METHOD AND DEVICE FOR EXTENDING THE DYNAMIC RANGE IN MEASUREMENTS CARRIED OUT WITH A DIRECTIONAL COUPLER

Abstract: The invention relates to a method for extending the dynamic range in measurements carried out with a directional coupler, such as antenna measurements, and to a device for carrying out the method. In the method a signal is supplied to the object under analysis, such as an antenna (10), through a first port (3) of the measuring directional coupler, part of the signal leaking as a leak signal (A4) through a second port (4) of the measuring directional coupler directly towards measuring means (12), and the signal returning from the object (10) under analysis is measured by the measuring means (12). To enable the measuring of signal levels lower than before, spurious signals, such as said leak signal (A4), summing to the signal to be measured are eliminated by supplying a phased compensation signal (B5) through a second directional coupler (DC2) to the measuring line (11).

Description: Method and device for extending the dynamic range in measurements carried out with a directional coupler The invention relates to a method for extending the dynamic range in measurements carried out with a directional coupler, such as antenna measurements. In the method a signal is supplied to the object under analysis, such as an antenna, through a first port of the measuring directional coupler, part of the signal leaking as a leak signal through a second port of the measuring directional coupler directly towards measuring means, and the signal returning from the object under analysis is measured by the measuring means. The invention also relates to a device for carrying out the method. The device comprises (i) transmitter means for generating an electric measurement signal, (ii) a measuring directional coupler, to which the transmitter means are coupled for coupling the measurement signal to the object to be measured, and (iii) measuring means for measuring the signal returning from the object under analysis. In particular, the invention can be applied to measuring the standing-wave ratio (S R) of an antenna in order to observe the condition of the antenna. In measurements such as mentioned above the situation is the following: the signal reflected back from the object to be measured, such as an antenna, is, when summing to the signal leaking from the directional coupler straight in the direction of the measuring means, at a random phase, e.g. depending on variations in the electric length of the antenna cable. The magnitude of the error caused by summation is thus dependent on the phase difference between the returning signal and the leaking signal, which varies e.g. as a function of the electric length of the antenna cable. Known measuring solutions aim at the best possible directivity of the directional coupler supplying the measurement signal to the antenna, in order to render as small as possible the spurious signal leaking from the directional coupler straight in the direction of the measuring means and summing to the actual signal to be measured. The effect of other spurious signals such as those reflected from the coupling points has been diminished by calibration and by means of modification tables and correction factors. The obtained directivity of the directional coupler is however rarely more than 30 dB; in general the directivity is in the range of 20...30 dB, as the directional coupler is to be realised on the printed circuit board as a micro strip structure, which is production-technically advantageous thanks to easy reproducibility. Owing to the limited directivity, the measuring of low signal levels, such as the signal reflected back from an antenna demonstrating a minor reflection loss, becomes difficult when the (useful) signal to be measured sums to a spurious signal leaking from the directional coupler. On the other hand, the useful signal to be measured sums also to other spurious signals passing to the measuring point, such as said signals reflected from the coupling points. In some cases these other spurious signals may be greater in magnitude than the above-mentioned leaking signal, thus preventing the measuring of low signal levels. The object of the present invention is therefore to obviate the drawbacks described above and to improve the dynamic range of measurements carried out with a directional coupler so that the measuring of low- level signals will be effected more successfully. This is achieved with the method according to the invention, which is characterised by the fact that spurious signals, such as said leak signal, summing to the signal to be measured are eliminated by supplying a phased compensation signal through a second directional coupler to the measuring line. The device for carrying out the method is, for its part, characterised by what is described in the characterising portion of attached claim 4. The basic concept of the invention is to eliminate the dominating spurious signals by supplying a compensation wave to the measuring line by means of the second directional coupler. The solution according to the invention extends the dynamic range of the measuring by 10...25 dB com¬ pared with