GB2181611A - Switching circuit for cable television distribution network - Google Patents

Abstract

A switching circuit for connecting an input (1) carrying a plurality of frequency distinguished signals to an output (5) comprises a first input diode (14) connected in series with a second output diode (15) between a tapping of an input transformer (16) and the output, the transformer winding being connected between the signal input and ground. A first resistor (26) is connected between and in series with the first and second diodes, a control input (3) is connected to the tapping of the input transformer by a second resistor (25) in series with a third diode (23), a fourth diode (24) is connected between ground and a point between the first and second diodes, and means are provided for controlling the polarity of the control input relative to ground. The output has a DC bias to ground, and the polarities of the diodes are such that the application of a potential of one polarity to the control input causes the first and second diodes to turn on and the third and fourth diodes to turn off, and the application of a potential of the opposite polarity to the control input causes the first and second diodes to turn off and the third and fourth diodes to turn on. The total impedance presented to the tap by the switch when the switch is off is the sum of the impedances of the second resistor and the third diode, and the total impedance presented to the tap by the switch when the switch is on is the sum of the impedances of the first resistor, the first and second diodes, and the output, the two total impedances being equal. <IMAGE>

Description

SPECIFICATION Switch circuit The present invention relates to a switch circuit, and in particularto a switch circuit for use in a cable television distribution network.

Cable television distribution networks are known in which a pluraiity of signals are transmitted from a head end to a switching centre. Located within the switching centre is a series of switch units remotely controlled by subscribers, each subscriber being able to control a respective switch unit so as to select any one of the available signalsfortransmission to the subscribers receiver. Networks of this type are generally known as star networks, and may use either conductors or optic fibres or combinations of the two to transmit signals.

British Patent Specification Nos. 2063026 and 2 121 656 describe networks in which signals are transmitted from the head end to switching centres on sixtrunktransmission lines, each trunk transmission line carrying five VHF television signals which are distinguished from each other by frequency. Thus the signals on each trunk line can occupy frequency channels having luminance frequencies of for example 69.2,75.2,93.2, 123.2 and 135.2 MHz respectively.At the switching centres, one ofthethirty available signals is selected by firstly selecting the packet of five signals on one trunk transmission line which includes the desired one signal, and then by selecting from the five signals on that transmission line the desired one signal by converting the desired signal to the frequency appropriate to the particular subscribers receiver.

With the above networks, in the first selection stage it is necessary to provide a switch which can connect any one of six inputs to a single output dedicated to a particular subscriber. The selected input is a VHF signal occupying a wide range of frequencies so that careful switch design is required is acceptable level of crosstalk and signal isolation areto be achieved.

tis an object of the Rresent inveotion ts p~ovide a switch circuit which provides good protection against crosstalk between a plurality of VHF inputs and outputs and good impedance characteristics.

According to the present invention, there is provided a circuitforconnecting an input carrying a plurality of frequency distinguished signals to an output, comprising a first input diode connected in series with a second output diode between a tapping of an input transformer and the output, the transformer winding being connected between the signal input and ground, a first resistor connected between and in series with the first and second diodes, a control input connected to the tapping of the input transformer by a second resistor in series with a third diode, a fourth diode connected between ground and a point between thefirstand second diodes, and means for controlling the polarity of the control input relative to ground, wherein the output has a DC bias to ground, and the polarities ofthe diodes are such that the application of a potential of one polarity to the control input causes the first and second diodes to turn on and the third and fourth diodes to turn off, and the application of a potential of the opposite polarity to the control input causes the first and second diodes to turn off and thethird and fourth diodes to turn on, the total impedance presented to the tap by the switch when the switch is off being the sum ofthe impedances ofthe second resistor and the third diode, and the total impedance presented to the tap by the switch when the switch is on being the sum of the impedances ofthefirst resistor, the first and second diodes, and the output, the said total impedances being equal.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa schematic illustration of the structure of a switch assembly for connecting any one of six inputstoanyoneofsixoutputs; Figure2 is a simplified partially exploded view of a switch assembly housing the general structure of Figure 1; Figure 3 illustrates the interconnection of components shown in Figure 2; Figure 4 is a view in the direction of arrow IV of the interleaved structure shown in Figure 2; Figures5to8arerespectivelyviews in the direction of arrows V, VI, VII and VIII in Figure 2, an outer screen shown in outline in Figure 2 having been removed;; Figure 9 is a diagram of a switch circuit in accordance with the invention supported by the structure of Figure 2; Figures 10 and 11 show the metalisation patterns formed on and components secured to the two sides of the input board of the structure of Figure 2; Figures 12 and 13 show the metallisation patterns formed on and components secured to the two sides of the output board ofthe structure of Figure 2; Figure 14 illustrates the metalisation pattern formed on and components supported by one side of an extended input board; and Figure 15is a diagram of the circuit supported by the portion ofthe extended input board of Figure 14.

