GB2264398A - High frequency coupled transmission lines - Google Patents

Abstract

High frequency coupled transmission line comprises a pair of separated conductive tracks (12, 14) formed upon one side of a insulating substrate (10), and a further pair of conductive tracks (20, 22) formed upon opposite sides of a second insulating substrate (18), mounted upon the first substrate, each track of the pair (12, 14) being aligned with and electrically connected along its length with the corresponding track of the other pair (20, 22). A conductive ground plane (30) is provided upon the lower face of the first substrate (10). The device may be employed as a signal splitter or combiner in radio frequency amplifiers for television transmitters. <IMAGE>

Description

Title: High frequency coupled transmission lines Field of invention The present invention relates to high frequency coupled transmission lines, formed at least in part as conductive tracks on insulating substrate.

Background to the invention High frequency coupled transmission lines of this type may be used as splitters and combiners for high frequency signals, for example for feeding a common input signal to, or combining the outputs signals from, paralleled radio frequency amplifiers such as are commonly employed in television transmitter equipment.

Conventionally, such high frequency coupled transmission lines may be made by forming appropriate conductive tracks, together with enclosing ground planes, upon opposite sides of an insulating substrate, such as a printed circuit board or, alternatively, by interleaving or overlaying conductive tracks upon the same side of the substrate.

In the first case, the provision of enclosing ground planes can be expensive and extremely accura-te registration of the conductive tracks is required. Any errors of registration of the tracks on a printed circuit board carrying not only the coupled line tracks but other circuits, could be costly to correct and could lead to the rejection of the whole board.

In the second case, with interleaved conductive tracks, close coupling requires narrow gaps between the "fingers" of the respective tracks reducing power handling capability and making manufacturing tolerances tighter.

With overlaid tracks, track registration can be difficult and the structure tends to be asymmetrical.

It is an object of the present invention to at least partly overcome these disadvantages of prior art devices.

Summary of the invention According to one aspect a high frequency coupled transmission line, formed at least in part as conductive tracks upon an insulating substrate, comprises a pair of separated but substantially parallel conductive tracks formed upon one side of a first insulating substrate, and a further pair of conductive tracks formed upon opposite sides of a second insulating substrate mounted upon the first insulating substrate such that one track of the pair of conductive tracks upon the first substrate is aligned with and electrically connected to one track of the further pair of conductive tracks, and the other track of the pair of conductive tracks upon the first substrate is aligned with and electrically connected to the other track of the further pair of conductive tracks, a conductive ground plane being provided upon the other side of the first insulating substrate.

The second substrate may be mounted upon the first substrate in a recess formed in the first substrate, between and substantially aligned with the pair of conductive tracks formed upon the first substrate, and preferably perpendicular to the first substrate.

The second substrate preferably extends beyond the perimeter of the tracks fomred upon it.

From another aspect the invention resides in a radio frequency amplifier incorporatinng a coupled transmission line in accordance with the invention. Two such amplifiers may be connected in parallel and the coupled transmission line employed as an input signal splitter or an output signal combiner.

The invention may also reside in a television transmitter incorporating a radio frequency amplifier or a pair of radio frequency amplifiers in accordance with the invention.

The invention will now be described by way of example with reference to the accompanying drawings of which Figure 1 is a perspective view of a high frequency coupled transmission line in accordance with the invention; Figure 2 is a section through the embodiment of Figure 1 along the line II-II looking in the direction indicated by the arrows; and Figure 3 and Figure 4 reproduce the section of Figure 2 with electric force lines inserted to illustrate respectively even-mode and odd-mode impedences of the coupled transmission line of Figure 1.

Detailed description of the drawings Referring to Figures 1 and 2, an insulating substrate 10, such as a printed circuit board, carries a pair of aligned identical conductive tracks 12 and 14, laid down upon its upper surface 16. The substrate 10 may be a discrete element or may form part of a larger substrate bearing other conductive tracks intended to form part of other electrical circuits.

A further insulating substrate 18, bearing a pair of aligned, overlying identical conductive tracks 20 and 22, on opposed sides, is mounted upon substrate 10 in a plane substantially perpendicular to the plane of substrate 10, by location within a suitably dimensioned recess 28, lying between and substantially parallel with tracks 12, 14, which electrically contact tracks 20, 22 respectively along their lengths, and are soldered or otherwise integrally electrically connected thereto.

The lower face of substrate 10 carries a conductive surface 30.

The tracks 12 and 20, and the tracks 14 and 22, respectively, constitute the coupled elements of the high frequency coupled transmission line, with conductive surface 30 constituting a common ground plane.

