Antenna supervising apparatus comprising a standing wave ratio measuring unit

Abstract: In an antenna supervising apparatus for supplying a multiplexed signal comprised of a plurality of carrier wave signals respectively outputted from a plurality of transmitters through a transmission line to an antenna, a directional coupler detects progressive signals of the multiplexed signal to the antenna and the reflected signals thereof from the antenna, and a band-pass filter respectively filters the progressive signal and the reflected signal of one carrier wave signal preselected among those of the detected multiplexed signal. Further, a detector respectively detects the filtered progressive signal and the filtered reflected signal of the one carrier wave signal, and a micro processing unit calculates a standing wave ratio with respect to the preselected one carrier wave signal based on the detected signals.

Description: BACKGROUND OF THE INVENTION The present invention relates to an antenna supervising apparatus, and more particularly, to an antenna supervising apparatus comprising a measuring unit for measuring a reflected signal and a standing wave ratio (referred to as an SWR hereinafter) in order to supervise an impedance matching state between transmitters and an antenna. A voltage standing wave ratio is referred to as a VSWR hereinafter. Recently, mobile radio communication systems such as an automobile radio communication system, an airplane radio communication system have been put into practical use accompanying developments of radio communication technique. Fig. 9 shows a conventional cellular radio zone type mobile radio communication system. As shown in Fig. 9, base stations 2 are provided within respective small radio zones 1 corresponding divided service areas, and radio communications are performed between mobile stations 3 located within a small radio zone 1 and a base station 2 of the small radio zone 1 through communication channels 4 and control channels 5 used for transmitting calling signals. The respective base stations 2 are connected through an exchange control station 6 and an exchange control subsystem 7 to a fixed telephone network 9 connected to fixed telephones 8. In such cellular radio zone type mobile radio communication system, each cell of the radio zone 1 is further divided according to increase in a number of subscribers. Then, according to the division of the cell, it is necessary to alter the frequencies to be used in the respective base stations 2, for example, using commands transmitted from the exchange control station 6. As shown in Fig. 10, the base station 2 comprises plural transmitters Tx1, Tx2, ..., Txi, ..., Txn (i is a natural number which is equal to or larger than one and equal to or smaller than a natural number n) for respectively transmitting carrier waves having frequencies f1 to fn different from each other. The respective outputs of the transmitters Tx1, Tx2, ..., Txi, ..., Txn are outputted, through corresponding respective isolators I1, I2, ..., Ii, ..., In constituted by three terminal type circulators each having one terminated terminal, to corresponding respective band-pass filters F1, F2, ..., Fi, Fn which are provided with the isolators I1, I2, ..., Ii, ..., In in a transmitter combiner 11, and are then band-pass-filtered. Thereafter, the outputs of the band-pass filters F1, F2, ..., Fi, ..., Fn are frequency-multiplexed, and a multiplexed wave comprised of respective carrier waves frequency-multiplexed is outputted through a directional coupler 12 for detecting a progressive wave transmitted to an antenna 13 and detecting a reflected wave transmitted from the antenna 13, and an antenna filter F0 of a band-pass filter for band-pass-filtering the multiplexed wave to the antenna 13, thereby projecting the multiplexed wave as a radio wave toward a free space. In order to establish a certain reliability of the radio communication and to prevent occurrence of radio interferences etc., a high frequency power projected from the antenna 13 toward the free space and an impedance matching state between a transmitter system and an antenna system are always supervised in the base station 2. As further shown in Fig. 10, in an antenna supervising apparatus 100 provided in the conventional cellular radio zone type mobile radio communication system, a progressive wave transmitted to the antenna 13 and a reflected wave transmitted from the antenna 13 are detected by the directional coupler 12 which is inserted between the antenna filter F0 and the transmitter combiner 11, and the progressive wave and the reflected wave outputted from the directional coupler 12 are respectively detected by detectors 14 and 15 of diodes so as to obtain direct-current voltages which are respectively in proportion to the progressive wave and the reflected wave. The detector outputs of the progressive wave and the reflected wave are inputted to a calculation circuit 16, and then, in response thereto, a high frequency power projected from the antenna 13 to the free space and a VSWR of the antenna system comprised of the antenna 13 and a transmission line located from the directional coupler 12 to the antenna 13 are calculated by the calculation circuit 16. Thereafter, the calculated result is displayed on a display unit 17. However, in the conventional antenna supervising apparatus 100, since the progressive wave and the reflected wave of the multiplexed wave comprised of respective carrier waves are detected by the directional coupler 12 comprising two coupling lines and are detected by the detectors 14 and 15, beat signals of frequencies (f1 - f2), (f1 - f3), etc. are superimposed in the outputs of the detectors 14 and 15. Due to the beat signals, the outputs of the detectors 14 and 15 change as a time passes as shown in Fig. 12, resulting in decrease in