GB2173077A - RF signal connection arrangement - Google Patents

Abstract

In an RF signal connection arrangement a number of sources of RF modulated television or like signals, such as an aerial (2), a video disc player (9), a home computer (17) or a video recorder (25) are connected to a coaxial transmission cable (1) through an associated coaxial cable (3, 11, 19, 27), an associated amplifier (5, 13, 21, 29), an associated isolating impedance (R2), a further associated coaxial cable (7, 15, 23, 31), and an associated connection or port (8, 16, 24, 32). The coaxial transmission cable (1) is additionally connected to equipment using television or like signals such as the video recorder (25) or a television receiver (47) by way of an associated connection or port (33, 40), an associated coaxial cable (34, 41), an associated isolating impedance (R2), an associated amplifier (36, 43), and a further associated coaxial cable (38, 45). The ends of the coaxial transmission cable (1) are terminated by resistors (R1) whose value corresponds to that of the characteristic impedance of the coaxial transmission cable (1). The isolating impedances (R2) prevent the coaxial transmission cable (1) from being loaded by the equipment (2, 9, 17, 25, 47) connected to it whilst the amplifiers (5, 13, 21, 29, 36, 43) compensate for the signal loss in the associated isolating impedance (R2). <IMAGE>

Description

SPECIFICATION RF signal connection arrangement The present invention relates to an RF signal connection arrangementforconnecting a source providing RF signals modulated by television or like information to apparatus utilising such modulated RF signals, said arrangement comprising a source of RF signals modulated by television or like information an amplifier whose input is connected to said source for amplifying said modulated RF signals and whose output is connected to a transmission cable which cable is additionally connected to television receiving apparatus which utilises said modulated RF signals.

The invention also provides an amplifier unit for use with such an arrangement.

Such an arrangement is known and has been employed when the incoming RFsignal from a television aerial is of low amplitude and needs to be amplified priorto being applied to a television receiver to ensure that the applied RFsignal has a signal to noise ratio sufficiently good to produce a reasonable display. These days sources of RF signals modulated bytelevision or like information otherthantelevision aerials are available, such as video disc players, video cassette recorders and home computers and when householders have one or more ofthese in addition to the television aerials as a source all of which can be optionally connected to one or more television receivers at different times, then connections have to be continuously changed depending on what is to be connecteed to what.Such connections are not assisted if a source is located in a room different to that in which the RF signals they produce are to be displayed or recorded.

It is an object ofthe invention to provide an RF signal connection arrangement which overcomes the above difficulties.

The invention provides an RFsignal connection arrangementforconnecting a source providing RF signals modulated bytelevision or like information to apparatus utilising such modulated RF signals, said arrangement comprising a source of RF signals modulated by television or like information, an amplifier whose input is connected to said source for amplifying said modulated RF signals transmission cable which cable is additionally connected to and whose output is connected to a television receiving apparatus which utilises said modulated RF signals, characterised inthatsaidtransmission cable has each of its ends terminated by an impedance the value of which substantially corresponds to the characteristic impedance of said transmission cable, said transmission cable having a plurality of ports by means of a first and a second of which the output of said amplifier and said receiving apparatus are respectively connected to said transmission cable, the connection between the output of said first mentioned amplifier and said first port being by way of a first isolating impedancewhilstthe connection between said second port and said receiving apparatus is by way of a second isolating impedance and a second amplifier with the second isolating impedance being connected between the second port and the input of said second amplifier whose output is connected to said receiving apparatus, further ports on said transmission cable being adapted for connection to a further source or sources of such modulated RF signals or further television receiving apparatus each through an associated isolating impedance and an appropriately directed amplifier.

With the present arrangement the isolating impedances preventthetransmission cable from being mismatched by the sources and receiving apparatus connected to it. It is of course necessaryforthese isolating impedancesto beconnected between the transmission cable and the associated amplifier.