Referring to Figure 1, six inputs 1 each carrying five frequency disting uished VHF sig nals are ppiiedt respective input boards 2. Each input board supports six solid state switches (not shown) and a control circuitfor selectively turning on any one of the six switches in response to signals applied to control inputs 3. Each of the switches on one input board is connected to a respective output board 4which supports six further switches (not shown). Any one of the output board switches on any one board may be selectively switched on to connect the signal received from one input board to an output 5 common to the board 4.

The structure of Figure 1 may be used to select any one of a series of packets of VHF signals received from the head end of a television distribution network and to pass the selected packet to a frequency selector whereby a single selected VHF signal can be converted to UHF and then transmitted to a subscriber to the network. The general structure of such a network may be appreciated by reference to the above mentioned British Patent Specifications.

Referring to Figure 2,the illustrated structure corresponds to the generalised structure of Figure 1.

Input boards 2 are interleaved with a first group of isolating plates 6 and output boards 4 are interleaved with a second group of isolating plates 7. An outer isolating screen 8 shown in broken lines is also provided. The assembly of boards and plates is mounted on a support board 9.

Although not shown in Figure 2, each input board 2 is connected by eight push in connectors to eight inputs on the support board. Thus each input board receives six DC switch control inputs, a DC supply voltage and a ground input. In addition, each output board is connected to each input board by two pairs ofinterengaging push terminals, one providing a signal path and the other ground. Figure3 showsthe interengagement between the support board 9, the input board 2furthest to the right in Figure 2, and the fourth output board 4 counting from the top in Figure 2. It will be seen thatfourterminals 10 at one end of the input board 2 make connections with four terminals 11 on the support board.A similar arrangement (not shown) is provided at the other end ofthe input board 2. In addition, two terminals 12 (only one ofwhich is visible in Figure 3) on the input board2engagetwoterminals 13 onthe outputboard 4. Each input board has eight terminals 10 and six sets of terminals 12, and each output board has six sets of terminals 13. The assembled input and output boards thus define thirty six cross-point switches controlled bythirty six inputs 3 (Figure 1).

Figure 4 is an end view in the direction of arrow IV ofthe assembly of boards and isolating plates.

Figures 5to 8 are respectively side views ofthe assembly in the directions farrows V, VI, VII and VI II.

Referring nowto Figure 9,the electrical characteristics of the components not previously shown on the input and output boards are illustrated in a circuit diagram. Each input board 2 supports six switches in the form of diodes 14 which are controlled via inputs 3 by respective subscribers, and each output board 4 which serves a respective subscriber supports six switches in the form of diodes 15, each diode 15 being connected to a different input board 2. A packet ofVHFsignals appearing at input 1 (Figure 1) is selected for transmission to individual subscribers outputs 5 by rendering the appropriate pair of diodes 14,15 conductive.It may be that from none to all six ofthe subscribers fed from the six illustrated output boards 4turns on the pairs of diodes 14, 15 simultaneously and itistherefore necessaryto carefully match the switching circuits to maintain the desired impedances.