detection precision of respective levels of the above-mentioned progressive wave and reflected wave. Therefore, there is such a problem that detection precision of the high frequency power and the VSWR calculated by the calculation circuit 16 in response to the progressive wave and the reflected wave are lowered. Further, when an interference wave having the other frequency is inputted from an external apparatus or system to the antenna 13, the interference wave is superimposed in the reflected wave, there is such a problem that the level of the reflected wave and the VSWR can not be correctly measured. Philips Telecommuniction & Data Systems Review, vol. 45, no. 3, September 1987, pages 69 - 80, which discloses the features of the preamble of claim 1, relates to a base station for a mobile telephone system. The base station comprises an antenna connected to coupling means for detecting progressive signals and reflective signals of multiplex signals stemming from a plurality of transmitters for measuring relevant parameters, such as the standing wave ratio. In a signal path subsequent to the couplers, detectors for detecting the progressive signal and to filter the signal are provided. A subsequent calculating unit calculates operational data, such as the standing wave ratio. DE 3317358 A1 relates to a reflectometer having a directional coupler 1 for detecting a progressive signal and a reflective signal. The progressive signal and the reflective signal are supplied to signal processing paths comprising mixers, low pass filters, amplifiers, and amplitude limiting circuit. A single side band generator is provided for generating a single side band signal for mixing a measuring frequency and the single side band signal of the frequency stemming from the single side band generator. It is the object of the present application to provide a reflected signal measuring apparatus capable of measuring a level of a reflected wave of particular high frequency signal transmitted on a transmission line independent of a phase of the reflected wave. This object is achieved by a reflected signal measuring apparatus according to claim 1. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: Fig. 1 is a schematic block diagram showing a VSWR measuring unit; Fig. 2 is a schematic block diagram showing a reflected signal measuring apparatus of a first preferred embodiment according to the present invention; Fig. 3 is a schematic block diagram showing a reflected signal measuring apparatus of a second preferred embodiment according to the present invention; Fig. 4 is a graph showing a relationship between a phase difference between respective signals inputted to a mixer of the VSWR measuring unit shown in Fig. 1 and an output voltage of an audio frequency signal outputted from a low-pass filter thereof, an audio frequency signal outputted from a phase modulator and an audio frequency signal outputted from the low-pass filter; Fig. 5 is a graph showing a relationship between a phase difference between respective signals inputted to the mixer of the VSWR measuring unit shown in Fig. 1 and an output voltage of an audio frequency signal outputted from the low-pass filter thereof, an audio frequency signal outputted from the phase modulator, an audio frequency signal outputted from the low-pass filter, and an audio frequency signal outputted from a full-wave rectifier; Fig. 6 is a flowchart showing a main routine executed by a micro processing unit of the VSWR measuring unit shown in Fig. 1; Fig. 7 is a flowchart showing a main routine executed by a micro processing unit of the reflected signal measuring apparatus shown in Fig. 2; and Fig. 8 is a flow chart showing a subroutine of the main routine shown in Fig. 7. Fig. 9 is a schematic diagram showing a conventional cellular radio zone type mobile radio communication system; Fig. 10 is a schematic block diagram showing a base station apparatus and an antenna supervising apparatus which are provided in the cellular radio zone type mobile radio communication system shown in Fig. 9; Fig. 11 is a graph showing a spectrum of a carrier wave used in the base station shown in Fig. 10; Fig. 12 is a graph showing a detector output including beat components in the antenna supervising apparatus shown in Fig. 10; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments according to the present invention will be described below with reference to the attached drawings. Fig. 1 shows a VSWR measuring unit 202. The features of the VSWR measuring unit 202 are as follows. A transmission signal outputted from a transmitter 201 is detected by a directional coupler 220 and is phase-modulated according to an audio frequency signal. On the other hand, a progressive signal of the transmission signal transmitted to an antenna 203 and a reflected signal thereof from the antenna 203 connected to an output terminal 230b of a directional coupler 230 are detected by the directional coupler 230 inserted between the directional coupler 220 and the antenna 203. Then, the progressive signal or the reflected signal which are detected by the directional coupler 230 is mixed with a phase-modulated progressive signal and is low-pass filtered. Thereafter, the filtered audio frequency signal is full-wave-rectified, and the peak of the rectified signal is held by a peak hold circuit 245. In response to the peak of the rectified signal, levels of the progressive signal and the reflected signal which are detected by the directional coupler 230 are calculated, and then, a VSWR on a transmission line located from the directional coupler 230 to the antenna 203 is calculated based on the respective levels of the calculated progressive signal and the reflected signal. Referring to Fig. 1, a transmission signal of a frequency f1 having a predetermined level outputted from the transmitter 201 is outputted through