WO2020079322A1

WO2020079322A1 - Signal splitter

Info

Publication number: WO2020079322A1
Application number: PCT/FI2018/050760
Authority: WO
Inventors: Marc Labro
Original assignee: Teleste Oyj
Priority date: 2018-10-17
Filing date: 2018-10-17
Publication date: 2020-04-23

Abstract

A signal splitter (300) for use in a cable television network, the splitter (300) comprising an input (302) connectable to a node of cable television network; at least a first output port (304) and a second output port (306), each output port being connectable to a subscriber equipment; signal splitting means for splitting an RF signal obtained from the input into a first frequency range and a second frequency range, the first frequency range being partly overlapping with lower frequencies of the second frequency range; wherein the signal splitting means are configured to provide the first output port (304) with the RF signal on the first frequency range and the second output port (306) with the RF signal on the second frequency range; and wherein the second frequency range comprises a first sub-frequency range and a second sub-frequency range and a frequency gap between the first and second sub-frequency range.

Description

SIGNAL SPLITTER

Field of the invention

The invention relates to a signal splitter for a cable television network.

Background of the invention

Multimedia wall outlets are used in cable television (CATV) networks in which RF signals, or at least the last part of the signal path, are transmitted via a coaxial cable. A multimedia wall outlet has traditionally comprised connectors for TV, radio and data signals for supplying various broadcast and Internet applications to the subscribers of the CATV network, but lately there has been a trend to deliver all signals via a common connector.

The CATV downstream signals (television signals from the headend to the customers) are typically modulated to the frequency range of 85 - 862 MHz and the CATV upstream signals (data and service control signals from the customers to the headend) are typically modulated to the frequency range of 5 - 65 MHz. In most of the current HFC (Hybrid Fiber Coax) networks, the downstream path has been frequency- limited such that the upper frequency edge is limited to, for example, 862 MHz or 1006 MHz, in any case typically below 1.2 GHz.

However, the frequency bandwidth used to offer services to subscribers in CATV networks is increasing to 1.8 GHz and beyond. Thereby, a larger extent of data services can be provided to a customer premise equipment (CPE) connected to the wall outlet in the subscriber premises. Nevertheless, the CATV operators must still be able to the CATV services at frequency range of 85 - 862 MHz.

For accessing both the CATV services and the data services, the wall outlets may be provided with CATV modules with two outlet connectors for different services. The increased frequency bandwidth of the data services poses challenges to the implementation of signal splitters used in the CATV modules. Brief summary of the invention

Now, an improved arrangement has been developed to reduce the above-mentioned problems. As aspects of the invention, we present a signal splitter adaptable to a cable television network and a CATV module, which are characterized in what will be presented in the independent claim.

The dependent claims disclose advantageous embodiments of the invention.

According to an aspect of the invention, there is provided a signal splitter for use in a cable television network, the splitter comprising an input connectable to a node of cable television network; at least a first output port and a second output port, each output port being connectable to a subscriber equipment; signal splitting means for splitting an RF signal obtained from the input into a first frequency range and a second frequency range, the first frequency range being partly overlapping with lower frequencies of the second frequency range; wherein the signal splitting means are configured to provide the first output port with the RF signal on the first frequency range and the second output port with the RF signal on the second frequency range; and wherein the second frequency range comprises a first sub frequency range and a second sub-frequency range and a frequency gap between the first and second sub-frequency range.

According to an embodiment, a lower frequency edge of the frequency gap substantially corresponds to a higher frequency edge of the first frequency range.

According to an embodiment, the signal splitting means comprise a first diplexer comprising a first highpass filter and a first lowpass filter, wherein said frequency gap is configured between a cutoff frequency of the first lowpass filter and a cutoff frequency of the first highpass filter. According to an embodiment, a lowpass filtered RF signal obtained from said first lowpass filter is conducted to a splitter; and the lowpass filtered RF signal obtained from a first output of the splitter is conducted to a second highpass filter in order to provide the first output port with the RF signal on the first frequency range.

According to an embodiment, the second highpass filter has a cutoff frequency above a frequency range of CATV upstream signals.

According to an embodiment, a highpass filtered RF signal obtained from said first highpass filter is conducted to a second diplexer; and the lowpass filtered RF signal obtained from a second output of the splitter is conducted to said second diplexer in order to provide, from an output of the second diplexer, the second output port with the RF signal on the second frequency range comprises the first sub-frequency range and the second sub-frequency range and the frequency gap between the first and second sub-frequency range.