As an optimum each isolating impedance will have avaluewhich isoftheorderof Ntimesthe characteristic impedance of the transmission cable where N is the number of ports on the transmission cable and where the gain of each amplifier is such asto substantially compensate for signal losses produced in the associated isolating impedance.

In orderto avoid using a separate powersupplyfor each amplifiertheterminals of a power-supply may be connected to respective conductors of thetransmission cable, each amplifier being energised by a resulting supply voltage present at its associated port.

In a preferred embodimentthetransmission cable is a coaxial cable and the ports are each formed buy a coaxial connector of a first type, each isolating impedance and its associated amplifier being mounted in a housing having a coaxial connector of a second type complementaryto the firsttype for making connection therewith, the second type connector being connected through the isolating resistor to the appropriate active terminal ofthe amplifier.

Each isolating impedance maybeformed by either the input or output impedance of its associated amplifier depending on which active terminal ofthe amplifier is connected two a port.

In orderto reduce the number of components located with the amplifier a pair of complementary connectors may be provided in the connection between a port and an associated amplifierwith the isolating impedance being located between the port and one ofthe connectors.

The invention also provides an amplifier unitfor use with an RFsignal connection arrangement as described above in which said unit comprises a housing, a first coaxial connector a first terminal ofwhich is connected to a first activeterminal of an amplifier whose second active terminal is connected to a first terminal of a second coaxial connector,thefirst terminal ofthefirst coaxial connector being addi tionallyconnectedthroughan RF choke to a first supply terminal of said amplifier whose second supplyterminal is connected to the second terminal of the first coaxial connector, characterised in that the connection between the firstterminal of said first coaxial connector and the first active terminal ofthe amplifier is byway of an isolating impedance whilst said amplifier unit may be of a first type in which the first and second active terminals are the input and output respectively or of a second type in which the first and second active terminals are the output and input respectively. With such an amplifier unit the isolating impedance may be formed by the terminal impedance of the associated active terminal.

The above and otherfeatures ofthe invention will now be described, by way of example, with reference tothe accompanying drawings, in which:-- Figure lisa diagram of an RF signal connection arrangement according to the invention.

Figure2 isa modification ofthe arrangement shown in Figure 1, Figures 3 and 4 are diagrams of amplifier units that can be used with the arrangement of Figures 1 or 2, Figure5 is a modification ofthe RF signal connectionarrangementshown in Figure 2, and Figure 6 is a modification of part of Figure 5.

Figure 1 shows diagrammatically an arrangement whereby a number of units which either produce or utilise RFsignalswhich are modulated by television or like information may be coupled to each other by way of a common line connection. Inthefollowing description it will be assumed thatallthe RFsignals lie in the UHF bands VI and V used forthetransmissions oftelevision signals in the United Kingdom although it is possiblethatthe RF signals will lie in these UHF band andlorthe VHF bands land Ill which are still used forthetransmission of television signals in Continental Europe.The common line connection is a coaxial transmission cable 1 which typically has a characteristic impedance Z0 of 75 ohms, the central conductor being shown as a continuous linewhilstthe outer conducting sheath is shown bythetwo adjacent broken lines. Each end ofthe coaxial cable 1 is terminated by a resistor R1 having a resistance of 75 ohms i.e. avaluewhich correspondstothecharacter- istic impedance ofthe cable to avoid reflections.

An aerial 2for receiving RF signals over at least part ofthe UHF bands is connected bya coaxial cable3to the input4 of an amplifier 5, the cable 3 again typically having a characteristic impedance Z0 of 75 ohms, whilst the input and output impedances ofthe amplifier 5 will also be 75 ohms to match that ofthe cable 3. The output 6 ofthe amplifier 5 is connected through an isolating resistor R2 and a further coaxial cable 7 to a port or connection point 8 on the coaxial cable 1. In this way RFsignals are applied from the aerial 2 to the coaxial cable 1, the aerial acting as a source of such RF signals.

Avideo disc player 9, typically one which repro ducestelevision signals recorded on an optical disc in digital form, has an output 10 which provides the recorded television signals modulated according to a television standard (the PAL standard in the United Kingdom) on an RF carrier in oneofthe UHF bands.