an isolator 210, a pass line 221 of the directional coupler 220 for detecting a progressive signal and a pass line 231 of the directional coupler 230 for detecting a progressive signal and a reflected signal, to the antenna 203, so as to be projected toward a free space. The directional coupler 220 comprises the pass line 221 for passing through the transmission signal inputted from the transmitter 201, and a coupling line 222 for detecting a progressive signal, which is provided separately at a predetermined distance from the pass line 221 on a side of the input terminal 220a of the directional coupler 220 so as to be electromagnetically coupled with the pass line 221 and to detect a part of the power of the transmission signal passing through the pass line 221, the coupling line 222 detecting the transmission signal (referred to as a progressive signal hereinafter) passing through the pass line 221. A terminal 222b of the pass line 222 on a side of an output terminal 220b of the directional coupler 220 is terminated with a termination resistance R1. On the other hand, a terminal 222a of the pass line 222 on a side of an input terminal 220a of the directional coupler 220 is connected to an input terminal of a phase modulator 240. In the direction coupler 220 constructed as described above, the transmission signal passing through the pass line 221 after being inputted from the isolator 210 to the input terminal 220a is inputted from the output terminal 220b to an input terminal 230a of the directional coupler 230, and also is detected by the coupling line 222 so that the detected progressive signal is outputted to the phase modulator 240. A signal generator 270 generates a sine-wave audio frequency signal of a frequency of, for example, 10 kHz having a predetermined level and outputs it to the phase modulator 240. The phase modulator 240 phase-modulates the progressive signal inputted from the directional coupler 220 according to the generated audio frequency signal, and outputs the modulated PM signal to a local oscillation signal input terminal of a mixer 241. The phase modulator 240 can be constituted by, for example, a three terminal type circulator, a transmission line connected to one terminal of which is connected to a varactor diode, or a 3 dB hybrid circuit, a pair of terminals of which is connected to varactor diodes. An operating point of each of the varactor diodes is biased by a negative direct-current bias voltage, and the generated audio frequency signal is inputted through a low-pass filter capable of passing through only the generated audio frequency signal to both ends of each of the varactor diodes, thereby phase-modulating the progressive signal of the frequency f1 passing through the other two terminals of the three terminal type circulator or another pair of terminals of the 3 dB hybrid circuit according to the generated audio frequency signal. The directional coupler 230 comprises a pass line 231 for passing through the transmission signal inputted from the transmitter 201 through the directional coupler 220, a coupling line 232 for detecting a progressive signal provided separately at a predetermined distance from the pass line 231 on a side of the input terminal 230a of the directional coupler 230 so as to be electromagnetically coupled with the pass line 231 and to detect a part of the power of the transmission signal passing through the pass line 231, and a coupling line 233 for detecting a reflected signal provided separately at a predetermined distance from the pass line 231 on a side of the output terminal 230b of the directional coupler 230 so as to be electromagnetically coupled with the pass line 231 and to detect a part of the power of a reflected signal (referred to as a reflected signal hereinafter) coming back from the antenna 203 to the output terminal 230b, the coupling line 233 detecting the reflected signal. A terminal 232b of the coupling line 232 on a side of the output terminal 230b of the directional coupler 230 is terminated with a termination resistance R2, and a terminal 233a of the coupling line 233 on a side of the input terminal 230a of the directional coupler 230 is terminated with a termination resistance R3. A terminal 232a of the coupling line 232 on a side of the input terminal 230a of the directional coupler 230 is connected through an a-side of the switch SW11 to a main signal input terminal of the mixer 241, and a terminal 233b of the coupling line 233 on a side of the output terminal 230b of the directional coupler 230 is connected through a b-side of the switch SW11 to the main signal input terminal of the mixer 241. In the directional coupler 230 constructed as described above, the progressive signal of the transmission signal passing through the pass line 231 after being inputted from the input terminal 230a from the directional coupler 220 is detected by the coupling line 232, and the detected progressive signal is inputted as a main signal through the a-side of the switch SW11 to the mixer 241. Further, the reflected signal of the transmission signal caused due to a miss-matching between an impedance seen from the output terminal 230b of the directional coupler 230 through a transmission line toward the antenna 203 and an output impedance of the directional coupler 230 is detected by the coupling line 233, and the detected reflected signal is inputted as a main signal through the b-side of the switch SW11 to the mixer 241. The mixer 241 constituted by a multiplier mixes the signal inputted to the main signal input terminal thereof with the signal inputted to the local oscillation signal input terminal thereof, and outputs the mixed signals to a low-pass filter 242. The mixed signals outputted from the mixer 241 include a PM signal of a frequency 2f1 (= f1 + f1), a PM signal of a direct-current voltage of a frequency (= f1 - f1) namely the above-mentioned audio frequency signal outputted from the signal generator 270 etc.. The low-pass filter 242 passes the above-mentioned audio frequency signal among the mixed signals therethrough, and outputs it through an audio frequency amplifier 243 for amplifying signals of frequencies from a direct-current to an audio frequency component, to a full-wave rectifier 244, for example, comprised of four rectifier diodes connected in a form of a bridge connection. The full-wave rectifier 244 full-wave rectifies the inputted audio frequency signal and outputs it to a peak hold circuit 245 constituted by an integrated circuit, for example, comprised of a capacitor and a resistance connected in parallel. The peak hold circuit 245 holds the peak of the inputted audio frequency signal, and outputs a signal for representing the peak thereof (referred to as a peak hold signal hereinafter) to an analogue to digital converter (referred to as an A/D converter hereinafter) 246. Thereafter, the A/D converter 246 converts the analogue peak hold signal into a digital signal and outputs the converted digital signal to a central processing unit (referred to as a CPU hereinafter) 251 through an interface 254 of an MPU 250. Fig. 4 shows a relationship between a phase difference between the respective signals inputted to the mixer 241 and an output voltage of an audio frequency signal outputted from the low-pass filter 242 under a such condition that an output voltage of a phase detector comprised of the mixer 241 and the low-pass filter 242 is zero Volt when the phase difference between the respective signals inputted to the mixer 241 is zero radian. As is apparent from Fig. 4, the output voltage of the phase detector comprised of the mixer 241 and the low-pass filter 242 changes depending on the phase difference between the respective signals inputted to the mixer 241. For example, when the progressive signal detected by the directional coupler 220 is phase-modulated according to a sine-wave audio frequency signal S1 outputted from the signal generator 270 so that the above-mentioned phase difference is within a phase range from - π/2 to +π/2, an audio frequency signal outputted from the phase detector becomes a sine-wave signal S2. The levels of the respective frequency components outputted from the mixer 241 comprised of a multiplier are in proportion to a product of the respective signals inputted to the mixer 241 and the level of the transmission signal inputted to the directional coupler 220 is a constant. Therefore, when the switch SW11 is switched over to the a-side, the level of the audio frequency signal S2 outputted from the low-pass filter 242 is in proportion to the level of the progressive signal detected by the directional coupler 230. On the other hand, when the switch SW11 is switched over to the b-side, the level of the audio frequency signal S2 outputted from the low-pass filter 242 is in proportion to the level of the reflected signal detected by the directional coupler 230. However, as shown in Fig. 5, when the phase difference between the respective signals inputted to the mixer 241 is shifted from a value of the state shown in Fig. 4 and the progressive signal detected by the directional coupler 220 is phase-modulated according to a sine-wave audio frequency signal Sla outputted from the signal generator 270 so that the above-mentioned phase difference is within a phase range from -3π/4 to +π/8, the audio frequency signal outputted from the above-mentioned phase detector becomes a signal S2a which is not a sine-wave signal. Therefore the audio frequency signal outputted from the low-pass filter 242 is amplified by the amplifier 243, is full-wave-rectified by the full-wave rectifier 244 so as to obtain a signal S3a of an absolute value of the audio frequency signal, and then, the peak of the signal S3a of the absolute value thereof is held by the peak hold circuit 245 so as to obtain the above-mentioned peak hold signal. When the switch SW11 is switched over to the a-side, the peak of the peak hold signal is in proportion to the level of the progressive signal detected by the directional coupler 230. On the other hand, when the switch SW11 is switched over to the b-side, the peak of the peak hold signal is in proportion to the level of the reflected signal detected by the directional coupler 230. As described above, in the VSWR measuring unit of Fig. 1, since both of the PM modulator 240 and the above-mentioned phase detector are used and there is provided the full-wave rectifier 244 of an absolute value circuit and the peak hold circuit 245 on the following step of the above-mentioned phase detector, the levels of the progressive signal and the reflected signal can be correctly measured even though the phase difference between the respective signals inputted to the mixer 241 becomes any value. Further, there will be described below such a case that an interference signal having a frequency fit (> f1) other than the frequency f1 is inputted from an external apparatus to the antenna 203. In this case, the above-mentioned interference signal and the reflected signal having the frequency f1 are detected by the coupling line 233 of the directional coupler 230, and the detected signals are inputted through the b-side of the switch SW11 to the mixer 241. On the other hand, the PM signal of the progressive signal having the frequency f1 detected by the directional coupler 220 is inputted to the mixer 241. Therefore, the mixed signals outputted from the mixer 241 include a PM signal having a frequency 2f1 (= f1 + f1), a PM signal of a direct-current voltage component (= f1 - f1) namely the above-mentioned audio frequency signal outputted from the signal generator 270, a PM signal having a frequency (fit + f1), and a PM signal having a frequency (fit - f1) etc.. However, the low-pass filter 242 passes only the above-mentioned audio frequency signal among the inputted mixed signals therethrough. Therefore, since the PM signal relating to the interference signal is prevented from passing through the low-pass filter 242 by the low-pass filter 242, the VSWR measuring unit 202 of the present preferred embodiment can measure the VSWR with respect to the transmission signal on the transmission located from the directional coupler 230 to the antenna 203, correctly, without receiving any influence of the interference signal even though the interference signal having a relatively large level is inputted from an external apparatus to the antenna 203. An MPU 250 of the VSWR measuring unit 202 comprises the CPU 251 for executing a VSWR measuring process, a read only memory (referred to as a ROM hereinafter) 252 for storing a control program of the above-mentioned VSWR measuring process and data required for executing the above-mentioned control program, and a random access memory (referred to as a RAM hereinafter) 253 used as a working area for storing data inputted through interfaces 254 and 256. The MPU 250 further comprises the interfaces 254 to 257, and the CPU 251, the ROM 252, the RAM 253 and the interfaces 254 to 257 are connected through a bus 259. Further, the interface 255 is connected to a control terminal of the switch SW11, the interface 256 is connected through a start switch SW12 to the ground, and the interface 257 is connected to a display unit 260. When the CPU 251 executes the above-mentioned VSWR measuring process, the CPU 251 controls the switch SW11 through the interface 255 so as to selectively switch over to the a-side or the b-side, and also outputs data of calculated results of the VSWR measuring process to the display unit 260 so as to display it thereon. Fig. 6 is a flowchart showing a main routine of the VSWR measuring process executed by the MPU 250 of the VSWR measuring unit 202, and the main routine includes a process for measuring the level of the progressive signal of the transmission signal transmitted from the transmitter 201 through the directional couplers 220 and 230 of the VSWR measuring unit 202 to the antenna 203 and the level of the reflected signal from the antenna 203, and thereafter, for calculating a VSWR based on the measured levels. As shown in Fig. 6, when a power switch (not shown) of the MPU 250 is turned on, the main routine of the VSWR measuring process shown in Fig. 6 is started, and then, first of all, it is judged at step S101 whether or not the start switch SW12 is turned on. Until the switch SW12 is turned on, the program flow becomes a stand-by state. When the switch SW12 is turned on (YES at step S101), the program flow goes to step S102. After the switch SW11 is switched over to the a-side at step S102, there is calculated an effective power of the progressive signal based on the level of the peak hold signal inputted from the A/D converter 246 through the interface 254 at step S103. Thereafter, the switch SW11 is switched over to the b-side at step S104, and then, there is calculated an effective power of the reflected signal based on the level of the peak hold signal inputted from the A/D converter 246 through the interface 254 at step S105. Further, there is calculated a VSWR based on the calculated effective powers of the progressive signal and the reflected signal at step S106, and then, there is displayed the calculated effective powers of the respective signals and the calculated VSWR on the display unit 260 at step S107. Finally, it is judged at step S108 whether or not the calculated VSWR is larger than a predetermined threshold value Ts such as Ts = 1.3. When the calculated VSWR is larger than the threshold value Ts (YES at step S108), display information "The antenna system is in an abnormal statel" is displayed on the display unit 260 at step S109, and then, the program flow goes back to step S101. On the other hand, the calculated VSWR is equal to or smaller than the threshold value Ts (NO at step S108), the program flow goes back to step S101, directly. All of the effective powers of the progressive signal and the reflected signal and the VSWR are converted into respective values on the output terminal 230b of the directional coupler 230 in a manner similar to that known to those skilled in the art, and the converted values are displayed on the display unit 260. If there is not provided the directional coupler 220, the progressive signal outputted from the output terminal 232a of the directional coupler 230 may be inputted to the PM modulator 240. Further, the level of the progressive signal may be measured by detecting the progressive signal detected by the directional coupler 220 or 230 using a detector comprised of a diode and converting the detected progressive signal into a digital signal using an A/D converter so as to output the converted digital signal to the MPU 250. Fig. 2 shows a reflected signal measuring apparatus 202a and a transmitter combiner 204 of a first preferred embodiment according to the present invention. In Fig. 2, the same sections as those shown in Fig. 1 are denoted by the same numerals as those shown in Fig. 1. Further, symbols of the directional couplers 220, 223, 226 and 230 are simplified, and the termination resistances connected to the directional couplers 220, 223, 226 and 230 are omitted in Fig. 2. The VSWR measuring unit 202a of the first preferred embodiment is characterized in that, three channel type transmitter combiner 204 is connected thereto, and VSWRs with respect to respective transmission signals outputted from respective transmitters 201a, 201b and 201c are measured using the measuring principle of the VSWR described with reference to the unit shown in Fig. 1. The differences between the first preferred embodiment and the unit