According to an embodiment, the second diplexer comprises a lowpass filter substantially similar to the lowpass filter of the first diplexer and a highpass filter substantially similar to the highpass filter of the first diplexer.

According to an embodiment, the first frequency range of the RF signal provided in the first output port substantially corresponds to a frequency range of CATV downstream signals, such as 85 - 862 MHz; the first sub-frequency range of the RF signal provided in the second output port substantially corresponds to a frequency range of CATV upstream and downstream signals, such as 5 - 862 MHz; and the second sub-frequency range of the RF signal provided in the second output port substantially corresponds to a frequency range of data signals above the cutoff frequency of the first highpass filter, such as 950 - 2000 MHz.

According to another aspect of the invention, there is provided a module attachable to a wall outlet of a CATV network, wherein said module comprises the signal splitter according to any of the above embodiments.

The other aspects, embodiments and advantages will be presented later in the detailed description of the invention.

Brief description of the drawings

The invention will now be described in more detail in connection with preferred embodiments with reference to the appended drawings, in which:

Fig. 1 shows an example of a CATV module connectable to a wall outlet;

Fig. 2 shows a prior art signal splitter in a simplified block chart;

and

Fig. 3 shows a signal splitter according to an exemplified embodiment.

Detailed description of the embodiments

Figure 1 shows an example of a CATV module 100 to be connected to a wall outlet. The CATV module 100 comprises a wall connector (at the rear side of the wall outlet, thus not shown), typically an IEC 169-2 connector to be plugged in a respective connector of the wall outlet. The CATV module 100 comprises two output connectors 102, 104, which may be, for example, IEC connectors. Inside the CATV module, there is an RF signal splitter circuitry dividing approximately 50 % of the RF signal to both ports underlying the two connectors 102,104. A skilled person appreciates that in addition to IEC connectors any type of connectors may be used in CATV modules, such as F type connectors.

Now for efficient delivery of both the CATV services and the data services on the increased frequency bandwidth, it would be desirable that a first port underlying the first connector 102 provides uni- directional communication of the CATV downstream signals at frequency range of 85 - 862 MHz, for example to a TV receiver, and a second port underlying the second connector 104 provides the whole frequency range 5 - 2000 MHz, wherein bi-directional communication includes the CATV upstream signals at frequency range of 5 - 65 MHz, the CATV downstream signals at frequency range of 85 - 862 MHz, and the data signals up to 2 GHz, for example to a CPE. It is noted that the naming convention of the first and the second connector, especially if referring to Figure 1 , can also be used vice versa; i.e. first connector 102 may be provided with the whole frequency range 5 - 2000 MHz and the second connector 104 may be provided with the CATV downstream signals at frequency range of 85 - 862 MHz.

Figure 2 shows an example of providing the frequency division using a conventional splitter circuitry configuration. The splitter circuitry 200 comprises an input port 202 for obtaining the RF signal on the whole frequency range 5 - 2000 MHz from a wall outlet. In an ideal world, the splitter 204 provided with a suitable ferrite would divide the RF signal into two branches such that one branch is connected directly to the second output port 206, thereby providing the frequency range of 5 - 2000 MHz via the second connector.

The other branch from the splitter 204 is connected to a first highpass filter 208 for filtering off the frequencies below 85 MHz, and further to a first lowpass filter 210 for filtering off the frequencies above 862 MHz. The filtered RF signal comprises the CATV downstream signals at the frequency range of 85 - 862 MHz and it is supplied to the first output port 212. A second lowpass filter 214 having a cutoff frequency of 65 MHz, which lowpass filter is further grounded via a resistor load 216, is used for providing a good return loss at the input in the attenuated frequency range of 5 - 65 MHz. Similarly, a second highpass filter 218 having a cutoff frequency of 950 MHz, which is further grounded via a resistor load 220, is used for providing a good return loss at the input in the attenuated frequency range of 862 - 950 MHz.

However, such frequency allocation between the ports poses some challenges for the splitter circuitry, whereupon designing the splitter in a conventional way, for example as shown in Figure 2, is very difficult, perhaps even impossible.

