduce the generation of a phase shift device and intermodulation products within the phase shift device. A pre-connection filter including a high-pass filter for stopping the voltage peak through reflection of energy contained in the voltage peak is provided in the prevention circuit. When such a signal splitter is used in a cable television network, the phase shift device ensures that the noise ingress into the network in the upstream signal, i.e. the signal originating from the customer, is substantially reduced. The upstream signal consists of signals from a number of different customers, each customer signal including data and noise components. The amplitude, phase, and frequency are not related because the data components from different customers come from different subscriber equipment. However, the noise components in each customer signal are mostly similar to each other because they originate from radio frequency electromagnetic radiation captured by the same source, namely the subscriber's equipment and the cable connecting such equipment to the output of the splitter. is doing. Introducing a phase shift in the upstream signal before the upstream signal reaches the output ensures that the noise components cancel each other when the upstream signal originating from the customer is combined by the splitter.

  During manufacture, the splitter and tap are so-called demagnetized to ensure that the ferrite core is magnetically neutral. In this case, the transfer function of the passive element is as linear as possible, reducing the amount of intermodulation product generated. Passive devices are exposed to all kinds of low and high voltage impulses during their operating lifetime. These impulses induce a direct current through the windings on the ferrite core. This current generates a magnetic field that will affect the magnetic neutrality of the ferrite. Depending on the ferrite material, the intensity of the pulse and the amount of pulse, the ferrite will become magnetized over time, the effect will be cumulative and the intermodulation performance of the element will be degraded. The prevention circuit reflects the energy in the pulse to prevent the pulse from reaching the ferrite core of the phase shift device, thus creating effective protection against intermodulation performance degradation. Intermodulation arises from the strong cable modem upstream signal present in the passive element, and is typically twice or three times the frequency of the original signal. This frequency is present in the CATV downstream spectrum and will interfere with the received radio and / or television signal.

  A signal splitter with a phase shift device reduces infusion in the upstream path by phase inverting half of the passive element ports. However, 180 ° phase shift transformers are susceptible to degradation due to voltage pulses. When more than one cable modem is connected to the same passive component (eg set top box or VoiP box), the generated intermodulation products are present in the upstream spectrum as well as in the CATV downstream spectrum become. The generated intermodulation product is considered as upstream infusion, which will seem to reduce the improvement in infusion level due to splitter technology.

  By combining the prevention circuit with a signal splitter and a phase shift device, the resulting signal splitter circuit is more effective when using more than one cable modem. This is because the phase shift transformer is effectively protected against intermodulation degradation due to voltage pulses.

  The prevention circuit has the foresight that the relatively low signal-to-noise ratio of the data signal in known communication systems is due in part to signal processing means or components incorporated in the (especially bidirectional) cable transmission network. Is based. The cable modem used generates a powerful RF signal capable of generating harmonics or intermodulation products within the component (especially in the passive component). These undesirable products interfere with normal data signals and adversely affect the signal-to-noise ratio of these normal data signals. The generation of harmonics or intermodulation products is prevented in whole or in part by providing the signal processing means with a prevention circuit for at least partially preventing the generation of intermodulation products in the signal processing means. Can have a positive effect on the signal-to-noise ratio of the data signal.

The prevention circuit is further based on the following foresight: the greatest problem with the degradation of harmonics or intermodulation products arises with passive components comprising ferrite transformers and / or connectors. The ferrite element usually has a non-linear transfer function due to ferrite saturation. The saturation can already occur with a relatively low level input signal when the ferrite is affected by a magnetic field. The magnetic field can be an external magnetic field or a magnetic field generated by current flowing through the transformer winding. Voltage peaks can introduce such currents into the transformer, causing the ferrite to already saturate at lower transmit or signal levels as a result of which harmonics or intermodulation products can be generated earlier. . This can be prevented by preventing the appearance of the voltage peak by means of a filter at the input of the signal processing means that reflects the energy contained in the voltage peak.

  In general, voltage peaks in cable transmission networks, such as cable television (CATV) networks, can be a major source of problems. The peak may be due to lightning strikes or equipment connected to the network. On the other hand, the voltage level of the voltage peak generated by the connected device is relatively low.

