WO1992003887A1

WO1992003887A1 - Modular amplifier system for television distribution

Info

Publication number: WO1992003887A1
Application number: PCT/AU1991/000385
Authority: WO
Inventors: Jan Raubiszko; Robert John Richards; Donald West
Original assignee: Hills Industries Limited
Priority date: 1990-08-23
Filing date: 1991-08-23
Publication date: 1992-03-05

Abstract

A signal combining apparatus particularly useful in television signal distribution systems of modular design providing an extendable and convenient means for adding and replacing modules without the need to disconnect large amounts of critical length co-axial cables. The apparatus comprises first (35) and second (36) connection means located on opposite sides of a module (18) where the connection means are adapted to releasably connect to adjacent modules (12-17) as required wherein said signal transmission means (40) is located between said connectors and each module (12-18) has a signal processing means having an output signal and a signal combiner means (90) adapted to match said output signal to the transmission means (40) and combine with a transmission line signal. Microstrip is one example of a suitable transmission means.

Description

MODULAR AMPLIFIER SYSTEM FOR TELEVISION DISTRIBUTION

This invention relates to radio and television signal distribution systems and in particular a modular receiver and amplifier system for the simultaneous distribution of radio and television signals to a number of receivers, and in particular a signal combining means which is integral with the modular components of the distribution system.

BACKGROUND OF THE INVENTION

Typically, television and radio signals for distribution to a large number of users within a building are received off air by antennas specifically designed to provide low noise, low interference, and minimum multipath distortion which otherwise is not obtainable or desirable at each of the user's locations within the building. For example, lodging establishments such as hotels and motels prefer to utilise a single television reception antenna and its associated television distribution system so that a multiplicity of television receivers located in each of the rooms of the establishment may be supplied with adequate television and radio signals in addition to providing produced video programming controlled by the establishment.

The typical elements of such systems comprise: an antenna, with which MW radio, VHF and UHF television broadcast signals are received; a signal processing and combining module; a distribution system used to distribute the combined signals to subscribers or individual television receivers located in each of the rooms of the establishment and standard radio and television receivers which are fed signals from the distribution system.

It is in the field of television distribution systems that the present invention is typically applicable. In such systems signal processing and combining modules comprise antenna pre-amplifiers, AM and FM hetrodyne repeaters, single channel amplifiers, UHF and VHF to IF down converters, IF to VHF and UHF up converters, vestigial sideband audio/video modulators and the associated mixing networks required to combine the individual outputs of various program sources to feed the distribution system.

The signal processing and combining modules are usually located close to the antenna where it is possible to receive the desired off air channels with a minimum of interference and at a sufficiently high level to obtain a high quality starting signal for combination and eventual distribution by the system.

It is also typical for the signal processing and combining modules to be interconnected with a complex array of radio frequency coaxial cables and various other signal and power carrying wires. In particular, it is known for the radio frequency signal combining modules to use pre-cut coaxial cable interconnections between units, so as to maximise signal coupling efficiencies between the processing and combining equipment. It is therefore important for installation and service personnel to maintain the correct interconnection arrangement and to not unduly interfere with the critical length of the coaxial cables thereof. It has also been known for the Mean Time Between Failure (MTBF) of the system to decrease with each use of interconnection cable, while it is also well established that for each disturbance of the installed system while being serviced, there is an increase in the likelihood of future failure.

It will be apparent therefore that each element of a signal distribution system is required to be connected to another element of the system and a large number of cables and wires is required to achieve this. Such an arrangement can be a difficult environment in which to trace a fault and it is a desirable requirement of such a system to have a signal combining device between signal processing modules which eliminates the need for a complicated array of interconnecting cables. It is a desirable requirement of such systems that individual elements be easily serviced or replaced and since technical assistance is not readily available, each element should ideally be capable of being monitored and modules replaced by untrained and unskilled users with minimum potential of disturbance to the combining device, so that simple faults may be easily identified and rectified prior to the attendance of repair personnel.

It has further been a desirable requirement of such systems that tuning of various signal processing and frequency combining elements is easily achieved at production and installation. Contemporary tuning techniques however have been limited by the standard nature of tuning related components such as variable inductive core positioning and variable resistance devices, which require the use of tuning tools while the system is being tested and installed. It has been found that the unpredictability of the final assembled configuration of these tuning elements and the variable length of connecting cables can unduly affect the performance of the individual modules and of the system itself.