shown in Fig. 1 will be mainly described below. The respective transmission signals of UHF band frequencies f1, f2 and f3 (f1 > f2 > f3) different from each other having predetermined levels respectively outputted from the transmitters 201a, 201b and 201c are inputted through respective pass lines 221, 224 and 227 of the directional couplers 220, 223 and 226 provided within the VSWR measuring unit 202a to the transmitter combiner 204. The respective directional couplers 220, 223 and 226 have structures similar to that of the directional coupler 220 of the eighth preferred embodiment. Namely, the directional coupler 223 comprises the pass line 224 and a coupling line 225 for detecting a progressive signal, wherein the pass line 224 has an input terminal 223a and an output terminal 223b on both ends thereof, and the coupling line 222 has an output terminal 225a. Further, the directional coupler 226 comprises the pass line 227 and a coupling line 228 for detecting a progressive signal, wherein the pass line 227 has an input terminal 226a and an output terminal 226b on both ends thereof, and the coupling line 228 has an output terminal 228a. The progressive signals outputted from the respective output terminals 222a, 225a and 228a of the coupling lines 222, 225 and 228 of the directional couplers 220, 223 and 226 are respectively outputted through an a-side, a b-side and a c-side of the switch SW13 selectively switched by the CPU 251 of an MPU 250a to the phase modulator 240. A control terminal of the switch SW13 is connected to the interface 255 provided in the MPU 250a. The transmitter combiner 204 is a frequency multiplexer for frequency-multiplexing the respective transmission signals of frequencies f1, f2 and f3 to output a multiplexed signal, and comprises an isolator 211a for passing the transmission signal of the frequency f1 therethrough in a direction from the transmitter 201a to the antenna 203, a band-pass filter 212a for passing only a frequency band component of the transmission signal of the frequency f1 therethrough, an isolator 211b for passing the transmission signal of the frequency f2 therethrough in a direction from the transmitter 201b to the antenna 203, a band-pass filter 212b for passing only a frequency band component of the transmission signal of the frequency f2 therethrough, an isolator 211c for passing the transmission signal of the frequency f3 therethrough in a direction from the transmitter 201c to the antenna 203, and a band-pass filter 212c for passing only a frequency band component of the transmission signal of the frequency f3 therethrough. Output terminals of the respective band-pass filters 212a, 212b and 212c are electrically connected to each other and are further electrically connected to the input terminal 230a of the directional coupler 230. The transmission signal outputted from the directional coupler 220 is inputted through the isolator 211a and the band-pass filter 212a to the directional coupler 230, the transmission signal outputted from the directional coupler 223 is inputted through the isolator 211b and the band-pass filter 212b to the directional coupler 230, and the transmission signal outputted from the directional coupler 226 is inputted through the isolator 211c and the band-pass filter 212c to the directional coupler 230. The multiplexed signal comprised of the respective transmission signals of the frequencies f1, f2 and f3 outputted from the transmitter combiner 204 is outputted through the pass line 231 of the directional coupler 230 and a transmission band-pass filter 205 for passing only frequency band components including the above-mentioned frequencies f1, f2 and f3 therethrough to the antenna 203 so as to be projected from the antenna 203 toward a free space. In the directional coupler 230, the progressive signal of the multiplexed signal is detected by the coupling line 232, and the detected progressive signal thereof is outputted from the output terminal 232a through the a-side of the switch SW11 to the mixer 241. Further, the reflected signal of the multiplexed signal caused due to a miss-matching between an impedance seen from the output terminal 230b of the directional coupler 230 toward the antenna 203 and an output impedance of the pass line 231 is detected by the coupling line 233, and then, the detected reflected signal thereof is outputted from the output terminal 233b through the b-side of the switch SW11 to the mixer 241. In the VSWR measuring unit 202a constructed as described above, for example, when the switch SW13 is switched over to the a-side and the switch SW11 is switched over to the a-side, the progressive signal of the multiplexed signal is inputted to the mixer 241, and also the PM signal of the progressive signal of the transmission signal of the frequency f1 detected by the directional coupler 220 is inputted to the mixer 241. Therefore, the mixed signals outputted from the mixer 241 include a PM signal of a frequency 2f1 (= f1 + f1), a PM signal of a direct-current voltage component (= f1 - f1) namely the above-mentioned audio frequency signal outputted from the signal generator 270, a PM signal of a frequency (f1 + f2), a PM signal of a frequency (f1 - f2), a PM signal of a frequency (f1 + f3), a PM signal of a frequency (f1 - f3) etc.. The low-pass filter 242 passes only the above-mentioned audio frequency signal in proportion to the level of the progressive signal of the frequency f1 among the mixed signals therethrough. Therefore, there can be correctly measured the level of the progressive signal of the transmission signal of the frequency f1 without receiving any influence of the progressive signals of the frequencies f2 and f3 other than the progressive signal of the frequency f1 selected by the switch SW13. Further, when the switch SW11 is switched over to the b-side, there can be