GB2266194A

GB2266194A - Frequency multiplexer

Info

Publication number: GB2266194A
Application number: GB9208237A
Authority: GB
Inventors: John David Rhodes
Original assignee: Filtronic Components Ltd
Current assignee: Teledyne Defence Ltd
Priority date: 1992-04-14
Filing date: 1992-04-14
Publication date: 1993-10-20
Also published as: GB9208237D0; WO1993021666A1

Abstract

In a multiplexer comprising two or more inputs (IN1, IN2) for connection to respective transmission sources and a common output for connection to an antenna (ANT), the inputs being connected to the output by respective filters (10, 12) tuned or tunable to separate passbands, each filter comprises at least two resonant stages (S1, S2) with both its final stage (S2) and its penultimate stage (S1) coupled to the common output. The multiplexer is arranged so that the transmission characteristic of each filter is substantially unaffected by a change in the operating frequency of the or any other channel.

Description

Multiplexer This invention relates to a frequency multiplexer and particularly to a high power multiplexer with tunable channels.

With the growth of cellular communications the need has arisen for the frequencies of transmission channels to be modified to accommodate growth and variations in daily demands.

However, a change in frequency of one channel affects the characteristics of the other channel or channels. Typically the different channels are connected to a common manifold through respective microwave cavity filters: a change in frequency of one channel causes distortion of the transmission characteristic of the filter of the adjacent channel and/or a displacement of the centre frequency of the passband of that filter.

We have now devised a multiplexer which overcomes the above problems, and wherein the transmission characteristic of one channel is substantially unaffected by a change in the frequency to which an adjacent channel is tuned.

In accordance with this invention, there is provided a multiplexer comprising two or more inputs for connection to respective transmission sources and a common output for connection to an antenna, the inputs being connected to the output by respective filters tuned or tunable to separate passbands, each filter comprising at least two resonant stages with both its final stage and its penultimate stage coupled to the common output.

The coupling between the two stages (or final two stages) of each filter, and between each of those two stages and the output, can be arranged so that the transmission characteristics of each filter is substantially unaffected by a change in operating frequency of the other (or any other) channel. In particular, the transmission characteristic of each filter is substantially unaffected by a change in operating frequency of the or any other channel to within 3 times (and preferably twice) the passband from its own centre frequency. For example, the usual requirement is for a minimum channel separation of 600 KHz between channels of 180 KHz bandwidth in the 935 to 960 NHz band.

The filter in each channel may comprise a microwave cavity filter: this filter may comprise separate cavities providing the two (or final two) resonant stages, or these two resonant stages may be supported in a single, dual-mode cavity.

Embodiments of this 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 an embodiment of diplexer in accordance with the invention; FIGURE 2 is an equivalent circuit diagram of the diplexer of Figure 1; FIGURE 3 is a simplified version of the equivalent circuit diagram, for use in explaining the characteristics of the diplexer of Figure 1; FIGURE 4 is a diagram showing the intringic response of each channel of the diplexer of Figure 1; FIGURE 5 is a diagram showing the reponse of one of the diplexer when the other channel is tuned to a centre frequency 600 RHz from the centre frequency of the one channel; and FIGURE 6 is an equivalent circuit diagram of a multiplexer in accordance with the invention, the multiplexer having n channels.

Figure S shows in schematic form a multiplexer in accordance with the invention. The example shown is a diplexer, i.e. with two inputs connected to a common output antenna. The multiplexer comprises two dual mode microwave resonant cavities 10, 12, each forming a tunable two-stage filter. The cavities 10, 12 have input probes IN1 and IN2 for connection to respective sources, to excite a first resonant stage in the respective cavities, indicated by SI in each case.

An adjustable screw K2 projects into each cavity and provides coupling to a second resonant stage S2, being a second mode orthogonal to the first mode S1. Respective probes 6 and 3 couple from the second resonant stages via coaxial cables 13, 14 to a node 2, and a coaxial cable 15 connects node 2 to a common node 5, to which the antenna is connected via a coaxial cable 18. Cables 13, 14 correspond in length to one wavelength (or multiple of half wavelengths) and cable 15 corresponds in length to a quarter wavelength. Probes 1 and 4 in the respective cavities couple from the first resonant stages via coaxial cables 16, 17 to the common node 5: cables 16, 17 correspond in length to one wavelength. Coaxial cable 18, which connects node 5 to the antenna, is of one wavelength.

The coupling screws K2 provide a selected impedance transformation between the first and second resonant stages S1, S2 in each cavity. Probes 1, 4 provide a selected impedance transformation between the respective second resonant stages S1 and the lines 16, 17. Probes 3 and 6 act as impedance inverters.

The arrangement of Figure 1 can be represented by the equivalent circuit diagram shown in Figure 2. Each channel is represented by a source S normalised by a series resistor and connected across the respective first stage of resonance S1 of the respective cavity 10 or 12. Each of the two stages of resonance in each filter is represented by a capacitor in parallel with an inductance: the inductance can be regarded as frequency-invariant, because the bandwidth of either channel is very small as compared with the operating frequency (typically two hundred KHz compared with an operating frequency of approximately 1000 MHz). The first and second resonant stages S1 and S2 are coupled by an impedance inverter having a characteristic admittance K2.The second resonant stages 52 are coupled to the node 2 by impedance inverters having characteristic admittances of minus unity, and node 2 is coupled to common node 5 by an impedance inverter of unity characteristic admittance. The first resonant stages S1 are coupled to the common node 5 by impedance inverters of characteristic admittance K1.

If one channel operates at a frequency of fo + ja and the other operates at a frequency of fo - ja, then ideally each channel effectively produces an all-pass network at the centre frequency of the other independent of the value of a.