It is yet a further consideration in the design of such systems that the various elements of the signal processing and combining unit are adequately screened from the radio frequency interference. Signal generation by each of the units required in a system of this kind is likely to affect adjacent units and other interconnected like units. Such screening is generally provided by individual encapsulation of resonant circuits which are located on various parts of the circuit boards of the system. There is however little consideration of the interfering effects which result from the lack of isolation between circuit boards that make up the system or that leakage of radiation that can result from poor connector termination practice, which becomes especially evident as the number of connectors used increases .

Another aspect of the invention comprises a frequency conversion system integrated within a module and its adjacent module, such that physical interconnection between the modules when located adjacent each other and interconnected via integral connection means allows a I.F. frequency selective amplifier in one module and working with a dual frequency conversion sub-system in another module to have a common output. This common output may then be combined into the radio frequency to be distributed through the system.

DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood and readily carried into effect, an embodiment will now be described by way of example only, with reference to the accompanying representations, wherein:

Fig. 1 depicts the front panel layout of a modular television signal distribution system;

Fig. 2 depicts a top perspective view of two connected circuit boards of the modular television signal distribution system and details in particular the configuration of the radio frequency connector used for connecting the modules together;

Fig. 3 depicts a sectional view of one half of the casing of a television signal distribution system module;

Fig. 4 depicts a sectional view of the casing of the other half of a television signal distribution system module orientated for fitment with the half depicted in Fig. 3;

Fig. 5 depicts a schematic representation of the radio frequency combiner configuration of the television signal distribution system; Fig. 6 depicts a schematic representation of the insertion loss of an embodiment of the signal combiner configuration;

Fig. 7 is a schematic representation of a helical filter element;

Fig. 8 shows a bottom view of an arrangement for varying the matching impedance of a tuning element;

Fig. 9.shows an arrangement for varying the coupling efficiency of closely coupled tuning elements.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment of the invention, there is depicted in Fig. 1 a typical configuration of modules of a television signal distribution system. Rack members 10 and 11 are spaced apart and fixed to a stable wall surface parallel to one another. Their cross-sectional profile is such as to allow reciprocally shaped guide members fixed to the top and bottom rear wall of each module to slidably engage with the rack member and be releasably fixable and locatable along the members 10 and 11. The power supply module 12 is locatable adjacent module 13 which is a VHF Channel 10 receiver/amplifier as per the Australian Broadcasting Band Plan. Reciprocal connectors 19 and 20 located at their lower ends and on opposing sides are releasably connectable to each other. Radio frequency and Intermediate Frequencies (IF) and power are connected between modules.

Power supply module 12 also has a radio frequency signal output connector 21 to which is connected a signal distribution system, typically a low loss coaxial cable 22 which is taped off and terminated as required in the vicinity of individual television receivers. The arrangement of modules depicted in this embodiment comprises individual VHF receiver modules 13-16 for channels 10, 9, 7 and 2 respectively of the Australian Broadcasting Band Plan and a UHF to IF down converter module 18 for channel 28 of the same plan which is located adjacent an IF to VHF up converter module 17 for channel 0 of the same plan.

Connection of the IF signal output of module 18 is via coaxial cable 32, a track and isolated pins (not shown) of the connectors 33 and 34, all located between modules 18 and 17. Connector 35 shown on the opposite side of module 18 to connector 33, may be used to append to the configuration addition modules, for example, pre-amplifiers for weak received antenna signals, vestigial sideband modulators for audio/video input and other like modules.

A UHF antenna 23 is connected to the RF input 24, while a VHF Band II antenna 25 of the plan is connected to the RF input 26 of module 16 and a VHF Band III antenna 27 is connected to a passive splitter (but which may also be an active splitter) 28 which in turn is connected to the RF inputs 29, 30 and 31 of modules 13, 14 and 15 respectively.

As will be described in greater detail, connectors between the modules provide a continuous path for radio frequency and power distribution between modules. To isolate a module for replacement its guide members are loosened so that the modules may move laterally along the rack members 10 and 11 towards their left or right. Further lateral movement of the desired module, releases its connector from the adjacent module. Having isolated the module the guide member allows the module to be raised and removed from engagement with the rack members. Replacement with a repaired or new module is achieved by reversing the above process . This method of module interconnection ensures constant standard of conduction between elements of the conductor and obviates the need for an untidy collection of pre-cut coaxial cable interconnection means between modules.