The output 10, which has an impedance of 75 ohms, is connected by a coaxial cable 11 ofthe same impe dance to an input 12 of an amplifier 13, the input 12 and an output 14 ofthe amplifier 13 also having an impedance of 85 ohms. The output 14 is connected through an isolating resistor R2 and a further coaxial cable 15 also of 75 ohms impedanceto a port orfurther connection point 16 on the coaxial cable 1. With this arrangementtelevision signals reproduced by the video disc player9 (acting as a source) are applied as modulated RF signals to the coaxial cable 1.

A homecomputer 17 produces a signalfordisplay so that it may by read by the operator or another viewer. This signal is converted to have the form like that of a television signal and modulated in accordance with a required television standard on an RF carrier in one of the UHF bands to appear at an output terminal 18 having an impedance of 75 ohms. This output is connected through a coaxial cable 19 ofthe same impedance to the input 20 ofan amplifier2l,the input 20 and an output 22 also having the same impedance as the coaxial cable (750hms). The output 22 is connected through an isolating resistor R2 and a further coaxial cable 23 of 75 ohms impedance to a furtherconnection point24 on the coaxial cable 1.In this way signals from the home computer (a source) are applied as modulated RF signals to the coaxial cable 1.

Avideocassette recorder25iscapableofreproduc- ing television signals magnetically recorded on tape in a cassette or of recording such signals onto such a tape cassette. The recorder 25 has an output 26 at which the recorded signals appear modulated according to the required television standard on an RF carrier in one of the UHF bands. This output 26, which has an impedance of 75 ohms, is connected through a coaxial cable 27 of the same impedance to the input 28 of an amplifier 29, this input 28 and an output 30 having an impedance of 75 ohms. The output 30 is connected through an isolating resistor R2 and a coaxial cable 31 Of 75 ohms impedance to yetafurther connection point 32 on the coaxial cable 1.In this way television signals produced by the video cassette recorder (acting as a source) are applied as modulated RF signals to the coaxial cable 1. Another port or connection point 33 on this cable 1 is connected through a coaxial cable 34 and an isolating resistor R2 to an input 35 of an amplifier36 which has an output 37 which is connected through a further coaxial cable 38 to an input 39 ofthe video recorder 25. The inputs 35 and outputs 36 ofthe amplifier as well as the input 39 ofthevideo recorder 25 all have the same impedance as the coaxial cables 34 and 38, namely 75 ohms.RF signals modulated by television or like information present on the coaxial cable 1 eitherfrom the aerial 2, the video disc player9 orthe home computer 17 are received atthe input 39 and which after suitable processing in the video recorder 25 are magnetically recorded onto a tape cassette for later reproduction.

Afurther connection point 40 on the coaxial cable 1 is connected through a coaxial cable 41 and an isolating resistor R2 to an input 48 of an amplifier43, an output 44 of which is connected through a coaxial cable 45 to an input 46 of a television receiver 47. The inputs42 and 46 and the output44 all have the same impedance as the coaxial cables 41 and 45, namely 75 ohms. RF signals modulated bytelevision or like information from the sources, that is the aerial 2,the video disc player 9, the home computer 17 orthe video cassette recorder 25, and which are applied to the coaxial cable 1 are applied from the connection point 40 to the television receiver 47 where one at a time is selectedfordisplay, it being preferablethatthe RF signals presentatthe outputs 10,18 and 26 are at different frequencies which frequencies also differ from the frequencies ofthe RF signals which are to be received at the aerial 2 to avoid co-channel interference.