Evaluating at the centre frequency of the first channel, the circuit of Figure 2 reduces to that shown in Figure 3. For two channels well separated in frequency, a is large and the two shunt elements tend to short circuits. Thus K2 can be regarded as being connected in parallel across K1. In order to ensure a perfect match: K2 = 1 - K, (1) For finite values of a, the transfer matrix of the circuit between nodes 2-3-4-5 is:

The corresponding admittance matrix is:

where

The transfer matrix between nodes 2-1-5 is:

with a corresponding admittance matrix of:

Connecting the two admittance matrixes (3) and (6) in parallel with the unity impedance inverter between nodes 2 and 5 yields an overall admittance matrix of::

Scaling the input node by (1-K) and extracting a 151resistor from the input leaves an admittance matrix of:

Expanding each entry in terms of 1/a and ignoring terms (1/a2) and above gives:

For the network to be an all-pass network to the order or 1/Q requires:

or

Consider an example where C2 = C1 and the positive solution of equation (10) is taken. The GSM system requires a minimum channel separation of 600 KHz and a 0.5dB bandwidth of 180KHz to be maintained, with variations in temperature, in the 935 to 960 NHz band. Figure 4 shows the response of a single channel (in the case of dual mode dielectric cavities of Figure 1 with a Q factor of approximately 14,000).Thus, Figure 4 shows the transmission characteristic A and the return loss characteristic LR. The 0.5dB bandwith is il25 KHz and the rejection is greater than 12dB at i600 KHz from the centre frequency fo.

Figure 5 shows the response of the channel if the second channel is tuned to fo + 600Hz (the minimum channel separation). Thus the modified transmission and return loss characteristics LA and LR are shown in comparison with the unmodified characteristics of Figure 4: the centre of the return loss characteristic has been displaced by about 10 Khz, the 0.5 dB bandwith is now -130 KHz to +120 KHz and the rejection at the centre of the adjacent channel is still greater than 12 dB. Accordingly, the transmission and return loss characteristics of the channel are substantially unaffected, for practical purposes, by turning the second channel to the minimum separation of 600 KHZ.

Whilst a diplexer (two channels) has been described, the principles are generally applicable to a multiplexer with more than two channels. Such an arrangement is shown in Figure 6. Here each channel has its final resonant stage coupled to a first common line 20 (instead of to the node 2 of Figure 2), and adjacent points of coupling to line 20 are separated by two unity impedance inverters. The first resonant stage of each channel is coupled to a second common line 22 (instead of the common node 5 of Figure 2), via an impedance inverter of admittance K1 (eg K11), and two further impedance inverters of admittances one and minus one: adjacent points of coupling to the line 22 are separated by two unity admittance impedance inverters. One end of the line 22 is coupled to the antenna by a unity admittance impedance inverter.Also each channel may include any number of resonant stages preceding the final two stages: this is represented in Figure 6 by lossless networks N.

At the centre frequency p = flr of the rth channel, to a first order of approximation the admittance reflected onto node 31 by all other channels is:

The equivalent admittance reflected onto node 31 is:

For Yff and Y2r approaching zero, to preserve the performance in the rth channel: Klr + K2r = 1 (13) i.e. K2r = 1 - K1r Also, if looking back into the lossless network N of the rth channel there is unity admittance at the frequency nr, then the additional admittance matrix between nodes 31 and 32 is:

Therefore the total admittance matrix between nodes 31 and 32 is::

For this to provide a lQ source impedance to the 1# load, to a first order of approximation it is required that: (1-K1r)2Y2r = Y1r r = 1 - n (16)

Typically for a tunable muliplexer, all prototype channels will be the same, i.e.

cii = C1 i = 1-n (18) C2j = C2 Substitution into the set of non-linear equations (17) gives: = = K1 r = 1-n (19) and

i.e.

Thus, with all channels the same apart from the frequencies to which they are tuned, to a first order of approximation the centre frequency and performance of any channel is independent of the frequencies of all other channels since, in the passband of any one channel, all other channels reflect all-pass networks into the path of that channel

Claims

Claims 1) A multiplexer comprising two or more inputs for connection to respective transmission sources and a common output for connection to an antenna, the inputs being connected to the output by respective filters tuned or tunable to separate passbands, each filter comprising at least two resonant stages with both its final stage and its penultimate stage coupled to the common output.
2) A multiplexer as claimed in claim 1, in which the filter in each channel comprises a microwave cavity filter either having separate cavities providing the two (or final two) resonant stages, or a single dual-mode cavity supporting both said stages.
3) A multiplexer as claimed in claim 1 or 2, the multiplexer being a diplexer, in which each said filter has its said penultimate stage connected to the common output to its said final stages by impedance inverters of predetermined characteristic admittances, the final stages of both filters are coupled to a common node by respective further impedance inverters, and said common node is coupled to said common output by an impedance inverter, the latter impedance inverter having a characteristic admittance equal but opposite to that of each said further impednce inverter.
4) A multiplexer as claimed in claim 1 or 2, in which the final stages of the filters are coupled to respective points spaced-apart along a first common line, and the penultimate stages of the filters are coupled to respective points spacedapart along a second common line, the first common line being coupled to the second common line at a point providing said common output.
5) A multiplexer as claimed in claim 4, in which each said filter has its said penultimate stage connected to said second common line and to its said final stage by impedance inverters of predetermined characteristic admittances, the final stages of the filters are coupled to said first common line by further impedance inverters, and said first common line is coupled to said second common lines by an impedance inverter having a characteristic admittance equal but opposite to that of each said further impedance inverter.
6) A multiplexer substantially as herein described with reference to Figures 1 and 2 or Figure 6 of the accompanying drawings