Fig. 2 depicts one embodiment of connectors 34-37 which comprise DB15 connectors, wherein connectors 35 and 36 are plugs and 34 and 37 are sockets. Printed circuit board 38 associated with module 18 and printed circuit board 39 associated with module 19 carry on their lower sides, a track 40 for conducting direct current power and a track 41 for conducting radio frequency signals. Larger surface area tracks 41, 42 and 43 are connected to earth as are the substantially unbroken conductive upper surfaces 44 and 44a (not shown) of the boards 38 and 39 and form in this embodiment a printed-board strip line (microstrip) in combination with track 41 which is particularly suited for the transmission of radio frequencies.

In this particular figure an IF signal is conducted between modules 18 and 17 via coaxial cable 32 via pin 45 of connector 35 and pin 46 of connector 34 to coaxial cable 32a onto module 17. This arrangement is peculiar to the down and up converter modules 17 and 18 otherwise respective pins of other connectors are used to connect to the earth^" track 42 of respective printed circuit boards of the modules.

Coaxial cable 47 carries VHF radio frequency channel 0 from the output of the up converter circuit of module 17 and is connected to track 40 on printed circuit 39. The output circuit of module 17 may comprise a variety of matching and combining means, in this embodiment a capacitive coupling circuit is used. Fig. 5 schematically depicts a variety of coupling means including inductive means.

The characteristic impedance, attenuation of microstrip 40 on printed circuit board 38 is dependant on the spacing between track 40 and its adjacent tracks 41 and 42 and the conductive surface 45 as well as the thickness of track 40 and the frequency of the signal being transmitted. Additionally, since the printed circuit board 38 is located at the lower portion of the module and track 40 faces the bottom metallic cover of the module a further RF barrier is created which isolates the transmission path created by the microstrip from the active circuit within the module and the effects of external radio frequency noise.

The signal strip 40 is etched to the appropriate size to provide the required impedance and attenuation performance, while the ground usually covers the opposite side of the printed circuit board, but is required only to be at least twice as wide as the signal strip.

Copper is the commonly used conductor material and the performance of icrostrips, like other high frequency signal-carrying transmission line devices, is very much dependant on the characteristics of the dielectric material between the conductor material.

For that reason plastics such as pure plastic, filled plastic, and fibre-reinforced plastic present some alternative materials. However, other factors are important such as heat dissipation and its variation with frequency and temperature, homogeneity, uniformity and isotropy, useful temperature range, dimensional stability, tensile and flexural strength and processing limitations.

It is important to be aware of these characteristics the higher the frequency used. However, the use of VHF and Low Band UHF makes these considerations less critical.

Although this embodiment depicts a microstrip to conduct signals between connectors, alternatives in the form of coaxial cable and twisted pair could also be used. It will be noted though that the use of a fixed length printed circuit board integrally matched to the connectors provides a stable, shielded and easily interconnected arrangement for adjacent modules as well as providing a convenient means for connecting the output from the signal processing circuit of the module via a matching circuit onto the signal transmission path.

It will be appreciated therefore that the rigid physical configuration of the interconnections created by the connectors located as shown in Figs. 1 and 2 provides for an equally spaced arrangement of modules and thereby a constant spacing relationship between the physical location of the radio frequency combining elements. This is advantageous for the optimum transfer of the received signal onto the transmission path created by the in-line radio frequency connectors .

Figs . 3 and 4 show sectional views of the profile of identical halves of an extruded metallic casing for the modular circuit boards previously described. The module enclosure comprises two identical halves which are assembled face to face to form a hollow rectangular housing.

The top half of a casing depicted in Fig. 3 comprises side walls 49 and 50 at right angles to each other joining along a reinforcing rib 68. The free edge 51 of side wall 49 is designed to snugly fit in grooved edge 56. An identical grooved marginal portion 52 on the other half, shown in Fig. 4 similarly receives the edge 57 of the first half. When the two halves are assembled together, a box-like structure is formed which has stability and more importantly a resistance to leakage and/or intrusion of radio frequency signals, other than those which are introduced via the radio frequency connectors as described and shown in Figs. 1 and 2. The overlap of surfaces of the housing ensures that the gaps attenuate and restrain the leakage and intrusion of radio frequency interference.