  It is a known fact that gas discharge tubes and very fast varistors can be used as protection against voltage peaks with high voltage levels. The known protection means have the following disadvantages. Firstly, the known protection means provide no protection against the harmful effects of voltage peaks with low voltage levels. Secondly, the known protection means generate a strong magnetic field to neutralize the voltage peak by a short circuit where a very high short circuit current of 1000 amperes or higher can flow through the protection means, If the protective means are incorporated in passive components, the magnetic field can adversely affect the intermodulation behavior of such passive components. In addition, if the load exhibits inductive and / or capacitive behavior (which will generally be the case), a short circuit causes a voltage peak (electrostatic discharge (ESD) pulse with a high voltage level and a low energy level. Occurrences) can be brought about. Under certain circumstances, these newly generated voltage peaks with high voltage levels and low energy levels can even cause permanent damage to certain components in the cable transmission network.

  Rather than using a gas discharge tube or varistor, the prevention circuit as used in the communication system according to the present invention uses a filter to prevent voltage peaks from entering the component by reflecting energy. In this configuration, there is no problem of a short circuit and the accompanying very high short circuit current (the voltage peak energy is reflected and there is no current flow), so there is no problem of generating a (high) magnetic field. As a result, the above voltage peak having a high voltage level and a low energy level is not generated.

  As a result of the high-pass behavior of the filter, protection is provided for both high voltage level and low voltage level voltage peaks.

  One embodiment of the communication system according to the invention is characterized in that the high-pass filter comprises an LC filter comprising at least one coil and at least one capacitor. In an advantageous embodiment, the pre-connection filter can be an LC filter composed of a coil and a capacitor, and it has been found that the capacitor is preferably a high-voltage capacitor having a low temperature coefficient.

  Preferably, the phase shifter introduces a 180 ° phase shift so that when the signals are combined, the noise components of the alternating outputs are out of phase and almost completely cancel each other.

Each phase shift device may comprise a phase shift transformer.
If the splitter has an even number of N outputs, N / 2 phase shifters are required, where N / 2 is an integer. If the splitter has an odd number of X outputs, the number of phase shift devices used will be the nearest integer above or below X / 2.

If desired, the phase shift device can be permanently connected to the respective output and can be fixed to the output in a common housing and is therefore built into the splitter.

Alternatively, the phase shift device may be separable from each output.
In accordance with another aspect of the present invention, a cable television network is provided that incorporates a plurality of signal splitter circuits, and the phase shift device in use is a noise stream that enters the network in an upstream signal, i.e., a signal originating from a customer. It acts to ensure that the bond is substantially reduced.

The invention will now be described by way of illustrative examples and with reference to the accompanying drawings in which:
The prior art signal splitter 10 of FIGS . 1 and 2 has an input 12 and multiple outputs, of which only a first output 14 and a second output 16 are shown for clarity. In use, these passive signal distributors 10 act as an interface between a local center or node and a number of customers, each customer connected to one output of the splitter 10 and the splitter input 12 connected to the node. Arrow 18 represents the transmission of a television signal (downstream signal) from the service provider to the input of the splitter, where the signal is distributed or branched for transmission to further customers, and arrows 18a and 18b are It represents the transmission of a branched television signal from the first and second outputs 14,16.

  Dotted arrows 20a and 20b represent the return transmission of the data signal (upstream signal) from the first and second subscribers to the first and second outputs of the splitter. The splitter combines the data signals from all connected subscribers and applies them to the input of the splitter. Dotted arrow 20c represents the transmission of all composite data signals from the splitter input to the service provider.

  Referring to FIG. 2, short dotted arrows 22a and 22b represent noise components present in the data signal transmitted from the subscriber to the first and second inputs of the splitter. The splitter 10 not only synthesizes the preferred data signal, but also synthesizes the noise components and applies them to the input 12 of the splitter. The long dotted arrow 22c represents the transmission of the composite noise signal from the splitter input to the service provider.

  If there are many outputs, the combined noise component applied to the splitter input (and thus sent from the splitter input to the service provider) will be large compared to the data signal, and thus up between the splitter and the service provider. The signal transmission capacity of the stream channel is reduced. As an example, assume that 1000 customers are connected to a single local center or optical node. If all customers produce the same amount of infusion, the total signal to noise ratio at the local center or light point will degrade by a factor of 1000, or 30 dB.

  A splitter circuit according to the present invention is shown in FIG. 3 and is connected to the input 26, a plurality of outputs in which only the first output 28 and the second output 30 are shown for ease of understanding, and the first output 28. A splitter 24 having a plurality of phase shift transformers connected to alternating outputs, only shown transformer 32 is shown. Each transformer is connected to only one output. The phase shift transformer can be built into the splitter and permanently associated with each output. Alternatively, the transformer can be externally connected to the existing output.