In one arrangement in which the illustrated circuit has given good results, the input and output lines 1 and 5 are both 50 ohm lines, and an input transformer 16 is formed by two ferrite cores having an upper section 17 with fourturns and a lower section 18 having five turns. When the switch is "off", the control inputs 3 (Figure 1) float unto the positive supply provided via resistors 19 and 20. When the switch is "on", a negative potential is appiied to the input 3, pulling the voltage on line 21 negative.

When the switch is off, line 21 is positive, turning off the first diode 14 and the second diode 15 via a DC bypass choke 22. A third diode 23 and a fourth diode 24 are turned on via resistor 25 and choke 22 respectively. When the switch is on, line 21 is negative, turning on diodes 14 and 15 and turning off diodes 23 and 24. The output 5 is DC biased to ground via the load which includes a further DC bias choke connected in parallel with a 50 ohm resistor between the output line 5 and ground. The DC bias circuit is shown foronly one ofthe outputs 5. Theturns ratio of thetransformer16givesita lSohmapproximately output.A series resistor 26 has an impedance of 39 ohms, and the resistor 25 has an impedance of 90 ohms. (In practice, a 91 ohm resistorwould be selected as such resistors are commercially available, the one ohm difference having a negligible effect).

Looking from the transformer 16 towards the outputs, there are six impedances in parallel. When a switch is on, the impedance is 50 ohms (line 5) plus 2 ohms (diodes 14 and 15) plus 39 ohms (resistor 26) to give a total of 91 ohms. When a switch is offthe impedance is 90 ohms (resistor 25) plus 1 ohm (diode 23)to give atotal of 91 ohms. Thus the impedance is substantiallythe same no matter how many switches are on. Looking from the output 5, there are again six impedances in parallel. When a switch is off, the impedance is effectively infinite. When a switch is on, the impedance is 39 ohms (resistor26), plus 2 ohms (diodes 14 and 15) to give a subtotal of41 ohms plus fiveswitchesin parallel each of 91 ohmsandthe transformer 16.If the impedance ofthe parallel circuit is Z, then liZ = 5/91 + 1/15,which is approximately equal to 8.2 ohms. The total impedance is thus 49.2 ohms which represents a good match to the 50 ohm line 5.

Capacitors 27 (8.2pF) and 28 5pF) deal with leakage inductance and capacitor 29 (1 nF) smooths the control current flowing through the control input 3.

Referring nowto Figures 10 and 11,thedetailed structure of one input board 2 is illustrated. The components shown in Figure 9 are indicated bythe same reference numerals. The transformer 16 is a simplewirewound binocularferrite core, and the chokes 22 are simple wire wound annularferrite core. The other components are surface mounted chips. The metalised surfaces ofthe boards are carefully designed to minimise crosstalk. In particularthere is a non-metalised portion 30 between the resistor 26 and the contact of the choke 22 to which it is connected. This is to ensure that signals on the metalised surface between the resistor 26 and choke 22 cannot bypass and thereby render ineffective the diode 24. Slots 31 are provided adjacent each of the terminals 12, these slots also being shown in Figure 3.

Referring nowto Figures 12 and 13,the detailed structure of one output board 4 is illustrated. The diodes 15 are positioned between earthed rectangular plates 32 which prevent crosstalk between the one diode 15 which can be carrying signals at any particulartime and the other conductive tracks. The selected signals are applied to a coaxial cable 33 which is secured to the board 4 by a clip 34. The board 4 is also provided with slots 35, and the slots 35 engage in the slots 31 ofthe input boards 2 when the boards are assembled together. Thus the edges of the input and output boards overlap by a distance equal to the sum of the depth of the slots 31 and 35.

The edges of the isolating plates 6 and 7 are also slotted, as can be seen in the case of plate 7 from FigureS. The slots in the isolating plates receive the edges of the boards and are of twice the depth of the slots 31 and 35 in the boards. Thus each ofthe pairs ofterminals 12,13 (Figure 3) is located within a four sided enclosure made up by elements offour isolating plates which extend into the spaces between the pairs of input and output boards. The signals appearing on any board are thus fully screened from every other board, and the terminals 12 and 13arealsofullyscreened.Appreciable crosstalk is thus effectively eliminated.

