1.  Field of the Invention
The present invention relates to an apparatus and a method for measuring a voltage standing wave ratio of an antenna of a base station, and more particularly to the apparatus and method for measuring the voltage standing wave ratio of a transferring terminal TX and a receiving terminal RX of the base station of a mobile communication, by using a base-station test unit for checking and testing the base station of the mobile communication. 
2.  Discussion of Related Art
In general, in the mobile communication system, it is an ideal that transmission output from the base station radiates in the air through a transmitting antenna.  But the location of each base station, the surroundings and equipment installation conditions are different from each other, even though the same equipment is used, the efficiency in the transmission and reception of the base station is affected by the surroundings. 
Thus, in order to make the base station perform at an optimal state, the state of the base station should be controlled in accordance with conditions around the base station and to minimize the minimize the effects of conditions by continuously monitoring the state of the base station. 
The voltage standing wave ratio is used to check the state of the base station. 
The standing wave is also called a stationary wave, which is generated by the combination of an incident wave from the signal source on the transmission line, and a reflected wave from the receiving terminal.  The standing wave has a constant ratio of an order value between points of "A" and "B", regardless of time, and ordinarily there are a lot of cases in which the amplitude of variation in the transmission is of the periodic function of the distance to the wave length direction. 
Also, the standing wave ratio is of the amplitude to both anti node and wave node.  If reflection coefficient is ".tau.",  the SWR is defined as (1+.tau.)/(1-.tau.).  Particularly, the SWR to a voltage is named VSWR, which is the ratio of the amplitude of voltage at the maximum point of the voltage caused around the waveguide, coaxial cable and transmission line at the minimum point adjacent thereto. 
Accordingly, by measuring the SWR, each base station can check if the transmitting signals are efficiently well transmitted through an antenna connected to its transmission terminal.  That is, it is determined whether the antenna and the transmission source are properly and efficiently matched.  This means that each base station has to measure the above VSWR consistently and periodically. 
In general, the large VSWR means that the transmission output at the base station doesn't radiate in the air, but returns to a transmission outlet of the system as much as the amount of mis-matching and has a bad effect on its transmission outlet. 
Namely, the mismatching means that VSWR is so large that the quality and effectiveness of the system equipment is substantially degraded.  Therefore, by measuring VSWR, the efficiency can be determined.  Accordingly, each base station can control the loss of transmission in order to make good electrical matching between the different mediums and to make it possible to send voltage without any loss. 
Also, even in case there is something wrong with a cable and antenna by the change of surrounding nearby, it triggers the big change of the VSWR so that each station can control its condition in a remote way. 
The coupler is widely used as a tool for measuring the VSWR, which consists of an input terminal, a coupling terminal, an isolation terminal, and an output terminal. 
The coupling terminal is where the signal inputted to the input terminal is outputted, after being attenuated as much as a fixed coupling coefficient C. The isolation terminal is where the signal inputted to the input terminal is theoretically held completely.  But, in reality, there happens to be a little leaking named the isolation coefficient I. Also, the input terminal of the coupling can be used as an output terminal, which means the coupling terminal and the isolation terminal exchange their role.  Namely, the terminal for generating an input signal from the input terminal is the coupling terminal and the opposite one to prevent output is the isolation terminal.  The difference between the coupling coefficient C and the isolation coefficient is marked "D". 
So, the output terminal is where the signals inputted to the input terminal are generated, namely, the transmission signals are generated and decreased as much as the coupling coefficient of the coupling terminal and the isolation coefficient of the isolation terminal.  For example, if the coupling coefficient is 30 dB and the isolation is perfect, 99.9% of the signals in the input terminal is transmitted.  Namely, the coupling coefficient of 30 dB is 1/1000, about 0.1%, which means that 0.1% of the entire transmitted signals are generated to the coupling terminal and 99.9% is generated to the output terminal without any leak into the isolation terminal. 
The equipment to measure VSWR of the output and input terminals at each base station according to the prior art will be explained in FIG.  1. 
The VSWR measuring equipment 15 of the prior art has a cellular or PCS station system 1, a transmission coupler 10 having an input terminal 11, a coupling terminal 12, an isolation terminal 13, and an output terminal 14 connected to the transmitting terminal of the base station 1, a transmission voltage standing wave ratio 15 for measuring the voltage standing wave ratio of the transmitting terminal under connection to the coupling terminal 12 and the isolation terminal 13 of the transmission coupler 10, and a transmitting antenna 16 connected to an output terminal of the transmission coupler 10. 