Ribs 64 and 65 on side 54 align in opposed relationship with ribs 66 and 67 on circuit board runner 59 when the structure is in its closed configuration. End caps (not shown) are used to seal the structure and screws are inserted through the end caps and into the grooves between ribs 64, 65 and 66, 67 wherein the peripheral edge of the helical screw contacts and frictionally engages both pairs of ribs . This arrangement provides further electrical connection between the parts of the housing obviating the need for separate straps and/or links to be provided while improving the radio frequency sealing of the constructed housing.

Similarly ribs 70, 71 are opposed to ribs 62, 63 when the sides are brought together.

Circuit board runner lugs 58 and 60 are also opposed to each other when the sides are brought together and allow sliding insertion and/or retention of the circuit board or removal of the side frame to provide access to the circuits therein.

Likewise circuit board opposed runner lugs 59 and 61 are similarly available for supporting a circuit board therein when the sides are brought together.

Fig. 5 shows a schematic representation of the signal combiner network configuration which results from the spaced relationship of the radio frequency connectors which are in¬ line along the base of the modular TV signal distribution system and which terminate into the distribution cable 22. A nominal mid-band frequency for the transmission line is determined and the spaced relationship of the radio frequency combiners is a combination of capacitive and inductive coupling devices which are arranged along the transmission line to maximise signal transfer efficiency as a result of their location along that transmission line.

A purely resistive 75 ohm or as desired value of the termination load may be located at an end of the combiner transmission line network 40. A capacitively coupled radio frequency source 1 is located adjacent an inductively coupled radio frequency source 2. It will be apparent that the modules which relate to source 1 and 2 respectively are located at fixed distances from the termination point by virtue of the fixed spaced relationship of the connectors and the location of the combining units therebetween when incorporated in the modular system described in Fig. 1. Also shown is a radio frequency source 3 which is capacitively coupled to the combiner transmission line network. A termination impedance of greater .value than the impedance of the transmission line allows a capacitive coupling to see low impedance at the termination point and conductively coupled sources effectively have an open circuit coupling at their locations . Radio frequency source 3 is shown as an IF source which is conducted to an adjacent module for further signal processing whereby radio frequency source 5 is capacitively coupled to the combiner/transmission line network.

A nominal direct current conduction path 39 is also depicted in Fig. 5, while earth 63 is depicted pictorially above and below the combiner/transmission line network 40.

Fig. 6 depicts the insertion loss characteristics of the combiner/transmission line which has six channels (2, 7, 9, 10, 28 and 52) impressed upon it. The vertical axis depicts loss in lO.OdBuV per graduation while the horizontal axis depicts frequency in 100MHz per graduation.

Fig. 7 depicts a pictorial section view of a helical filter element. These filters commonly comprise a helically wound coil located within a radio frequency sealed can- shaped encapsulation 90. The coil may have a high Q value and is shown as having a free end 110 located in close proximity to the inner surface of the can 111 and which exhibits a small amount of capacitance between the coil and the can 90. Helical filter coils of this general type generally have a separate earth connection 112, however some also have a preset tap 113 at which point radio frequency energy may be inserted into or extracted from the coil circuit element. This connection is preset and tuning of such coils is generally achieved by varying the longitudinal location of a metallic element 114 along the axis of the coil.

In this embodiment it has been found advantageous to extend the coils length by lengthening its end 96 into the form of a printed circuit track, as shown in Figs. 7 and 8, so that solder 97 or other conductor means may be used to bridge across from the extended coil portion 96 to earth and thus provide a convenient and otherwise unobtainable impedance matching mechanism and arrangement for such coil types .

Fig. 9 depicts an arrangement for varying the matching of closely coupled tuning elements. In particular the UHF modules of this device utilise helical filter elements. As shown pictorially in Fig. 9 can-shaped elements 90 and 91 are arranged adjacent one another. One end of the coil within these cans is connected to a track of the printed circuit board 106 at node 92 as shown in Figs. 7 and 8. The helical coil may then be shorted to the ground track 98 by providing a wire or other electrical connection link such as a solder bridge 97 to a suitable point from the circuit track 96 which forms part of the coil, so that suitable matching of the coil can be achieved.