The phase shift transformer 32 introduces a 180 ° phase shift into the signal passing therethrough. Thus, the branched television signal applied to the first output 28 is transmitted to the subscriber after the phase is shifted by 180 °, and the data signal transmitted by the subscriber's equipment connected to the first output 28 is It is applied to the first output 28 after the phase has been shifted by 180 °.

  As explained above, the data signal transmitted by the subscriber to the output of the splitter contains a noise component. The noise component has a variety of sources, the largest of which is radio frequency electromagnetic radiation that can be captured by the cable connecting the subscriber equipment and the output of the splitter to the subscriber equipment. In most cases, the source of radio frequency electromagnetic radiation captured by one such cable or subscriber's equipment will be captured by a number of other such cables or subscriber's equipment.

  The signal characteristics of the noise component will be very similar because most of them originate from the same source. The noise component will have almost the same frequency, amplitude and phase. Phase shift transformers connected to the alternating outputs of the splitter produce two groups of noise components. The noise components of both groups have almost the same frequency and amplitude, but the noise components of the first group are in antiphase with the noise components of the second group. When the noise components of both groups are combined, they cancel each other, thus greatly reducing the noise component of the combined signal applied to the splitter input.

  Preferred data signals originating from customers are unaffected because the amplitude, phase and frequency are not related because the data components from different customers originate from different subscriber equipment. They are therefore not reduced by synthesis after phase shifting. The downstream signal is also unaffected by the phase shift, so the use of a phase shift transformer mounted between the splitter output and the network connection branch attenuates the infusion, but the preferred downstream and upstream signals Is not affected.

  Of course, there are also several localized sources of radio frequency electromagnetic radiation that can only be captured by one subscriber's equipment or one cable, such as an electric motor in an appliance in the subscriber's home. Even if a phase shift is introduced, such a noise component cannot be reduced.

  Many houses have connections to the two outputs of the splitter, one connection for cable television and the other for telephone or internet services. If one connection is to the output of a splitter with a phase shift transformer and the other connection is to an output without such a transformer, then a localized source of radio frequency electromagnetic radiation Even the noise component caused by even can be reduced.

  A prevention circuit 40 is connected to the input 26 and each output 28,30. For each output 28, the transformer 32 is connected between the splitter 24 and the corresponding prevention circuit 40.

  4 and 5 show two embodiments of a prevention circuit in the form of a high-pass filter 40 that can be arranged in or in front of a signal processing means or component as a pre-connection filter (prevention means). The high-pass filter 40 shown in the figure is an LC filter each comprising an input 42, an output 44, and one or more coils 46 and a number of capacitors 48 disposed between the input and the output. With. Preferably, but not necessarily, the capacitors 48 are all high voltage capacitors having a relatively low temperature coefficient. Other filter configurations are also possible: a higher order filter based on the same principle (Chebyshev) or a filter based on other principles (Cauer filter or elliptical filter) may be used.

  The coil 46 and capacitor 48 in the high pass filter 40 shown in FIGS. 4 and 5 preferably have the following values: coil 46 is 3.3 μH, capacitor 48 is lnF / 2 kV / Y5E, capacitor 48 ′ is preferably Is 470.

  The high pass filter 40 shown in FIG. 5 provides more protection against voltage peaks than the high pass filter shown in FIG. However, the filter shown in FIG. 5 is more complex, expensive and takes up more space than the filter shown in FIG. In particular, the latter aspect is important when a filter is built in a CATV component.

  In order to evaluate the behavior of the filter shown in FIG. 4, the filter was built into a standard CATV insulator. Such insulators are generally used as terminal connection points between CATV networks and home appliances, thus forming a suitable point for construction in lightning protection measures. Home appliances may include, for example, amplifiers, cable modems, set top boxes, video recorders and televisions.

  In the first experiment, a standard IEC 1000-4-5 level 2 pulse (1 kV, 1.2 μs / 50 μs) was applied to the input of such a standard CATV insulator with and without the filter shown in FIG. ) Was sent. In the case of an unprotected CATV insulator (i.e. without a pre-connection filter), the voltage peak is transmitted to the house equipment without practically no attenuation by the CATV insulator (1 kV peak drops to 0 V at 180 μs) It appears that this can cause serious damage to the components and equipment in the installation. On the other hand, the protected CATV insulator (ie with the pre-connection filter) attenuated the voltage peak to a voltage peak with a low voltage level and a low energy level (40V peak drops to 0V in just 0.2 μs). Even very sensitive components or equipment are not damaged by this attenuated voltage peak.