In use, for example in a radio frequency signal splitter enabling a pair of parallel amplifying devices to be fed from a single incoming signal feed line, the line would be connected to tail 32 of the track 12, and the signal fed through the composite element comprised of track 12 and track 20, and out through tail 34 to the input circuit of one of the amplifying devices. An identical signal would be coupled through the substrate 18 into the composite element comprised of track 14 and track 22 and out through tail 36, connected to the input circuit of the other of the amplifying devices. The input signal to each amplifier circuit would be at approximately half the power of the original incoming signal to tail 32 of the splitter and the two signals would be in phase quadrature.

It will be appreciated that the same device may be employed in a reciprocal manner to combine two properly phase- and amplitude- related signals, for example from the outputs of a pair of paralleled amplifying devices, to provide a single combined output.

Coupled transmission lines of the type to which this invention relates are characterised by their coupling factor, characteristic impedance and working frequency.

The working frequency of a coupled transmission line in accordance with the present invention determines the length of the coupled elements and is substantially the same as in conventional practice.

The coupling factor and charateristic impedance of a coupled transmission line are conveniently described in terms of the even- and odd-mode impedance in the three conductor line which apply when the balanced conductors are energised in phase and in anti-phase respectively.

These impedancies are themselves functions of the line dimensions and the electrical properties of the interlying media. It will be seen from Figures 3 and 4, in which the electric force lines 40 and 42 for even- and odd- modes respectively are indicated, that the construction described substantially separates these notional impedances, so that substate material and track widths for each element of the two parts of a coupled transmission line in accordance with the invention may be selected independently.

By way of example, a radio frequency signal splitter working in the frequency range 470-850MH, and employed in the radio frequency power amplifying stage of a television transmitter comprises a coupled transmission line in accordance with the invention in which the length of each of the conductive tracks 12/14/20/22 is 75mm, the width of the tracks 12/14 carried by the base substrate 10 is 1.65mum, and the width of the tracks carried by the mounted substrate 18 is 2.2mm. The base substrate 10 is of glassreinforced PTFE of thickness 2.lmm and the mounted substrate 18 of the same material but of thickness 0.5mm.

With electro-deposited copper tracks and ground plane both 0.035mm thick, the power handling capacity of the device is in the region of 170W.

It will be appreciated that a coupled transmission line in accordance with the invention offers substantial advantages over those constructed in accordance with the prior art including: a device constructed on only one side of a substrate, which itself may carry other circuits, the ground plane remaining intact; a physical structure for the device which is physically symmetrical and therefore electrically balanced; voltage handling which is enhanced since dielectric material separates individual conductors; registration of tracks on the subsidiary board which is less critical and inexpensive to correct; and in many applications, a device cheaper to manufacture than prior art devices.

Claims (9)

Claims

1. A high frequency coupled transmission line, formed at least in part as conductive tracks upon an insulating substrate, comprises a pair of separated but substantially parallel conductive tracks formed upon one side of a first insulating substrate, and a further pair of conductive tracks formed upon opposite sides of a second insulating substrate mounted upon the first insulating substrate suc that one track of the pair of conductive tracks upon the first substrate is aligned with and electrically connected to one track of the further pair of conductive tracks, and the other track of the pair of conductive tracks upon the first substrate is aligned with and electrically connected to the other track of the further pair of conductive tracks, a conductive ground plane being provided upon the other side of the first insulating substrate.

2. A coupled transmission line as claimed in Claim 1 in which the second substrate is mounted upon the first substrate in a recess formed in the first substrate, between and substantially aligned with the pair of conductive tracks formed upon the first substrate.

3. A coupled transmission line as claimed in either Claim 1 or Claim 2 in which the second substrate is arranged substantially perpendicular to the first substrate.

4. A coupled transmission line as claimed in any preceding claim in which the second substrate extends beyond the perimeter of the tracks formed upon it.

5. A coupled transmission line as described herein which specific reference to the accompanying drawings.

6. A radio frequency amplifier incorporating a coupled transmission line in accordance with any preceding claim.

7. A pair of radio frequency amplifiers connected in parallel comprising a coupled transmission line in accordance with any one of Claims 1 to 5 employed as an input signal splitter.

8. A pair of radio frequency amplifiers connected in parallel comprising a coupled transmission line in accordance with any one of Claims 1 to 5 employed as an ouput signal combiner.

9. A television transmitter incorporating a radio frequency amplifier or a pair of radio frequency amplifiers as claimed in any one of Claims 6 to 8.