The coaxial cable 1 and the associated connections form a distribution arrangement for the RF signals by means ofwhich signals from a number of sources may be conveyed over a common transmission cable to one or more television receivers and it is obvious that whenmorethanonetelevision receiverisconnected to the cable 1 signals from different sources may be viewed on differenttelevision receivers. The coaxial cable may run between different rooms in a residence such thatthe ports or connection points 8, 16,24 and 40 may be located in different rooms though the connection points 32 and 33 would need to be located in the same room.The amplifier 5,13,21,29,36 and 43 may all be identical and could be of a type currently sold for amplifying or boosting signals from a television aerial provided it has the required gain. If the sources 2,9,19 and 25 and the television receiver 47 were all directly connected to the coaxial transmission cable 1 (via their own coaxial cables) then their output impedances in the case ofthe sources and the input impedances in the case of receivers and the video recorder 25 would all load the coaxial cable 1 such that the cable would be mismatched and there would be reflections. The isolating resistors R2 serve to isolate each source, receiver or recorderfrom the coaxial cable 1 to avoid this.In Figure 1 six connections are made to the coaxial cable 1 and to avoid such mismatch each isolating resistor R2 would need to have a resistance value which as an example is six times the characteristic impedance ofthe cable 1 (6x750hms) i.e. 450 ohms. However, it is anticipated that for convenience any distribution system would require say 10 or 12 connection points in which case each isolating resistor R2 could be chosen to have a resistance value of1 Kohms.

The inclusion of an isolating resistor R2 between each connection point 8, 16,24,32,33,40 and its associated source, receiver or recorder input produces a drop in signal level which without any further step would worsen the signal to noise ratio. This is overcome by the inclusion of the amplifier5, 13, 21, 29, 36 and 43 each of which has a gain chosen to compensate for the loss caused by the associated isolating resistor R2. In the case where resistor R2 has a value of 1 Ohms then this gain would need to be 20 dB.

As previously stated the amplifiers 5, 13, 21, 29. 36 and 43 may all be identical and some booster amplifiers used for amplifying signals from television aerials often incorporate a battery as their source of power supply, the battery life being long as the power consumption by the amplifier is low. Such amplifiers however have a limited gain which may be sufficient if the number of connection points and hence the resistance value of isolating resistor R2 is limited.

Where the gain has to be higher and the power consumption also higher rather than power each amplifier from its own individual supply the amplifiers may be energised from a common supply by way of the coaxial line 1 as shown in Figure 2. In this Figure like components appearing in Figure 1 are given the same reference symbol.

In Figure2 a dc powersupply48 is indicated as containing a batterythough it could of course be mains energised, the supply 48 being preferably current limited to make it safe should a short circuit be present at anytime across the cable 1 or its connections. The resistors R2 can provide this limiting action and also protectthe cable 1 from being short circuited at the operating frequency if a length of coaxial cable (a stub) were directly connected to a port with a length of n x4T.he positive terminal "+" of the supply48 is connected through an RF choke Lto the central conductor of the coaxial cable 1 whilst its negative terminal "this connected to the outer sheath of this cable, the choke L preventing the low impedance of the supply 48 from loading the cable 1.The junction of the central conductor of the coaxial cable 7 and the isolating resistor R2 is connected through an RF choke Lto a positive supply terminal "+"forthe amplifier5 whilst the sheath of the coaxial cable 7 is directly connected to a negative terminal"-" forthe amplifier 5. Thus a supply for energising the ampl ifier 5 is conveyed from the dc power supply 48 via the coaxial cables 1 and7andtheirconnection point8tothe amplifier5. In a similarwaythe other amplifiers 13,21, 29,36 and 43 may be energised from the supply 48 via the coaxial cable 1, their associated coaxial cables 15, 23,31,34 and 41 and RFchokes L.With the arrangement of Figure 2 each terminating resistor R1 is connected to the central conductor ofthe cable 1 via a capacitor C which has substantially zero impedance at UHFto prevent the resistors R1 drawing current from the supply 48.

