GB2205995A - Automatic aerial matching circuit - Google Patents

Abstract

A circuit for matching the reactance of an aerial (2) to the output (4) of an r.f. source (3) is in the form of a feed-forward system. An analog r.f. bridge (6) detects the phase difference between signal voltages and the corresponding signal currents in the aerial feeder. Successive ranges of values of the analog output signal of the bridge are converted to respective switching signals by means of a processing circuit (13) and these are used to switch reactive elements included in a controllable reactance circuit (5) into and out of circuit. <IMAGE>

Description

AUTOMATIC AERIAL MATCHING CIRCUIT This invention relates to a circuit arrangement for automatically matching the reactance of an aerial to the output of an r.f. signal source, which arrangement provides an r.f.

signal path from said output to said aerial and comprises, situated along said path, (a) a controllable reactance circuit comprising a plurality of switchable reactive elements and (b) an analog phase-difference detector for producing at an output thereof an analog output signal representative of the phase difference between r.f. signal voltages in said signal path and the corresponding r.f. signal currents, the arrangement further including a signal processing circuit having an input coupled to the said output of the phase-difference detector and an output coupled to a control input of the reactance circuit, for switching said reactive elements in dependence upon the output signal of the phase-difference detector.

In an arrangement of this kind disclosed in US-A-4 380 767 the phase-difference detector and the reactance circuit are situated along said r.f. signal path in that order, and the signal processing circuit includes a microcomputer. When the microcomputer is instructed to enter an aerial match routine the phase-difference detector output is sensed to determine whether the aerial, as "seen" by the output of the r.f. signal source, is capacitive or inductive, and successive ones of the reactive elements are switched into or out of circuit under the control of the microcomputer until any such reactance seen becomes substantially tuned out.The use of such a routine is necessary inter alia because of the fact that the reactance of the reactance circuit is variable only in finite steps, and any such variation gives rise to a corresponding step change in the output signal of the phase-difference detector. The switching of the reactive elements therefore has to be initiated in response to some stimulus, and progressed until the phase-difference detector output signal indicates that a satisfactory match has been obtained. In effect a record has to be kept of the current switched state of the reactance circuit; there cannot be a one-to-one relationship between the value of the phase-difference detector output signal and the switching state required of the reactance circuit.This tends to make such an arrangement more complicated and expensive than is justified in some possible applications, with the result that the disadvantages of not providing an automatic aerial matching arrangement are often tolerated in, for example, portable transceivers.

It is an object of the invention to provide an automatic aerial matching circuit arrangement which can be less complicated and expensive than the known arrangement while still giving an acceptable performance in at least some applications. To this end an arrangement of the kind defined in the first paragraph is characterised in that the reactance circuit and the phase-difference detector are situated along said r.f. signal path in that order, and the signal processing circuit is constructed to convert successive ranges of values of the detector output signal into respective switching signals for the reactive elements.

It has now been recognised that reversing the order in which the phase-difference detector and the controllable reactance circuit are situated along the r.f. signal path results in the effect on the phase-difference occurring at the phase-difference detector of switching the reactance of the reactance circuit being very considerably reduced or eliminated.

In effect a feedback system is transformed into a feed-forward system, with the result that there can now be a one-to-one relationship between the value of the output signal of the phase-difference detector and the reactance required of the reactance circuit. This in turn means that the signal processing circuit can be constructed to merely convert successive ranges of values of the detector output signal into respective switching signals for the reactive elements, which can be done, for example, by means of relatively inexpensive circuitry such as a set of comparators fed with respective reference voltages.

It should be noted that US-A-4 311 972 discloses an antenna coupler which provides an r.f. signal path from the output of an r.f. signal source to an aerial. The coupler comprises a tuning stub and a so-called "power transfer sampler" situated along the path, in that order. The power transfer sampler detects the signal voltages occurring at three equidistant points along the path, one of these being situated substantially at the aerial.

The detected voltages are converted to digital form and are then processed by means of an arithmetic unit and look-up tables to produce switching signals for the elements of the tuning stub to adjust its length and position on the signal path to thereby match the aerial to the r.f. source. A signal indicative of its output frequency is applied by the r.f. source to the arithmetic unit, this being necessary because the frequency has to be taken into account in the calculation performed by the arithmetic unit. This known arrangement is very complicated, employing as it does detectors, analog-to-digital converters, digital absolute-value devices, digital squarers, digital dividers, a digital processor and a pair of look-up tables, or alternatively a suitably programmed microcomputer, and hence tends to be expensive.In addition to tuning out the reactive component of the aerial impedance it also adjusts the resistive component to the required value. However, in many applications such adjustment of the resistive component is not in fact required; it has been found, for example, that if an aerial is in the form of a helix then in many applications the resistive component of its impedance remains substantially constant over an appreciable operating frequency range and with changes in its immediate environment, so that any matching of this resistive component to the output of the r.f. source which may be required can be achieved by means of a pre-set adjustment.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which Figure 1 is a block diagram of the embodiment.