Also, as the combination of the VSWR measuring equipment 23 of the reception terminal, the reception coupler 20 connected with the reception terminal of the above station has the following 5 terminals, namely the input terminal 24, a first coupling terminal 25, a second coupling terminal 26, an output terminal 27, and isolation terminals 28.  The first coupling terminal 25 of the reception coupler 20 is coupled with the Phase Locked Loop 21, the second coupling terminal 26 of the reception coupler 20 with the voltage standing wave ratio 23 of the receiving terminal, respectively. The input terminal 24 of the reception coupler 20 is coupled with the receiving antenna 22, the output terminal 27 of the reception coupler 20 with the receiving terminal of the base station 1, and the isolation terminal 28 with the voltage standing wave ratio 23 of the receiving terminal, respectively. 
There will be explained an operation of the antenna of the base station VSWR measurement equipment as follows. 
First, the measuring procedure of the VSWR on the transmission terminal is fully explained.  The signal from the output terminal of the station system 1 is transmitted to the input terminal 11 of the transmitting coupler 10, diminishes as much as coupling coefficient C and becomes an output X1, which is transmitted to the SWR measuring equipment 15 of the reception terminal. 
Also, the signal of the above station inputted to the above transmitting coupler 10 is transmitted to the transmitting antenna 16 through the output terminal 14.  But, if the transmission terminal and transmitting antenna 16 of the base station 1 are not fully matched, all the signals transmitted to the above transmission antenna 16 will not radiate to the air and some transmitted signals are reflected due to the mis-matching.  This reflected signal is applied to the output terminal 14 of the transmission coupler 10 and becomes the output X2 through the isolation terminal 13 of the above transmission coupler 10 to thereby be applied to the transmission terminal VSWR equipments 15 which measure the ratio of the VSWR of the transmission terminal coupler comparing the values of outputs X1 and X2. 
There will be explained hereinafter a measuring procedure of the VSWR of the reception terminal. 
First, an oscillator having PLL 21 connected to the first coupling terminal 25 of the reception coupler 20 oscillates some frequency either from Frequency Band of the reception frequency of the above station system with some power.  The first coupling terminal 25 of the reception coupler 20 having the oscillated frequency, generates a transmission signal X4 reduced as much as the couple coefficient C by the second coupling terminal 26.  And, the rest of non-reduced signals are generated to the receiving antenna 22. 
The receiving antenna 22 reflects signal due to the mis-match with the reception part of the above station.  The reflected signals generate the transmission signal X3 reduced by the couple coefficient C of the isolation terminal 28 of the reception coupler 20.  Also, the VSWR equipment 23 of the reception terminal will measure the ratio of the VSWR of the reception terminal by comparing the outputs X3 and X4. 
As mentioned above, the part of the previous reception terminal can't measure the VSWR in the same manner with that of the output terminal because it is not where signal is generated but where the signal is inputted.  Accordingly, the receiving part generates some random signals and sends them to the receiving antenna, after it measures the output X3 reflected from the receiving antenna 22 and calculates the ratio of the VSWR through the output X4 coupled with the above PLL 21 signals.  At this time, the equipment to measure the VSWR on the receiver's side was quite complicated because the oscillator with the PLL was widely used to generates signals. 
In other words, in measuring VSWR according to the prior art, there has been a big problem that an exclusive VSWR equipment only for measuring the VSWR and exclusive oscillator with the PLL and the reception coupler separately manufactured to measure the VSWR of the reception terminal become larger. 
Furthermore, when measuring the VSWR of the reception part, there has been a problem that noise caused by the PLL oscillator or mixer could affect the precision of measurement of the VSWR ratio. 
And, the PLL signal is applied to the first coupling terminal 25, and after reduced in accordance with the couple coefficient C of second coupling terminal 26 and reflected as much as mis-matching at the receiving antenna 22.  And also reduced in accordance with the couple coefficient C of isolation terminal 28 after that inputted to reception terminal of the base station 1.  Therefore the base station cannot discern the above PLL signal from the signal received by the ordinary user's terminals, because the signal reduced again as much as the couple coefficient C of the isolation terminal is directly inputted to the above reception terminal of the base station.  Accordingly, the operation of the reception terminal had a problem that it had to be stopped until the VSWR measuring is finished, which caused the entire base station system to be held up by the end of the VSWR measurement. 
Also, when manufacturing the reception coupler 20, except the coupling terminal and the isolation terminal, another coupling port to receive a signal of the random PLL circuit is needed.  In general, it is more difficult for a coupler with three coupling ports to match each port's function than a coupler with two coupling ports.  Also, there is a problem in that it is difficult to manufacture the system and its manufacturing cost rises.  Further, there is provided a problem that it is difficult to measure the accurate characteristic of the voltage standing wave ratio owing to an operation of the oscillator.