In certain circuits of the UHF modules of the television signal distribution system, coupling occurs between helical filter coils which are in close proximity to each other. In such cases a complicated arrangement of fixed and variable cavity sizes which contain at least two helical coils are arranged so that variable coupling can be achieved between the coils. In this embodiment as shown in Fig. 9 two adjacent cans 90 and 91 have apertures 101 and 102 (not shown) in opposed relationship to each other. In one embodiment a metallic flap member.103 is soldered to the printed circuit board at 104 which is then connected to an earth track 98. The flap member then acts as a deflector and attenuator of signals being coupled between coils.

Adjustment of the flap 103 in the directions of arrows 105 and 106 varies the degree of signal that may be coupled from one coil to the other. Coupling of the helical filters using this mechanical adjustment method has been found to be easy and convenient. The flap 103 may be soldered permanently into position or alternatively if certain parameters have changed the flap may have its location relocated to adjust to the new parameters by adjusting its orientation and resoldering it into position.

An alternative arrangement for adjusting the coupling between helical filters is provided by soldering two thin and stiff wires upstanding between the adjacent cans 90 and 91.

The wires can then be moved into place to suit the required coupling between helical filter elements .

Claims

The claims defining the invention are as follows:

1. A signal combining apparatus comprising a plurality of signal combining modules wherein at least one of said modules comprises: first and second connection means located on opposed sides of said module wherein each of said connection means is adapted to releasably connect to connection means of an adjacent module, a signal transmission means adapted to connect between said connection means to form a transmission line for the transmission of a signal between said first and second connection means, a signal processing means having an output signal, and signal combiner means adapted to match said output signal to said transmission means, wherein said transmission device is adapted to receive said matched signal processing means output signal to thereby combine said output signal with a transmission line signal.

2. A signal combining apparatus according to claim 1 further comprising a printed circuit board, wherein said first and second connection means are fixedly located at respective ends of said printed circuit board, said signal transmission means comprises a microstrip integral with said printed circuit board.

3. A signal combining apparatus according to claim 2^' wherein said printed circuit board has a plurality of tracks located thereon whereby at least one of said track is adapted to carry direct current between said first and second connection means.

4. A signal combining apparatus according to claim 1 wherein said signal combiner means comprises inductive or capacitive elements to transfer said signal processing means signal output onto said transmission device.

5. A signal combining apparatus according to claim 1 wherein said signal transmission means comprises a microstrip connected between said first and second connection means.

6. A signal combining apparatus according to claim 1 wherein said signal transmission means comprises a planar printed circuit track connected between said first and second connection means.

7. A signal combining apparatus according to claim 1 wherein said signal transmission means comprises a coaxial cable connected between said first and second connection means .

8. A signal combining apparatus according to claim 1 wherein said first and second connection means comprise complementary radio frequency signal connectors.

9. A signal combining apparatus according to claim 8 wherein said connection means comprises DB15 connectors.

10. A signal combining apparatus according to claim 1 further comprising a signal combining module housing having two identical halves which releasably fit together in face to face relation to form a hollow rectangular cross-section housing said housing further having opposed openings to allow said connectors to be positioned for releasable connection to a connector of an adjacent module.

11. A signal combining apparatus according to claim 6 wherein said DB15 connector has a plurality of integral connector pins some of which are adapted to carry signals from one said module to an adjacent module.

12. A signal combining apparatus according to claim 1 wherein said signal combiner means comprises a helical coil element adapted to match said output signal to said transmission device, said helical filter further comprising an extended earth potential connection wire formed from a metallic circuit board track which is adapted to provide a plurality of connection points between said extended earth connection and an earth potential.

13. A signal combining apparatus according to claim 1 wherein said signal combiner means comprises a two conductively enclosed helical filter elements in proximity to one another so as to interact by mutual induction adapted to allow variable interaction by manual adjustment of induction interference means located between said two filter elements.

14. A signal combining apparatus according to claim 13 wherein said adaption to said filter elements comprises apertures in the enclosure of each said filter element having aperture to aperture orientation and further having said induction interface means comprising of conductive wire means adjustably fixed between said filter elements.