Figure 2 shows a possible construction for one of the blocks of Figure 1, Figure 3 shows a possible construction for another of the blocks of Figure 1, and Figure 4 shows possible a construction for another of the blocks of Figure 1.

In Figure 1 a circuit arrangement 1 for automatically matching the reactance of an aerial 2 to the output of an r.f.

signal source 3 provides an r.f. signal path from the source output 4 to the aerial. The arrangement comprises, in order along the path, a controllable reactance circuit 5 and an analog phase-difference detector 6, the path extending from the output 4 of source 3 to an input 7 of circuit 5, through circuit 5 to an output 8 thereof, from output 8 to an input 9 of detector 6, and through detector 6 to an output 10 thereof, output 10 being connected to the aerial 2. Detector 6 is constructed to produce an analog output signal at a second output 11 thereof, this signal being representative of the phase difference between r.f.

signal voltages occuring in the signal path between input 9 and output 10 and the corresponding signal currents (this being a measure of the reactance of aerial 2). Output 11 is connected to the input 12 of a signal processing circuit 13 an output 14 of which is connected to a control input 15 of reactance circuit 5.

Processing circuit 13 converts successive ranges of values of the analog signal appearing at output 11 of detector 6 into respective switching signals for reactive elements included in circuit 5. These switching signals appear at its output 14 (which will normally be a multiple one, as will input 15 of the circuit 5, the coupling from output 14 to input 15 then consisting of a set of parallel conductors).The respective reactive elements, e.g. inductors and/or capacitors, included in circuit 5 are switched into and out of circuit, for example by means of p-i-n diodes, switching transistors or relays, in accordance with the output signals of circuit 13 and their values are chosen so that, for each range of values of the signal appearing at output 11 of detector 6, i.e. for each range of reactances of the aerial 2, the reactive element or combination of reactive elements switched into circuit is such as to substantially tune out the aerial reactance as seen at output 4 of source 3, and thereby match the reactance of the aerial 2 to the output 4.If aerial 2 is, for example, in the form of a helix having a resonant frequency lower than the output frequency of source 3, its reactance as seen at output 10 will be series-inductive so in such a case the reactive elements in circuit 5 may conveniently take the form of either capacitances switchable in series with the signal path from input 7 to output 8 or inductances switchable in parallel therewith.

Figure 2 shows a possible construction for the detector 6 of Figure 1. It takes the known form of a phase or r.f. bridge comprising a sensing coil 16 inductively coupled to the r.f.

signal path from input 9 to output 10 and a sensing capacitor 17 connected between the signal path 9-10 and ground via a resistor 18, the junction point of capacitor 17 and resistor 18 being connected to the junction point of equal-value resistors 19 and 20 connected in series across coil 16. The junction points of coil 16 with resistors 19 and tO respectively are connected to output 11 via a series combination of a resistor 21 and the anode-cathode path of a diode 22, and via a series combination of a resistor 23 and the anode-cathode path of a diode 24, respectively. Resistors 21 and 23 have equal values and the junction point of diodes 22 and 24 are connected to ground via the parallel combination of a resistor 25 and a decoupling capacitor 26.The detector 6 of Figure 2 produces an analog signal at output 10 which has a minimum value when the phase difference between the r.f. signal voltages in the path 9-10 and the corresponding signal currents is zero, and which increases with increasing values of this phase difference, whatever its sign. Such a detector is satisfactory provided that the reactance of aerial 2 is always either inductive or capacitive, whatever its actual value, as will normally be the case if for example aerial 2 is in the form of a helix resonant at a lower frequency than the output frequency or range of output frequencies of source 3, because no ambiguities can then occur.

If, however, the reactance of aerial 2 can change between inductive and capacitive the polarity of one of the diodes 22 and 24 is perferably reversed, so that the sign of the signal at output 11 will then become indicative of the nature of the aerial reactance. Whether or not this is the case the detector construction shown in Figure 2 has the useful property that its performance can be substantially independent of frequency over a wide range. The value of its analog output signal does, however, also depend on the signal level in the path 9-10, so that it is suitable for use in an arrangement as shown in Figure 1 only if the output signal level of source 3 is substantially constant, or if compensation is provided in some way for the changing level of the signal at output 11 with change in the output signal level of source 3.One possible way of providing such compensation will be described in the context of the following description with reference to Figure 3.

