Cable network device

Abstract

There is provided a cable network device, 50, such as a multitap device, comprising a first series of interconnected directional couplers, 56, connected to a downstream signal input, 52, with each dir

Description

A diagram of a prior art amplifier 10 used in a broadband, data and / or telecommunications network is shown in Figure 1, amplifier 10 using directional couplers 12, 14 instead of diplex filters to separate upstream and downstream signals for amplification. Input connector 16 receives downstream signal 20 and splits this into separate downstream and upstream paths using directional couplers 12, 14. Amplifier elements 22, 24 amplify signals conveyed along the uni-directional signal paths between directional couplers 12, 14. At output 26, amplified downstream signal 30 passes to splitter 32 which is connected to two 12-way multitaps 34, 34’, each tap providing twelve tap ports for connecting users to the network. Splitter 32 has a finite return loss and as such some of amplified signal 30 is reflected back to amplifier 10. The reflected signal 40 travels upstream and is amplified by upstream amplifier element 24. At input 16 this amplified reflection signal 41 will again be partly reflected due to the finite return loss associated with input 16. The secondary reflected and amplified signal 42 travels back downstream, in the same direction as the original signal 20. Depending on the gain of amplifier elements 22, 24, signal levels, return loss, modulation and timing of the signals, these signal reflections can lead to degradation of original signal 20. In order to address these disadvantages, the present invention provides a 12-way multitap 50 as shown in Figure 2 which provides twelve tap ports as with prior art multitaps shown in Figure 1. Multitap 50 comprises a separate downstream input 52 and upstream output 54, with an interconnected series or cascade of twelve directional couplers 56 in the downstream path providing a connection to each tap port 60, 60’, 60”, 60’” and an interconnected series or cascade of twelve directional couplers 62 in an upstream path returning upstream signals to separate upstream output 54. Each cascade is terminated by a resistor 55, 55’, typically having a value of 75 Ohms. For multitaps with fewer ports, for example a 4-way tap, each cascade will consist of the same number of directional couplers a there are tap ports. Directional couplers 56 are microstrip couplers, also known as tilted taps, with the output 57 of each microstrip coupler 56 connected to the input 58 of the next microstrip coupler 56 so as to allow the downstream signal received at input 52 to reach all tap ports. Each tap port 60 is connected to a coupled port 64 of microstrip coupler 56 with a proportion of the downstream signal from input 52 drawn off to supply each tap port which is in turn connected to a coaxial cable routed to a connected home. The isolated port 66 of each microstrip coupler 56 is connected to a single directional coupler 62 in the interconnected cascade of directional couplers 62 in the upstream return path leading to upstream output 54. Each directional coupler in the first cascade associated with downstream input 52 is paired with a single directional coupler in the second cascade associated with upstream output 54. Thus an interconnected pair of directional couplers is provided for each tap port. Directional couplers 62 are of preference ferrite transformer- type directional couplers. If desired an optional high pass filter 70, with a pass frequency above the lowest expected downstream frequency, typically in the range of 100 or 200MHz, is disposed between input 52 and a first directional coupler 56. An optional low pass filter 72, with a pass frequency below the highest expected upstream frequency usually in the range of 400-700MHz, is disposed between output 54 and directional coupler 62 closest to output 54. In use, a downstream signal present at downstream input 52 travels along the first cascade formed from microstrip couplers 56 and a small, defined, portion of the signal energy is coupled to each tap port 60. The downstream signal does not appear on isolated port 66 due to the nature of the directional coupler. An upstream signal generated in a connected home travels from tap port 60 to isolated port 66 of microstrip coupler 56 with little insertion loss, and thence to directional coupler 62 connected to isolated port 66. The upstream signal is not present at output port 57 of microstrip directional coupler 56 due to the nature of the directional coupler ensuring tap to tap isolation. The upstream signals from the respective tap ports 60 travel along the upstream path connecting the second cascade of directional couplers 62 and route through the optional low pass filter 72 to upstream output port 54. This configuration results in a multitap with separate and RF isolated downstream input and upstream output ports connectable to an amplifier 80 with a separate downstream output 82 and upstream input 84, see Figure 3. Any reflected downstream signal does not reach upstream amplifier element 86 as there is isolation between the 5 separate upstream and the downstream connectors of multitap 50 and is therefore not amplified and cannot reach input connector 88 of amplifier 80. Therefore, there is no reflection signal travelling in the same direction as the original downstream signal. The reflected signal cannot travel in the opposite direction through the downstream amplifier element 90. io For a 12-way tap with a highpass filter at 200MHz and a lowpass filter set to 400MHz, the isolation between the downstream input port 52 and upstream output port 54 is shown in Figure 4 on a scale from 12 to 1900MHz and 0 to -lOOdB and is well over 60dB. 15 The high isolation avoids reflection problems and also prevents issues with amplifier oscillation. For a typical amplifier, an upstream to downstream isolation in the multitap of >40dB would be sufficient to avoid reflection problems.

Claims

1. A cable network device comprising a first series of interconnected directional couplers connected to a downstream signal input with each directional coupler in the first series connected to a separate tap port, and a second series of interconnected directional couplers connected to an upstream signal output, wherein the downstream signal input and upstream signal output are separate and pairs of directional couplers created from one directional coupler from each of the first series and second series are connected together to enable an upstream signal path routed through the tap port and directional coupler in the first series to reach the upstream signal output.

2. A cable network device according to claim 1, wherein each directional coupler connects to an adjacent coupler in the same series and also connects to a single directional coupler in the other series.

3. A cable network device according to claim 1 or claim 2, wherein the directional couplers of the first series are microstrip couplers.

4. A cable network device according to any of the preceding claims, wherein a coupled port of the microstrip directional coupler is connected to the tap port.

5. A cable network device according to any of the preceding claims, wherein an isolated port of the microstrip coupler is connected to the other directional coupler in the pair of directional couplers.

6. A cable network device according to any of the preceding claims, wherein the directional couplers of the second series are ferrite directional couplers.

7. A cable network device according to any of the preceding claims configured to have an isolation from the downstream signal input to an upstream signal output of greater than 40 dB.

8. A cable network device according to any of the preceding claims configured to have an isolation from the downstream signal input to an upstream signal output of greater than 60 dB.5 9. A cable network device according to any of the preceding claims being a multitapdevice.

10. A cable network device according to any of the preceding claims when used in combination with an amplifier comprising a directional coupler to create separate io upstream input and downstream output.

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