conventional methods of measuring. Another advantage of the solution provided by the invention is that it enables even the use of a directional coupler with a poor directivity, since the spurious wave leaking from the directional coupler is eliminated in any case. The circuit is also inexpensive to realise, and the actual measuring is quick to carry out, as the error correction is effected in real time. The possible tuning of the compensation signal needs to be effected only in the production phase. In the following the invention will be described in more detail with reference to the example depicted in the attached figure which discloses a measuring arrangement according to the invention in measuring the condition of an antenna. The figure discloses the measurement arrangement according to the invention when the condition of a receiving antenna 10 of the base station of a cellular network is to be checked. The condition is checked by defining the standing-wave ratio describing the electric matching of the antenna to the rest of the transmitter- and receiver system by measuring the power of the signal reflected back from the antenna with the measuring means 12 at the end of a measuring line 11 and by comparing this with the power transmitted to the antenna. Only that part of the measuring arrangement which pertains to the invention will be set out. In whole this kind of measuring arrangement is disclosed in Finnish Patent 80,817. At the front end of the measuring line 11 there is disposed a measuring directional coupler DCl whose input port 1 is coupled to a divider 14, a travelling wave port 3 is coupled through an antenna line 10a to the antenna 10, a port 2 is terminated by resistance R and a gate 4 is coupled to the measuring line 11. Between the measuring means 12 and measuring directional coupler DCl there is disposed a second directional coupler DC2 whose input port 1' is coupled through a delay unit 13 to the divider 14, a travelling wave port 3' is coupled to the measuring line in the direction of the measuring means 12, a port 2' is terminated by resistance R and a port 4' is coupled to the measuring line 11 in the direction of the antenna 10. Transmitter means 15 are coupled to the divider 14, supplying an electric measurement signal Al through the divider 14 to the directional couplers. The measurement signal Al is formed as described in the above-cited Finnish Patent 80,817 from the transmitter signal by mixing it with a transfer oscillator to the desired measuring frequency. In addition to the measurement signal Al, in the figure signals from and to the directional coupler DCl are marked with reference signs An (n=2...4) and signals from and to the directional coupler DC2 with reference signs Bn (n=2...5). Next to the reference signs tiny arrows are drawn indicating the travelling direction of the signal in question. The measurement signal A2 which is supplied to the antenna 10 through the divider 14 and the measuring directional coupler DCl leaks in the measuring directional coupler DCl straight in the direction of the measuring means 12. This leak signal A4 sums to the signal returning from the antenna at a random phase causing an error whose magnitude is dependent on the reciprocal phase of the leak signal and the signal returning from the antenna. In addition to the leak signal, error is caused e.g. by the summing of reflec¬ tions caused by cables and connectors in the antenna line to the signal returning from the antenna. The correction of the error (improving the dynamics) can be divided into two situations: (1) a situation in wh ' ich the leak signal A4 of the measuring directional coupler DCl is stronger than the other spurious waves and (2) a situation in which the other spurious waves are stronger than the leak signal A4. In situation (1) , which is more general, the leak wave A4 is eliminated by supplying with a second directional coupler D2 in the direction of the measuring means 12 a compensation signal B5 which has an amplitude which is equal to that of the leak signal A4, but in relation thereto delayed in its phase by half a wave length (λ /2) or its odd multiplicity. The compensation signal is formed of the measurement signal Al by branching the measurement signal by means of the divider 14 through a delay unit 13 to the second directional coupler DC2. In order for " the amplitude of the compensation signal to be equal to that of the leak wave, the coupling of the second directional coupler DC2 is equal in magnitude to the sum of the coupling and the directivity of the measuring directional coupler DCl (no losses in the measuring line) . If thus for instance the directivity of the measuring directional coupler DCl (D^IO log A3/A4) is 20 dB and the coupling (C-^IO log A2/A3) 20 dB as well, the coupling of the second direc- tional coupler DC2 (C 2 = 10 log B3/B5) will be 40 dB. In order for the phase of the compensation signal to be transferred by half a wave length (or its odd multiplicity) with regard to the leak wave, a delay dl of the delay unit 13 would be of such magnitude that a delay d2 of the signal between the directional couplers DCl and DC2 deducted from the delay dl has a magnitude of