correctly measured the level of the reflected signal of the transmission signal of the frequency f1 without receiving any influence of the reflected signals of the frequencies f2 and f3 other than the progressive signal of the frequency f1 selected by the switch SW13, similarly. Furthermore, the levels of the progressive signal and the reflected signal of the other frequencies f2 and f3 can be correctly measured by sequentially switching over the switch SW13 to the b-side or the c-side, similarly. Furthermore, even though an interference signal having a relatively large level is inputted from an external apparatus to the antenna 203, the VSWR measuring unit 202a of the present preferred embodiment can correctly measure the VSWRs with respect to the respective transmission signals without receiving any influence of an interference signal, in a manner similar to that of the eighth preferred embodiment. The MPU 250a of the present preferred embodiment executes a VSWR measuring process which will be described below. Fig. 7 is a flowchart showing a main routine of the VSWR measuring process of the MPU 250a of the VSWR measuring unit 202a of the first preferred embodiment, and the main routine is a process for measuring the levels of the progressive signals of the respective transmission signal outputted from the respective transmitters 201a, 201b and 201c through the respective directional couplers 220, 223 and 226 of the VSWR measuring unit 202a, the transmitter combiner 204, the directional coupler 230 of the unit 202a and the transmission band-pass filter 205 to the antenna 203, and the levels of the reflected signals of the respective transmission signals from the antenna 203, and thereafter, for calculating the VSWRs with respect to the respective transmission signals based on the calculated levels of the progressive signals and the reflected signals thereof. As shown in Fig. 7, when a power switch (not shown) of the MPU 250a is turned on, the main routine of the VSWR measuring process shown in Fig. 7 is started, and first of all, it is judged at step S111 whether or not the start switch SW12 is turned on. Until the start switch SW12 is turned on, the program flow becomes a stand-by state at step S111. Thereafter, when the start switch SW12 is turned on (YES at step S111), the program flow goes to step S112. After the switch SW13 is switched over to the a-side at step S112, a subroutine of a VSWR measuring process shown in Fig. 8 with respect to the transmission signal of the frequency f1 is executed at step S113. Thereafter, after the switch SW13 is switched over to the b-side at step S114, the subroutine of the VSWR measuring process shown in Fig. 8 with respect to the transmission signal of the frequency f2 is executed at step S115. Further, after the switch SW13 is switched over to the c-side at step S116, the subroutine of the VSWR measuring process shown in Fig. 8 with respect to the transmission signal of the frequency f3 is executed at step S117, and then, the program flow goes back to step S111. Fig. 8 is a flowchart showing the subroutine of the VSWR measuring process (steps S113, S115 and S117). Processes of steps S122 to S129 are executed in a manner similar to that of the respective processes of steps S102 to S109 shown in Fig. 6. Fig. 3 shows a reflected signal measuring apparatus 202b and the transmitter combiner 204 of a second preferred embodiment according to the present invention. In Fig. 3, the same sections as those shown in Fig. 2 are denoted by the same numerals as those shown in Fig. 2. The VSWR measuring apparatus 202b of the second preferred embodiment is characterized in that, the progressive signals of the respective transmission signals are not detected by the directional coupler 230 of the ninth preferred embodiment, and there is measured the levels of the progressive signals of the respective transmission signals based on those detected by the respective directional couplers 220, 223 and 226. The differences between the present preferred embodiment and the ninth preferred embodiment will be described below. Referring to Fig. 3, as is apparent from comparison between the second preferred embodiment and the first preferred embodiment shown in Fig. 2, it is unnecessary to provide the coupling line 232 for detecting the progressive signal within the directional coupler 230, and the switch SW11, the progressive signal outputted from the common terminal of the switch SW13 is detected by a detector 247 of a diode, and the detected progressive signal is inputted through an A/D converter 248 and an interface 258 connected to the bus 259 provided in an MPU 250b. The MPU 250b further comprises the interface 258 in addition to the circuits of the MPU 250a of the ninth preferred embodiment, and the MPU 250b executes the main routine shown in Fig. 7 and the subroutine shown in Fig. 8, similarly, except for a process of step S123 for calculating the level of the progressive signals of the respective transmission signals based on the signal inputted from the detector 247 through the A/D converter 248 and the interface 258 to the CPU 251. The VSWR measuring unit 202b of the second preferred embodiment has the above-mentioned advantages similar to those of the VSWR measuring unit 202 of the first preferred embodiment. At step S108 of Fig. 6 of the second preferred embodiment and step S128 of Fig. 8 of the first preferred embodiment, an abnormal state of the antenna system is detected by judging the calculated VSWR, the present invention is not limited to this. An abnormal state of the antenna system may be detected by judging the measured level of the reflected wave. In the first and second preferred embodiments, there is used the phase modulator 240, however, the present invention is not limited to this. There may be used an angle modulator such as a frequency modulator.

