GB2306792A - Microwave multiplexer - Google Patents

Abstract

A microwave multiplexer comprising a line arranged as a ring. The multiplexer has the filters BP of a plurality of channels connected to it at intervals and also has a common antenna connected to it (point A).

Description

Microwave Multiplexer The present invention relates to a frequency multiplexer or manifold for microwave signals, particularly for use in communications stations.

Mobile radio communications require careful allocation of frequency bands by the relevant national or international authorities to providers, who in turn divide these frequency bands into narrower bands for allocation to individual users.

Because the electromagnetic spectrum is limited, providers must make frequency allocations to users with considerable care and must have sufficient flexibility to re-allocate frequencies rapidly.

It is a requirement to achieve maximum performance at minimum cost. At bast stations, a critical component is the circuit which connects several transmitters to a common antenna, each transmitter serving one user at a time, each operating at a different frequency from the others and each being rapidly tunable. Such connecting circuit is referred to as a frequency "multiplexer" or as a "manifold". It is an objective to design such a multiplexer to have the best possible electrical performance compatible with simple and flexible mechanical construction: this flexibility is needed so that stations can expand to serve more customers. It is an object of the present invention to provide a multiplexer which meets these requirements and also exhibits improved performance over conventionai multiplexers.

There are many known types of multiplexer. Those under consideration in the present discussion require tunable filters operating in ncn-contiguous channels: their band-edge frequencies, at any given time, are spaced far enough apart that they do not interfere with one another. Some examples of known multiplexers which are applicable are shown in Figures 1 to 3 of the accompanying drawings. Figure 1 shows a "star configuration", in which the bandpass filters BP of all channels are connected to a common junction, to which the antenna is also connected. Figure 2 shows an arrangement in which complementary bandpass-bandstop filter pairs (BP and BS) are connected in cascade up to the antenna.Figure 3 shows an in-line configuration in which the filters BP are connected at carefully selected points to a single section of line which has the antenna at one end and a short circuit at the opposite end.

In a typical mobile telephone system, there are a plurality of transmitters (24 being a typical number) all connected to the same antenna. At high frequencies, the "star configuration" of Figure 1 suffers from the fact that there is usually not enough space to connect all filters directly (i.e.

within a small fraction of a wavelength) to the common junction with the antenna: accordingly, line lengths must be introduced to achieve the required physical separation; these line lengths result in undesirable features such as frequency sensitivities causing mismatch within a user band as well as mismatches which vary among user bands. Usually it is possible to achieve a satisfactory compromise between circuit complexity and performance, for example by using the arrangement of Figure 2: however, complex circuits tend to be bulky, difficult to adjust, difficult to design, relatively inflexible and expensive. The configuration of Figure 3 is often chosen for its simplicity, even though it generally has inferior performance.

We have now devised a multiplexer or manifold which offers the simplicity and flexibility of the in-line configuration, but can also provide significant improvements in electrical performance.

In accordance with the present invention, there is provided a microwave multiplexer which comprises a line arranged as a ring and having the filters of a plurality of channels connected to it at intervals and also having a common antenna connected to it.

As will be explained in this specification, this "ring" configuration provides the advantage that the filter furthest from the antenna junction is closer to that junction than in the above-descred in-line configuration (assuming the same overall number of filter connections).

In one embodiment of the present invention, adopting a first design approach, the ring forms the first resonator of a filter. In order to eliminate possible spurious resonances due to standing waves in the ring, a lossy element may be provided at an appropriate point. For example, the junction to which the antenna is connected may comprise a hybrid junctions (e.g. a Wilkinson divider) to absorb the unwanted resonance.

In another embodiment, two isolators are provided in the ring adjacent the junction to the antenna, having their forward (low-loss) direction towards the antenna.

An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: FIGURE 1 is a schematic diagram of a first known type of multiplexer or manifold; FIGURE 2 is a schematic diagram of a second known type of multiplexer; FIGURE 3 is a schematic diagram of a third known type of multiplexer; and FIGURE 4 is a schematic diagram of a multiplexer in accordance with the present invention.

For given filter or console physical dimensions, the separations between adjacent filters are substantially predetermined. If the minimum necessary centre-to-centre spacing is denoted by L, then when the filters are connected to a single line, as in Figure 3, with one end of the line connected to the antenna and the other end short-circuited, then the line length to accommodate n filters must be of the order of nL. This dimension is a major limitation in determining the frequency sensitivity.

Figure 4 shows a multiplexer or manifold in accordance with the present invention, comprising a line in the form of a ring, to which the filters BP are connected at intervals.

Instead of the filters being connected to a line of length nL extending from the antenna port A to a short circuit, the filters are connected to a ring with the result that the filter junction furthest from the antenna junction A is only half as far away as in the in-line configuration of Figure 3, i.e. a distance of the order of nL/2 instead of nL.

Further, the ring configuration makes it practical to design matching circuitry at the junction A leading to the antenna, which is thus applicable to all filters at once. This results not only in a better match for all channels but, just as importantly, reduces the variability of insertion loss from channel-to-channel: thus, as channels are switched as needed from frequency to frequency, there is no significant variation in range or reception, as sometimes occurs with many conventional arrangements.

In a first design approach, the ring is treated as the first resonator of a filter, and the output circuit at the antenna junction A is designed accordingly. We have built and tested a multiplexer using this approach.

The above design approach can however lead to spurious resonances which are due to standing waves in the ring, which are unable to couple out because a standing wave minimum falls at the output junction. If such a spurious resonance occurs within the system's operating band, then it can be effectively eliminated by providing a lossy element where the spurious resonance has a maximum and the amplitude of the standing wave is zero, or substantially zero: this is possible where the ring has been made into the first resonator of a filter. For example, the output T-junction A, leading to the antenna, can be replaced by a hybrid junction, e.g. a Wilkinson divider, to absorb the unwanted resonance. We have built and tested a multiplexer using this design approach.

A third design approach can also be made, requiring very little or no matching structure at the output junction, as follows. Two ferrite isolators are placed inside the ring adjacent the output T-junction A, i.e. at the positions indicated at B in Figure 4, these isolators having their forward (low-loss) direction towards the antenna. The isolators accordingly absorb reflected waves (i.e. waves inside the ring, travelling away from the output junction A). It will be noted that, in the in-line configuration of Figure 3, it would be unacceptable to provide isolators between coupling junctions, as they would introduce a 3-dB loss over and above the short-circuit alone.

The above-described ring configuration of the present invention provides a simple, flexible and relatively inexpensive arrangement to connect a large number of filters to a common antenna, whilst making it possible to approach the theoretical performance of multiplexers which are considerably more complex, expensive and cumbersome.

Claims (7)

Claims

1) A microwave multiplexer which comprises a line arranged as a ring and having the filters of a plurality of channels connected to it at intervals and also having a common antenna connected to it.

2) A microwave multiplexer as claimed in Claim 1 wherein said ring forms the first resonator of a filter.

3) A microwave multiplexer as claimed in Claim 2, in which said line incorporates a lossy element.

4) A microwave multiplexer as claimed in Claim 3, wherein the junction to which said antenna is connected comprises a hybrid junction, said hybrid junction providing said lossy element.

5) A microwave multiplexer as claimed in Claim 4, wherein said hybrid junction comprises a Wilkinson divider.

6) A microwave multiplexer as claimed in Claim 1, wherein two isolators are provided in the ring adjacent the junction to the antenna, having their forward (low-loss) direction towards the antenna.

7) A microwave multiplexer substantially as herein described with reference to Figure 4 of the accompanying drawings.