The described arrangement thus fully screens the various signal pathsfrom each otherand maintains good impedance matching regardless ofwhich signal packet is selected by a subscriber.

The isolating plates 6 and 7 may be formed from individual plates suitabíy slotted to enablethe structure illustrated to be assembled. The plates may be formed from tin plate hot dipped after assembly to form an integral assembly into which the input and output boards are inserted. Alternatively, the plates 6 and 7 may be formed in aluminium by extruding a single integral structure and then machining out those parts not required.

The preceding drawings illustrate an arrangement in which each screened switch unit comprises six input boards and six output boards. Any array of such switch units can be provided within a suitable housing to serve any number of subscribers. It is possible howeverto reduce the volume required by extending the input boards 2to carry a pluralityofthe circuits shown in Figure 9. For example, whereas the boards illustrated in Figures 10to 13 aretypically 58mm in length, a double input board can be provided with a length of 1 46mm, that is two end sections each indentical to the input board of Figures 10 and 11, and a central section 30mm in length. Such a double input board can be engaged by each oftwo sets of six output boards identical to that shown in Figures 12 and 13.

To avoid the need for two input cables to the input board, a 3dB hybrid splitter may be mounted on the central section of the double board. The layout of such a splitter is shown in Figure 14 and its electrical circuit is shown in Figure 15. The structure and principle of operation of hybrid splitters is known fromthe priorart and further information inthis regard can be obtained from our earlier British Patent Specification No. 317244.

Referring to Figures 14 and 15, the input coaxial cable 1 is connected via a metalised pad 36 to an input winding of a transformer37. Two output windings of the transformer are connected via metalised pads 38 and 39 to short coaxial cables 40 and 41 which are connected to the input transformers 16 of two circuits such as illustrated in Figure 9.

Capacitors 42, 43 and 44 and a resistor 45 are also provided. The capacitors 42,43 and 44 compensate for leakage inductance.

The illustrated arrangement improves the response of the circuit at VHF frequencies whilst also resulting in a relatively fast fall-off in the circuit response at higher frequencies. This gives a better attenuation at UHFfrequencies which is desirable given that the VHF signals passed by the switch are converted to UHFfortransmission to a subscriber.

Figure 11 shows the capacitors 27 and 28 mounted on the reverse side of the input board 2. In production versions of the input board the capacitors 27 and 28 will be mounted on the front surface of the board which surface is shown in Figure 10, the capacitors being in the form of surface mounted components similarto capacitors 27 and 28 illustrated in Figure 10.

Attention is directed to British Patent Specification No.2152762 which claims a switch assembly ofthe type as hereinbefore described.

Claims (2)

1. Acircuitfor connecting an input carrying a plurality of frequency distinguished signals an output, comprising a first input diode connected in series with a second output diode between a tapping of an inputtransformer and the output, the transformer winding being connected between the signal input and ground, a first resistor connected between and in series with the first and second diodes, a control input connected to the tapping of the input transformer by a second resistor in series with a third diode, a fourth diode connected between ground and a point between the first and second diodes,and meansforcontrollingthepolarityofthe control input relative to ground, wherein the output has a DC bias to ground, and the polarities ofthe diodes are such that the application of a potential of one polarity to the control input causes the first and second diodes to turn on and the third and fourth diodes to turn off, and the application of a potenlial of the opposite polarity to the control input causes the first and second diodes to turn offend the third and fourth diodes to turn on,thetotal impedance presented to the tap by the switch when the switch is off being the sum of the impedances of the second resistor and the third diode, and the total impedance presented to the tap by the switch when the switch is on being the sum of the impedances ofthefirst resistor, the first and second diodes, and the output, the said total impedances being equal.

2. A circuit substantially as hereinbefore described with reference to the accompanying drawings.