The usage of frequencies of 1800 - 2000 MHz or even more requires a revolutionary ferrite to be used together with the splitter 204 for maintaining the impedance matching while splitting the signal. One issue that needs to be considered in designing the RF signal splitter is passive intermodulation (PIM). PIM is a form of intermodulation distortion that may occur even when no active components are present, such as in splitters. PIM may occur in a variety of areas from coaxial connectors to cables or any joint where dissimilar metals occur. PIM occurs when two or more signals are present in a passive non linear device or element, such as in connectors, switches, isolators of the like, and the signals will mix or multiply with each other to generate other unwanted signals.

Conventional ferrites, such as the ones used in current CATV frequencies, have rather low PIM performance. This is problematic especially when using DOCSIS 3.x CPE’s, which may supply high upstream RF signal levels, even up to 125 dBpV. Such upstream RF signal levels could destroy the CATV services, if ferrites with poor PIM performance are used.

The splitter 204 should cover the whole frequency range 5 - 2000 MHz, which is quite challenging. There is a trade-off between good insertion loss at 2 GHz and low passive intermodulation when driven by high level upstream signals. Good behavior at 2 GHz requires very small ferrites, whereupon isolation between the branches is poor. On the other hand, good PIM performance requires bigger ferrites with special ferrite materials, but it may help having a good isolation in 85- 862 MHz range. Moreover, after some surge pulses, small ferrite intermodulation increases a lot. Manufacturing a sufficiently small size ferrite with PIM performance high enough for managing with the frequencies of 1800 - 2000 MHz or even more has turned out to be too challenging so far. A further challenge in designing the desired RF signal splitter circuitry arises from the fact that the frequency ranges used for CATV signals and data signals are adjacent to each other. For example, there may be a 6 or 8 MHz wide CATV downstream channel up to 862 MHz in the first port and a data signal transmitted to/from a CPE at a very close frequency range, such as 880 MHz, in the second port. For implementing the sufficient directivity between the ports, there must be very high isolation between the output ports. Considering the requirements for the size, number and cost of the components usable in RF signals splitter, such isolation is almost impossible to reach.

Therefore, in the advent of higher frequency ranges for data services an improved signal splitter is needed to enable the CATV operators to provide simultaneous access to both the CATV services and the data services.

Accordingly, a new signal splitter for use in a cable television network is herein introduced, the splitter comprising: an input connectable to a node of cable television network; at least a first output port and a second output port, each output port being connectable to a subscriber equipment; signal splitting means for splitting an RF signal obtained from the input into a first frequency range and a second frequency range, the first frequency range being partly overlapping with lower frequencies of the second frequency range; wherein the signal splitting means are configured to provide the first output port with the RF signal on the first frequency range and the second output port with the RF signal on the second frequency range; and wherein the second frequency range comprises a first sub-frequency range and a second sub-frequency range and a frequency gap between the first and second sub-frequency range.

Thus, a frequency gap is arranged in the broadband RF signal to be provided in the second output port, thereby dividing the broadband RF signal into a first sub-frequency range and a second sub-frequency range, wherein there is a frequency gap between the first and second sub-frequency range. Since the RF signal on the first frequency range to be provided in the first output port are partly overlapping with lower frequencies of the second frequency range; i.e. the first sub-frequency range, the frequency gap enables to provide sufficient isolation between the first output port and the second output port. More, with the signal splitting means as disclosed herein, the usage of a ferrite at the input is advantageously avoided, as explained more in detail below.

According to an embodiment, a lower frequency edge of the frequency gap substantially corresponds to a higher frequency edge of the first frequency range. Consequently, by using a frequency gap having the lower frequency edge substantially corresponding to the higher frequency edge of the first frequency range, it can be further ensured that RF signals on the second sub-frequency range to be provided in the second output port are sufficiently remote from the RF signals on the first frequency range to be provided in the first output port, thereby providing sufficient isolation between the first output port and the second output port.

According to an embodiment, the first frequency range of the RF signal provided in the first output port substantially corresponds to a frequency range of CATV downstream signals, such as 85 - 862 MHz; the first sub-frequency range of the RF signal provided in the second output port substantially corresponds to a frequency range of CATV upstream and downstream signals, such as 5 - 862 MHz; and the second sub-frequency range of the RF signal provided in the second output port substantially corresponds to a frequency range of data signals above an upper frequency edge of the frequency gap, such as 950 - 2000 MHz.