Figure 3 shows a possible construction for the signal processing circuit 13 of Figure 1. As shown in Figure 3, processing circuit 13 comprises a set of three comparators 27, 28 and 29, the inverting inputs of comparators 27 and 29 and the non-inverting input of comparator 28 being fed with respective d.c. reference voltages from respective taps on a resistive potential divider 30, 31, 32, 33 connected between the positive supply rail +V and ground. The non-inverting inputs of comparators 27 and 29, and the inverting input of comparator 28, are connected to the output of a buffer amplifier 34 which is provided with a negative feedback resistor 35 from its output to its inverting input, and a further resistor 36 from its inverting input to ground. The input 12 of the processing circuit is connected to the non-inverting input of amplifier 34 via a resistor 37.The output of comparator 27 is connected to the base of an npn switching transistor 38 via a resistor 39, and the output of comparator 29 is connected to the base of an npn switching transistor 40 via a resistor 41. The output of comparator 28 is connected both to the base of an npn switching transistor 42 via a resistor 43 and to the base of a pnp switching transistor 44 via a resistor 45. The collector of transistor 44 is connected to a first conductor 14A of the output 14 and the emitters of transistors 38 and 40 are commoned and connected to a second conductor 14B of this output. The emitters of the transistors 42 and 44 are connected to the collector of transistor 40 and to the positive supply rail respectively. The collectors of the transistors 38 and 42 are connected to the positive supply rail via resistors 46 and 47 respectively.

When the voltage applied to input 12 and hence the output voltage of amplifier 34 lies within the lowest part of its range (between zero and +V1 say) the output voltages of comparators 27, 28 and 29 are low, high and low respectively, so that transistors 38 and 40 and 44 are turned off and transistor 42 is turned on (assuming a d.c. path exists from conductors 14A and 14B to ground). The potentials on both conductors 14A and 14B are therefore both low. If the potential applied to input 12 lies within the next higher part of its range (between +V1 and +V2 say), the outputs of comparators 27 and 28 are still low and high respectively, but that of comparator 29 is now high, turning on transistor 40. As transistor 42 is also on the potential on conductor 14B is now high, that on conductor 14A remaining low.

It can be deduced in a similar manner that if the potential on input 12 lies within the next higher part of its range (between +V2 and +V3 say) the potentials on conductors 14A and 14B are high and low respectively, whereas if the potential on input 12 lies within the highest part of its range (above V3 say) the potentials on conductors 14A and 14B are both high. The processing circuit of Figure 3 therefore converts successive ranges of analog signal applied to its input 12 from the phase-difference detector 6 into respective signal combinations on the conductors 14A and 14B.

As mentioned above, the analog output signal of the possible construction for detector 6 shown in Figure 2 is dependent on the magnitude of the output signal from source 3, so if this magnitude is variable the output signals of the processing circuit of Figure 3 will not necessarily correspond in the desired way to the phase-differences sensed by detector 6. In order to compensate for this effect the processing circuit 13 of Figure 3 may be provided with a further input 48 connected to the non-inverting input of amplifier 34 via a resistor 49 (shown in dashed lines) input 48 being connected to the output of a square law detector (not shown) fed from the r.f. signal path from the output 4 of source 3 to the aerial 2 in Figure 1.If this square law detector is referenced to the positive supply rail and arranged to produce an increasingly negatively directed output voltage with increasing signal level in the r.f. signal path then a positive offset voltage will be applied to amplifier 34 through resistor 49, this offset voltage decreasing with increasing signal level in the r.f. signal path and thereby compensating for the increasing analog voltage applied to input 12 due to the increasing signal level if the relative values of resistors 37 and 49 are suitably chosen.

Figure 4 shows a possible construction for the controllable reactance circuit 5 of Figure 1, this being appropriate if the nature of the aerial 2 is always series-inductive as will normally be the case if, for example, it takes the form of a helix resonant at a frequency lower than the maximum output frequency of source 3.