  The reduction in the intermodulation behavior of the passive CATV component (in this case a standard CATV splitter) is due to the 40 MHz, 118 dBμV (75 ohm) input signal after all the gates of the passive CATV component have undergone a 25 V DC / 500 μs voltage peak. This was experimentally determined in the second experiment by supplying the second harmonic. It has been found that CATV splitters protected by a pre-connection filter as shown in FIG. 4 do not show any reduction in intermodulation behavior compared to CATV splitters that were not previously fed with voltage peaks. On the other hand, an unprotected CATV splitter exhibits a 10 dB intermodulation behavior degradation.

Even if a series of five consecutive IEC 100 0 -4-5 level 2 voltage peaks are pre-applied to the protected CATV splitter, no decrease in intermodulation behavior can be observed. In the case of an unprotected CATV splitter, this situation leads to at least 25 dB degradation of the intermodulation behavior.

  Further experiments have shown that a filter as shown in FIG. 4 has an extremely low insertion loss from 0.5 dB to less than 1000 MHz and at the same time has a very good return loss greater than 20 dB.

  The successful operation of the signal splitter circuit of the present invention relies on the similarity between the noise components of the data signal applied to the output of the splitter. If the noise components are of different amplitudes or experience different phase shifts during transmission from the subscriber's equipment to the output of the splitter, the reduction of the noise components in the composite data signal will be less noticeable. However, if the noise component is reduced by only 3 dB, the data transmission capacity of the upstream signal channel can be doubled.

If the output of the splitter is an odd number, the reduction of the noise component is not so noticeable. In this case, the number of phase shifters attached to the output should be as close as possible to half of the number of outputs, eg two or three phase shifters for a splitter with five outputs. Of course, for a splitter with a larger odd number of outputs, the effect of having a phase shifter attached to an output that is slightly less or more than half of the output of the splitter decreases as the number of outputs increases.

1 is a schematic diagram of a prior art signal splitter. FIG. 1 is a schematic diagram of a prior art signal splitter. FIG. 1 is a schematic diagram of a signal splitter circuit according to the present invention. FIG. 4 is a circuit diagram of a prevention circuit used in the signal splitter circuit of FIG. 3. It is a circuit part of the prevention circuit used in the signal splitter circuit of FIG.

Explanation of symbols

  24 splitter, 26 inputs, 28 first output, 30 second output, 32 transformer, 40 prevention circuit.

Claims (11)

A signal splitter circuit for use in a cable television network, comprising a signal splitter having an input connectable to a cable network and a plurality of outputs,
It said plurality of outputs that are alternately arranged in the output is connected to a protection circuit for reducing the generation of intermodulation products of the phase shift device and in the phase shift device,
Each of the phase shift devices is connected between a corresponding alternating output and the prevention circuit such that the phase shift device and the prevention circuit are downstream of the output,
A signal splitter circuit, wherein the prevention circuit is provided with a pre-connection filter comprising a high-pass filter for stopping the voltage peak moving upstream . The signal splitter circuit of claim 1, wherein the prevention circuit is connected to a downstream of each of the plurality of outputs . The is coupled to the input of the upstream signal splitter further comprises a further prevention circuit for stopping voltage peaks to move downstream, the signal splitter circuit of claim 1. The signal splitter circuit according to claim 1, wherein the high pass filter comprises an LC filter including at least one coil and at least one capacitor. The signal splitter circuit according to claim 4, wherein the capacitor is a high voltage capacitor. 6. The signal splitter circuit of claim 5, wherein the high voltage capacitor has a relatively low temperature coefficient. The signal of claim 1, wherein each of the phase shift devices introduces a 180 ° phase shift such that when the signals are combined, the alternating output noise components are out of phase and substantially cancel each other. Splitter circuit. The signal splitter circuit of claim 1, wherein each phase shift device comprises a phase shift transformer. The signal splitter circuit according to claim 1, wherein the phase shifter is permanently connected to each corresponding output, fixed to the corresponding output in a common housing, and built in the signal splitter circuit. The signal splitter circuit of claim 1, wherein the phase shift device is separable from its corresponding output. A cable television network, each incorporating a plurality of signal splitter circuits according to claim 1 .