WO1994021042A1

WO1994021042A1 - Auto-tuning microwave filter

Info

Publication number: WO1994021042A1
Application number: PCT/GB1994/000460
Authority: WO
Inventors: John David Rhodes
Original assignee: Filtronic Limited
Priority date: 1993-03-09
Filing date: 1994-03-09
Publication date: 1994-09-15
Also published as: GB2276051A; GB9304764D0

Abstract

A microwave filter tuning arrangement comprises a directional coupler (DC) for coupling with an input line to a microwave filter (C1), so as to provide a first output (1) representing the power of a signal (f1) passing forwardly along the input line and a second output (2) representing the power of signals passing in the opposite direction along the input line, and means for combining the two outputs to form a control signal for tuning the filter. A 180° phase shifter (PS) is provided for one of the outputs, giving the control signal an appropriate characteristic to provide closed-loop control to tune the filter (C1).

Description

AUTO-TUNING MICROWAVE FILTER

This invention relates to an arrangement for automatically tuning a microwave filter, particularly in a channel combiner.

In multi-carrier high power transmitters, it is frequently necessary to amplify each channel separately and then combine the channels at high power, rather than transmit using a single amplifier. For fixed tuned systems, many designs of manifold combiners may be used. In most low loss designs, each channel filter, when tuned, interacts with other channels which are close in frequency: however, by adjusting each cavity in an interactive manner, the whole combiner can be aligned correctly.

In cellular telephone base stations, periodic changes to the frequency distribution are required and it is desirable to achieve this rapidly and possibly remotely. Changes to the transmit frequencies can be achieved in fractions of a second using synthesisers, but adjustment of the combining cavities is more difficult.

We have now devised an arrangement for automatically tuning a microwave filter, enabling the synchronous auto-tuning of a multi-channel combiner.

In accordance with this invention, there is provided a microwave filter tuning arrangement, comprising means coupled with the input line to a microwave filter to provide a first output representing the power of a signal passing forwardly along said line to the filter and a second output representing the power of signals passing in the opposite direction along said line, means for phase shifting one of said outputs and combining said outputs together to form a control signal for tuning the filter.

Preferably the means for providing the first and second outputs comprises a directional coupler. Preferably the phase shifter provides a 180° phase shift: with this arrangement the control signal has an appropriate characteristic to provide closed-loop control to tune the microwave filter to the frequency of a signal being fed to the filter along its input line .

The arrangement enables a plurality of microwave channels to be combined, each channel including a tunable filter and a tuning arrangement as just described, allowing the frequency of any channel to be altered and the filters of all channels then automatically tuning to the respective frequencies of their input signals.

An embodiment 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 a typical channel combiner;

FIGURE 2 is a schematic diagram of an arrangement in accordance with this invention; FIGURE 3 is a plot of the control signal developed in the arrangement of Figure 2;

FIGURE 4 is a plot of the control signal which can be developed in the arrangement of Figure 2 when the phase shifter is by-passed; and FIGURE 5 is a more detailed diagram of an arrangement in accordance with the invention.

Referring to Figure 1, there is shown diagrammatically a typical combining network, comprising a plurality of microwave filters (cavities Cl to CN) in respective channels f_j, f₂ .... f_n, the outputs of the microwave cavities being connected together and to an antenna. It is desired to be able to change the frequency of any one or more of the channels, without adversely affecting the other channels. For example if the first channel f* is considered, then in addition to the input signal at frequency f, which has to pass through the cavity with low loss, all of the remaining signals f₂ to f_n have to be reflected from the output. Normally, the power variations in the channels can be up to 30db: with a large number of signals to be reflected from the output of any given channel, the total power level to be reflected could be significantly greater than the transmitted power. Thus any direct detection of power in any given channel would be significantly affected by the signals from the other channels and would therefore be unreliable for automatic tuning purposes .

Figure 2 shows schematically an arrangement in accordance with this invention. A directional coupler DC is provided in the input line (shown including an isolator I) to a microwave filter cavity Cl. The upstream port 1 of the directional coupler is connected to an amplifier Al with automatic gain control (AGC) , and the downstream port 2 of the directional coupler is connected to a phase shifter PS: the outputs of the amplifier Al and phase shifter PS are connected to a mixer M,* the output of which has a low pass filter LPl.

The output of port 1 of the directional coupler is a low power representation of the input signal at frequency f,: if the coupler has a high directivity, very little energy at frequencies f₂ to f_n is present in the output of port 1. The output of port 2 is a low power representation of the power at frequency f_{ reflected from the cavity, plus power penetrating the cavity at all other frequencies f_x to f_n.

If ω = 2π (f - fι)α where f. is the resonant frequency of the cavity, f is the input frequency to port (1) and α is a constant, then the output voltage from the low pass filter LPl is:

V₀= ^Λω sindμ+tan^ω) V +ω²

where A is a constant . This expands as \ -

V₀= (ω²cosψ+ωsinψ)

1+ω²

If ψ=0 then ;

V₀ = -^ (1)

1+ω² and this function is plotted in Figure 3

If ψ =- then : - 2

V Ό_n = -^ (2)

1+ω² and this function is plotted in Figure 4 . In order to tune the cavity onto the centre frequency f = f_j, both of the functions (1) and (2) above are zero at the required point. Based on the function (2) shown in Figure 4, a closed-loop control can be formed, providing |ω| does not greatly exceed unity: thus, close to resonance the cavity will lock to f = f,. Furthermore, due to all other frequencies f₂ to f_n being offset in frequency, the low pass filter LPl will reflect any mixed products and prevent them from producing an output voltage. If, before tuning, the cavity is sufficiently detuned from f,, then according to the function shown in Figure 4, the output will tend to zero and possibly imply that the cavity is tuned. In order to overcome this, the phase shifter PS may be switched from

= π/ 2 to φ = 0, to produce the function shown in Figure 3. Then, an output voltage is produced in an off- tune position and the cavity can then be tuned into the range of Figure 4 (when |ω|<l) to enable auto-tuning to commence.

Figure 5 shows the arrangement more fully, with a switch S for the phase shifter PS, and the output of the low pass filter LPl being used to tune the cavity: this tuning may be achieved by electromechanical, magnetic or electronic means. Figure 5 further shows variable attenuators V, and V₂ in the inputs to the amplifier Al and to an amplifier A2 prior to the phase shifter PS. In an on-tune situation, a zero output is achieved for both functions (1) and (2) (Figures 3 and 4) assuming there is an input signal. A detector D is provided on the output from amplifier Al to control the variable attenuators VI and V2 via a low pass filter LP2, thus forming an automatic gain control system. Also a check is made that the output from detector D is above a predetermined level, to indicate the presence of an input signal.

Claims

1) A microwave filter tuning arrangement, comprising means for coupling with an input line to a microwave filter and arranged to provide a first output representing the power of a signal passing forwardly along said line to the filter and a second output representing the power of signals passing in the opposite direction along said line, and means for combining said outputs together with or without phase shifting one of said outputs, to form a control signal for tuning the filter.

2) A microwave filter tuning arrangement as claimed in claim 1, in which said means for providing said first and second outputs comprises a directional coupler.

3) A microwave filter tuning arrangement as claimed in claim 1 or 2, in which said means for combining said outputs comprises means for introducing a 180° phase shift on one said output.

4) A microwave filter tuning arrangement as claimed in claim 3, in which said means for combining said outputs further comprises switching means for selectively bypassing said phase shifting means.

5) A microwave channel combiner, comprising a plurality of tunable microwave filters each with a tuning arrangement as claimed in any preceding claim.