Claims: A reflected signal measuring apparatus comprising a transmission line, one end of which is connected to a load (203), said apparatus measuring a level of a reflected signal coming back from said load to said transmission line when a high frequency signal having a predetermined level is inputted to another end of said transmission line, said apparatus comprising a plurality of first coupling means (220, 223, 226) being constituted by directional couplers; second coupling means (230; 233) provided on said transmission line for detecting said reflected signal; calculating means (250); frequency multiplexing means (204) for multiplexing a plurality of high frequency signals having frequencies different from each other and for supplying a multiplexed signal comprised of said plurality of high frequency signals through said transmission line to said load (203); and switching means (SW13); characterized by the provision of the plurality of said first coupling means (220, 223, 226) before said multiplexing means (204) for detecting the respective progressive high frequency signals when the respective high frequency signals are input, wherein said switching means (SW13) selects one of said progressive high frequency signals detected by one of said first coupling means (220, 223, 226) and outputs said selected progressive high frequency signal; modulating means (240) for angle-modulating said selected progressive high frequency signal outputted from said switching means (SW13) according to a signal having a predetermined level and for outputting a modulation signal; mixing means (241) for mixing said reflected signal detected by said second coupling means (233) with said modulation signal outputted from said modulating means (240) and outputting the mixed signals; low-pass filtering means (242) for filtering a signal from said mixed signals outputted from said mixing means (241); rectifying means (244) for rectifying said signal filtered by said low-pass filtering means (242) and outputting a first signal for representing an absolute value of said filtered signal; integrating means (245) for integrating said first signal outputted from said rectifying means (244) and outputting a second signal for representing a peak value of said first signal, wherein said calculating means (250a, 250b) respectively calculates levels of said reflected signals on said transmission line with respect to said high frequency signals based on said second signal outputted from said integrating means by controlling said switching means (SW13) to sequentially select said respective progressive high frequency signals. Apparatus according to claim 1, characterized in that said second coupling means (230; 232) further detects the multiplexed progressive signal; said mixing means (241) respectively mixes said multiplexed progressive signal outputted by said second coupling means (232) and said reflected signal detected by said second coupling means (233) with said modulation signal outputted from said modulating means (240) and outputs the respective mixed signals; said low-pass filtering means (242) respectively filters respective signals from said respective mixed signals outputted from said mixing means (241); said rectifying means (244) respectively rectifies said respective signals filtered by said low-pass filtering means (242) and outputs respective first signals for representing absolute values of said filtered respective signals; said integrating means (245) respectively integrates said respective first signal outputted from said rectifying means (244) and outputs respective second signals for representing peak values of said respective first signals; said calculating means (250a) comprises: means for respectively calculating a level of said multiplexed progressive signal and a level of said reflected signal based on said respective second signals outputted from said integrating means; and means for respectively calculating standing wave ratios on said transmission line with respect to said high frequency signals based on the calculated level of said multiplexed progressive signal and the calculated level of said reflected signal by controlling said switching means to sequentially select said respective progressive high frequency signals. Apparatus according to claim 1 or 2, characterized in that said modulating means (240) is a phase modulator. Apparatus according to any of claims 1 to 3, characterized in that said second coupling means (230) is constituted by one or two directional couplers (232, 233).

Inventors: ISHIKAWA YOUHEI, WADA HIDEKAZU, YAMASHITA SADAO, ISHIKAWA, YOUHEI, WADA, HIDEKAZU, YAMASHITA, SADAO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1991
Main classification: G01R27
IPC codes: G01R27