half a wave length or its odd multiplicity (dl-d2 = n λ /2, n=l, 3, 5... ). As the compensation signal B5 eliminates the leak wave A4, the signal to be measured is no longer dependent on the electric length of the antenna line (antenna cable) ; in other words, the summing of the signal returning from the antenna at a random phase to the leak signal no longer causes measurement error. Measurement error is, however, still caused to some extent by other factors such as cables and connectors. The dynamic range of the measurement can nevertheless be extended by 10...25 dB compared with conventional methods. In situation (2) the compensation signal must be tuned separately to equal in amplitude the spurious signal in the measuring line and to be in its phase opposite to this spurious signal. The tuning may be accomplished e.g. by terminating the antenna line with a matched resistance (test load) and carrying out in time domain in a manner known per se a measurement (time domain analysis) which gives the amplitude and phase of the reflected spurious signal, in other words, the loca¬ tion of the point causing reflection and the magnitude of the reflection. Before the measurement, already at the planning stage, all points (reflection points) having an effect on the standing-wave ratio in the antenna line should be taken into account by adjusting said points at the measuring frequency in question at intervals of half a wave length or its multiplicity. This enables elimination of all factors causing spurious signals by means of one additional directional coupler DC2. Even if the invention was described above with reference to the example depicted in the attached drawing, it is clear that the invention is not restricted thereto but can be modified in many ways within the scope of the inventive concept set forth above and in the attached claims. In principle in place of the directional couplers there could be other directional means, e.g. a capacitively coupled hybrid. For this reason the used term 'directional coupler' should be understood in a larger sense. Neither is measuring limited to sole antenna measurements but the principle according to the invention can also be used in other measurements carried out with directional couplers. It is also possible to change the mutual order of the directional couplers.

Claims: Claims 1. A method for extending the dynamic range in measurements carried out with a directional coupler (DCl) , such as antenna measurements, in which method - a signal is supplied to the object under analysis, such as an antenna (10) , through a first port (3) of the measuring directional coupler, part of the signal leaking as a leak signal (A4) through a second port (4) of the measuring directional coupler directly towards measuring means (12) , and - the signal returning from the object (10) under analysis is measured by the measuring means (12) , c h a r a c t e r i s e d in that - spurious signals, such as said leak signal (A4) , summing to the signal to be measured are eliminated by supplying a phased compensation signal (B5) through a second directional coupler (DC2) to the measuring line (11). 2. A method according to claim 1, c h a r a c ¬ t e r i s e d in that the compensation signal (B5) is delayed so that the phase difference between it and the signal (A4) leaking from the measuring directional coupler (DCl) has a magnitude of half a wave length or its odd multiplicity (n λ/2, n=l, 3, 5...), and that the amplitude of the compensation signal (B5) is adjusted to be equal to the amplitude of the leak signal by adjusting the coupling of the second directional coupler (DC2) to be equal in magnitude to the sum of the coupling and directivity of the measuring directional coupler (DCl) . 3. A method according to claim 1, c h a r a c ¬ t e r i s e d in that the spurious signal occurring in the measuring line (11) is determined by a separate measurement and the compensation signal (B5) is tuned on the basis of said spurious signal measurement so that the amplitude of the compensation signal is equal to the amplitude of the spurious signal and the phase of the compensation signal is opposite to the phase of the spurious signal. 4. A device for carrying out the method according to claim 1, which device comprises - transmitter means (15) for generating an electric measurement signal, - a measuring directional coupler (DCl) to which said transmitter means (15) are coupled for coupling the measurement signal to the object to be measured (10) , and - measuring means (12) for measuring the signal returning from the object to be measured (10) , c h a r - a c t e r i s e d in that a second directional coupler (DC2) is coupled to the measurement line (11) , and said transmitter means (15) are coupled to said second directional coupler (DC2) through a delay unit (13) . 5. A device according to claim 4, c h a r a c - t e r i s e d in that the coupling of the second directional coupler (DC2) is equal in magnitude to the sum of the directivity and coupling of the measuring directional coupler (DCl) .

Inventors: KNUUTI MIKA, KNUUTI, MIKA
Applicants: TELENOKIA OY
Application year: 1992
Main classification: G01R27
IPC codes: G01R27, G01R29