Hence, with the signal splitter according to the embodiments, a CATV standard-specific signal splitter can be implemented by selecting frequency ranges appropriately, for example by using the appropriately dimensioned filters. It is noted that the frequency values and ranges mentioned herein are only examples, which may be suitable for one or more contemporary CATV systems. A skilled person appreciates that in different of in future CATV systems, the frequency ranges of upstream and downstream CATV signals, as well as data signals may differ from what is disclosed herein. Therefore, the exact frequency values are not significant for implementing the embodiments.

Figure 3 shows the basic operational principle of a signal splitter according to an embodiment. The signal splitter 300 comprises an input 302 connectable to a node of cable television network, a first output port 304 and a second output port 306. Considering, for example the CATV module 100 shown in Figure 1 , the first output port 304 and the second output port 306 may be connected to the output connectors 102, 104, respectively.

According to an embodiment, the signal splitting circuitry comprises a first diplexer comprising a first highpass filter 308 and a first lowpass filter 310, wherein said frequency gap is configured between a cutoff frequency of the first lowpass filter and a cutoff frequency of the first highpass filter. Thus, the required frequency gap is created upon inputting the broadband signal into the splitter circuitry, wherein the processing of the RF signals can be advantegously divided into two branches: a first branch comprising RF signals to be provided both in the first and the second output ports 304, 306; and a second branch comprising RF signals to be provided only in the second output port 306.

By arranging the frequency gap already at the input and dividing the RF signal into two branches with frequency ranges below and above the frequency gap, the usage of a ferrite at the input is avoided. Moreover, the processing of the RF signals to be provided both in the first and the second output ports can be significantly facilitated.

According to an embodiment, a lowpass filtered RF signal obtained from said first lowpass filter 310 is conducted to a splitter 312; and the lowpass filtered RF signal obtained from a first output of the splitter is conducted to a second highpass filter 314 in order to provide the first output port with the RF signal on the first frequency range. Thus, the first branch of the signal splitting circuitry comprises a splitter 312, which splits the RF signals to be provided both in the first and the second output ports in further branches. The splitter 312 may be provided with a ferrite, but since the signals are processed on conventional frequencies, no specific requirement for the ferrite are set. The RF signals to be provided in the first output port are conducted to the second highpass filter 314 for filtering off the unwanted lower frequencies.

According to an embodiment, the second highpass filter 314 has a cutoff frequency above a frequency range of CATV upstream signals. Herein, the lower frequency edge of the CATV downstream signals, such as 85 MHz, may be used as the cutoff frequency of the second highpass filter 314. Thus, the CATV upstream signals are filtered off, and only the CATV downstream signals are provided in the first output port 304. A second lowpass filter 316 having a cutoff frequency of 65 MHz, which lowpass filter is further grounded via a resistor load 318, is used for providing a good return loss at the input in the attenuated frequency range of 5 - 65 MHz. The second highpass filter 314 and the second lowpass filter 316 complementing each other practically form a 65/85 MHz diplexer in the first output. .

According to an embodiment, a highpass filtered RF signal obtained from said first highpass filter 308 is conducted to a substantially similar highpass filter 320 of a second diplexer; and the lowpass filtered RF signal obtained from a second output of the splitter 312 is conducted to a lowpass filter 322 of said second diplexer , the lowpass filter 322 being substantially similar to the lowpass filter 310 of the first diplexer. Thus, the possible problems caused by low PIM performance of the ferrite of the splitter 312 are addressed. Now, from an output of the second diplexer, the second output port 306 is provided with the RF signal on the second frequency range comprising the first sub frequency range and the second sub-frequency range and the frequency gap between the first and second sub-frequency range.

The second diplexer is preferably similar to the first diplexer, thus having the cutoff frequencies of the highpass filter and lowpass filter similar to the first diplexer. Thus, the RF signals coming from the splitter 312 comprise both the CATV upstream signals and the CATV downstream signals at frequency ranges below the frequency gap, and the RF signals coming from first highpass filter 308 comprise data signals at frequency ranges above the frequency gap. Thus, the second output port 306 may be provided with the RF signal on the second frequency range comprising the first sub-frequency range 5 - 862 MHz and the second sub-frequency range 950 - 2000 MHz, wherein the frequency gap 862 - 950 MHz is provided between the first and second sub-frequency range.