In the circuit 5 shown in Figure 4 the signal path from input 7 to output 8 includes three d.c.-blocking capacitors 50, 51 and 52 in series. An inductor 53 is connected between the junction point of capacitors 50 and 51 and ground via a p-i-n switching diode 54. Similarly an inductor 55 is connected between the junction point of capacitors 51 and 52 and ground via a p-i-n switching diode 56. The common point of inductor 53 and the anode of diode 54 is connected to a first conductor 15A of the control input 15 via an r.f.-isolating choke 57 and a resistor 58, the common point of choke 57 and resistor 58 being connected to ground via a decoupling capacitor 59.Similarly the common point of inductor 55 and the anode of diode 56 is d.c.-connected to a second conductor 15B of the control input 15 via an r.f.-isolating choke 60 and a resistor 61, the common point of choke 60 and resistor 61 being connected to ground via a decoupling capacitor 62. The input conductors 15A and 15B are connected to the output conductors 14A and 145 respectively of the processing circuit 13 of Figure 3 so that a high level on output conductor 14A turns on diode 54, thereby connecting inductor 53 between the signal path 7-8 and ground. Similarly a high level on output conductor 14B turns on diode 56, thereby connecting inductor 55 between the signal path 7-8 and ground.

In consequence, when the voltage applied to the input 12 of the circuit 13 of Figure 3 lies within the four successive ranges O-V1, V1-V2, V2-V3, and above V3 as discussed above, neither inductor 53 or 55 is connected in circuit, only inductor 55 is connected in circuit, only inductor 53 is connected in circuit, and both inductors 53 and 55 are connected in circuit respectively. The inductance of inductor 55 is greater than that of inductor 53 so that a decreasing amount of parallel inductance is switched into circuit for increasing values of the analog voltage applied to the input 12, i.e. for increasing values of series-inductance of the aerial 2. The inductance values of the inductors 53 and 55 are chosen so that the result is substantial compensation for the inductance of the aerial 2 as seen at the output 4 of source 3.

Obviously controllable reactance circuit 5 could be provided with suitably valued capacitors which are switchable in series with the signal path 7-8 rather than suitably valued inductors switchable in parallel therewith. If the reactance of aerial 2 were capacitive rather than inductive or could vary between the two, circuit would have to be provided with switchable parallel capacitors and/or series inductors, or combinations of these with switchable series capacitors and/or parallel inductors.

Phase-difference detector 6 is preferably positioned in the signal path to aerial 2 as close as possible to aerial 2, particularly if the output frequency of source 3 is variable, because of the change in the relative phase of the signal voltages and currents which is likely to occur along the signal path. In a particular application satisfactory results were obtained provided that the distance between detector 6 and aerial 2 was no greater than one-tenth of the wavelength of the signal in the signal path.

It will be evident to those skilled in the art that there are many possible alternatives to the particular constructions described for the various blocks of Figure 1. For example, depending on the accuracy of compensation required processing circuit 13 may be constructed to respond to more or less than four ranges of values of the signal at output 11 of detector 6, the number of switchable reactive elements in circuit 5 being modified accordingly. In a particular application to a portable transceiver transmitting at 172 MHz and provided with a helical aerial the provision of four such ranges was found to be sufficient to keep the voltage standing wave ratio (VSWR) presented to output 4 of source 3 below 2:1 for all possible locations of the transceiver relative to the user.

Claims (4)

CLAIMS:

1. A circuit arrangement for automatically matching the reactance of an aerial to the output of an r.f. signal source, which arrangement provides an r.f. signal path from said output to said aerial and comprises, situated along said path, (a) a controllable reactance circuit comprising a plurality of switchable reactive elements and (b) an analog phase-difference detector for producing at an output thereof an analog output signal representative of the phase difference between r.f. signal voltages in said signal path and the corresponding r.f. signal currents, the arrangement further including a signal processing circuit having an input coupled to the said output of the phase-difference detector and an output coupled to a control input of the reactance circuit, for switching said reactive elements in dependence upon the output signal of the phase-difference detector, characterised in that the reactance circuit and the phase-difference detector are situated along said r.f. signal path in that order, and the signal processing is constructed to convert successive ranges of values of the detector output signal into respective switching signals for the reactive elements.

2. An arrangement as claimed in Claim 1, wherein the signal processing circuit comprises a set of comparators and means for supplying respective reference voltages to first inputs of said comparators, the detector output being coupled to a second input of each comparator and the comparator outputs being coupled to control inputs of switching elements for said reactive elements.

3. An arrangement as claimed in Claim 1 or Claim 2, wherein the aerial is in the form of a helix.

4. A circuit arrangement for automatically matching the reactance of an aerial to the output of an r.f. signal source, substantially as described herein with reference to Figure 1 of the drawings, or to said Figure 1 in combination with Figure 2 and/or Figure 3 and/or Figure 4 of the drawings.