The port connection points 8,16,24,32,33 and 40 may be made via standard coaxial plugs and sockets designed for operation with coaxial cables having a characteristic impedance of 75 ohms. Thus the connection points 8, 16,24,32,33 and 40 to the coaxial cable 1 may take the form of a numberofstandard wall mounted coaxial sockets whilstthe coaxial cables 7, 15,23,31,34 and 41 may each be terminated with a standard coaxial plug. In a preferred embodiment these latter coaxial cables may each be replaced bythe coaxial plug mounted directly onto a housing containing the isolating resistor R2, the amplifier, the RF choke Land the associated supply connections.Such a housing may also contain a further coaxial termination e.g. a coaxial socket to which the connection from the source 2,9, 17 or 25, the video recorder 25 or the television receiver 47 may be made, the housing and its contents forming a unit which may be connected between the coaxial wall sockets and the associated source or receiver/recorder. Figure 3 shows a unit of a firsttype for the situation where a source (2,9, 17 or 25) applies a signal to the coaxial cable 1 ,the unit described being that containing the amplifier 5 though units containing the amplifier 13,21 or 29 would be identical. In Figure 3 the housing which is indicated by the reference 49, has at its right hand end a coaxial socket indicated by the input terminal reference 4 as it serves thatfunction. The inner contact of this input socket is connected to the input of the amplifier 5 whose output is connected through the isolating resistor R2 to the inner contact of a coaxial plug located atthe left hand side ofthe unit and given the reference 7 as it provides the coaxial connection to the cable 1. The Junction between the plug 7 and the isolating resistor R2 is connected to the RF choke L which in turn is connected to the positive supply terminal "+" forthe amplifier 5.The negative supply terminal"-" forthe amplifier 5 is connected to the outercontactofthe coaxial plug 7which is also connected to the outer contact ofthe coaxial socket 4.

Figure4 shows a unit of a second type for where signalsareto be supplied from the coaxial cable 1 to equipment which uses such signals e.g. the video recorder 25 or the television receiver 47, the unit to be described being that containing the amplifier 36 though that containing the amplifier 43 will be the same. In Figure 4the housing is again given the reference 49 and as with Figure 3 it also has at its left hand end a coaxial plug indicated by the reference 34 and at the right hand end a coaxial socket indicated by the reference 37 forsimilar reasons as with Figure 3.

The connections and components are the same as in Figure 3 save that the amplifier 36 has its direction of amplification opposite to that for amplifier 5. Itwill then be seen that units of the first and second type only differ in the orientation of the amplifier.

In the above description reference has been made to an isolating resistor R2 shown in the form of a discrete component either at the input or output of an amplifier. This has been done to showthat a common amplifier having the same input and output impedance may be used for any function (c.f. Figures 3 and 4). However, this is not essential and the resistor R2 may be replaced by the amplifier input or output impedance, where appropriate, raised to the required level for isolation.

With the arrangement described in relation to Figure 2the ports on the coaxial cable 1 and the associated connecting cables maytaketheform of coaxial sockets and plugs. In Figure 5, which uses the same references forthe same components as in Figure 2, the isolating resistors R2 are moved to be located in the wall mounted coaxial sockets which avoids the need to place each resistoradjacentan amplifier.This does mean that the dcsupply has to be taken through an isolating resistor which causes a voltage drop in the supply received by the amplifier.

However, with the small currenttalcen bythe amplifier this could be compensated by a small increase in the voitage provided by the supply 48. If the voltage drop across the isolating resistors R2 cannot be accepted then each resistor may have an RF choke placed in parallel with itto provide a dc path for the supply.

Such an arrangement is shown in Figure 6 for one of the ports 16 and associated items butwould bethe sameforother ports. In Figure Gthe parallel RFchoke is indicated by the reference L2whilstthe RF choke connected to the "+" supply terminal ofthe amplifier is given the reference L1. Forfrequencies in the UHF band the RFchokes only require a small numberof turns of copper wire and so that choke L2 could readily be wound around its associated isolating resistor R2.

It is stated in .he above description thatthe gain of each amplifiers 5,13,21,29,36 and 43 is such asto overcome the loss in signal level produced as a result ofthe presence ofthe associated isolating resistor R2.