It is noted that 950 MHz is only mentioned as one example of the upper frequency edge of the frequency gap, and a skilled person appreciates that many other values could be used as the upper frequency edge of the frequency gap, as well. However, in the design of diplexers, there are many constraints set e.g. by required spacing and shielding plates between the cells, the type of cells to be used, etc. Thus, while trying to minimize the K factor of a diplexer (defined as a ratio between cutoff frequencies of highpass and lowpass filters; F_stop/F_Pass), it typically becomes an optimization task in terms of the various constrains. Herein, 950 MHz provides an excellent value for the K factor (950/862=1.1 ) and achieving K factor values below 1.1 is very difficult.

It is further noted that the usage of 950 MHz as the lower frequency edge of the second sub-frequency range enables the splitter circuitry to be used also for receiving satellite TV (SATV) broadcasts.

As becomes evident from what has been described above, the exemplified splitter circuitry as disclosed in Figure 3 may provide significant advantages over a splitter circuitry implemented in a conventional manner, such as the splitter circuitry disclosed in Figure 2. For example, the avoidance of using 5 - 2000 MHz splitter in the input enables to focus on optimizing the more easily manageable 5 - 862 MHz splitter. Further, the splitter circuitry of Figure 3 is less sensitive to 85 - 862 MHz branch impedance loading. Moreover, the splitter circuitry of Figure 3 provides very low insertion loss at 2 GHz where the coaxial losses are very important for in-home installations. In good conditions, the 950 MHz highpass filter of the diplexer may provide as low as 1 dB insertion loss at 2 Ghz. As another aspect, there is provided a module attachable to a wall outlet of a CATV network, wherein said module comprises the signal splitter according to any of the embodiments described above. It will be obvious for a person skilled in the art that with technological developments, the basic idea of the invention can be implemented in a variety of ways. Thus, the invention and its embodiments are not limited to the above-described examples, but they may vary within the scope of the claims.

Claims

Claims:

1. A signal splitter for use in a cable television network, the splitter comprising

an input connectable to a node of cable television network; at least a first output port and a second output port, each output port being connectable to a subscriber equipment,

signal splitting means for splitting an RF signal obtained from the input into a first frequency range and a second frequency range, the first frequency range being partly overlapping with lower frequencies of the second frequency range;

wherein the signal splitting means are configured to provide the first output port with the RF signal on the first frequency range and the second output port with the RF signal on the second frequency range; and

wherein the second frequency range comprises a first sub frequency range and a second sub-frequency range and a frequency gap between the first and second sub-frequency range.

2. The signal splitter according to claim 1 , wherein a lower frequency edge of the frequency gap substantially corresponds to a higher frequency edge of the first frequency range.

3. The signal splitter according to claim 1 or 2, wherein the signal splitting means comprise

a first diplexer comprising a first highpass filter and a first lowpass filter, wherein said frequency gap is configured between a cutoff frequency of the first lowpass filter and a cutoff frequency of the first highpass filter.

4. The signal splitter according to claim 3, wherein a lowpass filtered RF signal obtained from said first lowpass filter is conducted to a splitter; and

the lowpass filtered RF signal obtained from a first output of the splitter is conducted to a second highpass filter in order to provide the first output port with the RF signal on the first frequency range.

5. The signal splitter according to claim 4, wherein

the second highpass filter has a cutoff frequency above a frequency range of CATV upstream signals.

6. The signal splitter according to claim 4 or 5, wherein a highpass filtered RF signal obtained from said first highpass filter is conducted to a second diplexer; and

the lowpass filtered RF signal obtained from a second output of the splitter is conducted to said second diplexer in order to provide, from an output of the second diplexer, the second output port with the RF signal on the second frequency range comprises the first sub-frequency range and the second sub-frequency range and the frequency gap between the first and second sub-frequency range.

7. The signal splitter according to claim 6, wherein the second diplexer comprises a lowpass filter substantially similar to the lowpass filter of the first diplexer and a highpass filter substantially similar to the highpass filter of the first diplexer.

8. The signal splitter according to any preceding claim, wherein

the first frequency range of the RF signal provided in the first output port substantially corresponds to a frequency range of CATV downstream signals, such as 85 - 862 MHz;

the first sub-frequency range of the RF signal provided in the second output port substantially corresponds to a frequency range of CATV upstream and downstream signals, such as 5 - 862 MHz; and the second sub-frequency range of the RF signal provided in the second output port substantially corresponds to a frequency range of data signals above the cutoff frequency of the first highpass filter, such as 950 - 2000 MHz.

9. A module attachable to a wall outlet of a CATV network, wherein said module comprises the signal splitter according to any of claims 1 - 8.