In the case ofthe amplifier 5 this gain could be much greater in order to cater for signals received by the aerial 2 which have a low level. This increase of gain would also assistto achieve the situation where the signals applied to the coaxial cable 1 from the various sources all have a signal level which is substantially of thesameorderwhichwouldpreventon display intermodulation products being visible for sources where the signal level is low. In addition, this would minimise any possible interference between two such RF connecting arrangements positioned in adjacent locations should there be any leakage in one ofthe arrangements. To caterforvariations in signal level for differenttransmissions received by the aerial 2 the amplifier5 could be provided with automatic gain control.

Claims (10)

1. An RF signal connection arrangementforconnecting a source providing RF signals modulated by television or like information to apparatus utilising such modulated RF signals, said arrangementcomprising a source of RFsignals modulated bytelevision or like information, an amplifierwhose input is connected to said source for amplifying said modulated RF signals and whose output is connected to a transmission cable which cable is additionally connected to television receiving apparatus which utilises said modulated RF signals, characterised in that said transmission cable has each of its ends terminated by an impedance the value of which substantially correspondstothe characteristic impedance of said transmission cable, said transmission cable having a plurality of ports by means of a first and a second of which the output of said amplifier and said receiving apparatus are respectively connected to said transmission cable, the connection between the output of said first mentioned amplifier and said first port being by way of a first isolating impedance whilstthe connection between said second port and said receiving apparatus is byway of a second isolating impedance and a second amplifierwith the second isolating impedance being connected between the second port and the input of said second amplifier whose output is connected to said receiving appar atus, further ports on said transmission cable being adapted forconnection to afurthersourceorsources of such modulated RF signals orfurthertelevision receiving apparatus each through an associated isolating impedance and an appropriately directed amplifier.

2. An arrangement as claimed in Claim 1, characterised inthateach isolating impedance hasavalue which is of the order of N times the characteristic impedance of the transmission cable where N is the number of ports on said transmission cable and where the gain of each amplifier is such asto substantially compensate forsignal iosses produced in the associated isolating impedance.

3. An arrangement as claimed in Claim 1 or2, characterised in thattheterminals of a power-supply are connected to respective conductors of said transmission cable, each amplifier being energised by a resulting supply voltage present at its associated port.

4. An arrangement as claimed in Claim 1,2 or3, in which said transmission cable is a coaxial cable, characterised in that said ports are each formed by a coaxial connector of a firsttype, each isolating impedance and its associated amplifier being mounted in a housing having a coaxial connector of a second type complementa ry to the first type for making connection therewith, the second type connector being connected through the isolating resistor to the appropriate active terminal ofthe amplifier.

5. An arrangement as claimed in Claim 1,2,3 or 4, characterised in that each isolating impedance is formed by eitherthe input orthe output impedance of its associated amplifier depending on which active terminal of said amplifier is connected to a port.

6. An arrangement as claimed in Claim 1,2,3 or4, characterised in that a pair of complementary connectors is provided in the connection between a port and its associated amplifier, the isolating impedance being located between the port and one of the connectors.

7. An RF signal connection arrangement substan tally as herein described with reference to the accompanying drawings.

8. An amplifierunitforusewith an RFsignal connection arrangement as claimed in 4 or 5, in which said unit comprises a housing, a first coaxial connector a firstterminal of which is connected to a first active terminal of an amplifier whose second active terminal is connected to a first terminal of a second coaxial connector, the first terminal of the first coaxial connector being additionally connected through an RF choke to a first supply terminal of said amplifierwhose second supply terminal is connected to the second terminal ofthe first coaxial connector, characterised in thatthe connection between thefirstterminal of said first coaxial connector and the first active terminal of the amplifier is by way of an isolating impedance whilst said amplifier unit may be of a firsttype in which the first and second active terminals are the input and output respectively or of a second type in which the first and second active terminals are the output and input respectively.

9. An amplifier unit as claimed in Claim 8, characterised in that the isolating impedance is formed by the terminal impedance of the associated active terminal.

10. An amplifier unit substantially as herein described with reference to Figures 3 or4 ofthe accompanying drawings.