Abstract:
A radio frequency signal folding-back test circuit shifts a test transmission away from a normal transceiver frequency, and selects a test filter to receive the loop-back transmission. The test circuit is controlled by automatic logic which shifts a transmission frequency, causes the transmission to loop-back to the receiver, and connects an appropriate band pass filter to the receiver to obtain the test signal. The circuit provides a simple, automatic means for testing a transceiver without interfering with normal operation. The circuit avoids manual intervention to perform a test, and eliminates the need to shut down a remote transceiver while the test is performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a radio frequency signal folding-back transmitting/receiving circuit for testing the operation of a radio transmitting/receiving apparatus. The present invention also relates to a radio transmitting/receiving apparatus having such a circuit. 
     2. Description of the Related Art 
     In recent years, to allow users to use a communication line at a low cost, it is desired to improve the efficiency of a maintenance operation periodically or non-periodically performed after a communication apparatus is installed. Thus, it is required to easily perform characteristic tests for the communication apparatus in the state in which it is operating. 
     FIG. 1 is a block diagram showing the structure of an opposite communication system having two radio transmitting/receiving apparatuses oppositely disposed. With reference to FIG. 1, an example of a system that allows a characteristic test to be effectively performed will be described. 
     Referring to FIG. 1, in a local station radio transmitting/receiving apparatus  1 , a multiplexed signal is input from, for example, a public telephone line to a local station transmission signal input terminal  10 . A local station transmitter  3  amplifies the power of the multiplexed signal. A radio wave corresponding to the amplified signal is transmitted from an antenna through a local station branching filter  5 . In addition, a local station receiver  4  receives a radio wave from the antenna  2  through the local station branching filter  5 . For example, a multiplexed signal corresponding to a transmission signal is output from a local station reception signal output terminal  11 . 
     On the other hand, referring to FIG. 1, in an opposite radio transmitting/receiving apparatus  6 , a multiplexed signal is input from for example a public telephone line to an opposite station transmission signal input terminal  12 . An opposite station transmitter  8  amplifies the power of the multiplexed signal. A radio wave is transmitted from an antenna  2  through an opposite station branching filter  7 . In addition, an opposite station receiver  9  receives a radio wave from the antenna  2  through the opposite station branching filter  7 . For example, a multiplexed signal corresponding to a transmission signal is output from an opposite station reception signal output terminal  13 . 
     In the opposite communication system (in particular, a radio communication system using a microwave circuit routed nationwide), a transmission side telephone unit is connected to a reception side telephone unit through a transmission side subscriber line exchange, a microwave repeating station, a repeating exchange, and a reception side subscriber line exchange. The transmission side telephone unit transmits a signal of audio, data, video, or the like to the receiving side telephone unit through a designated telephone network. Thus, a radio repeating station is used as one important elements of the network. 
     Conventionally, an indoor radio transmitting/receiving apparatus and an outdoor antenna are connected with a waveguide capable of conducting a radio signal frequency of the equipment signal or the like. As the radio frequency rises, the power loss of the communication apparatus increases. In addition, installation cost of higher frequency increases. Thus, a transmission frequency converting portion and a reception frequency converting portion of the radio transmitting/receiving apparatus are disposed adjacent to the antenna to omit a connecting portion of waveguide. Consequently, the installation costs and power loss can be decreased. 
     In the opposite communication system of the radio transmitting/receiving apparatuses shown in FIG. 1, when a radio frequency signal folding-back test is performed for the local station radio transmitting/receiving apparatus  1 , a radio frequency signal folding-back test circuit is disposed in the radio transmitting/receiving apparatus shown in FIG.  2 A. In the radio frequency signal folding-back test circuit, a frequency shifter  14  is disposed between the local station antenna  2  and the local station radio transmitting/receiving apparatus  1  so as to perform a radio frequency signal folding-back test of the local station radio transmitting/receiving apparatus  1 . The frequency shifter  14  converts a transmission frequency into a reception, frequency. 
     In FIG. 2A multiplexed signal that is input to the local station transmission signal input terminal  10  is compared with a multiplexed signal that is folded back by the frequency shifter  14  to the local station reception signal output terminal  11 . When the same multiplexed signal is correctly obtained, it can be determined that the local station radio transmitting/receiving apparatus is operating normally. When the same multiplexed signal is not correctly obtained, it can be determined that the local station radio transmitting/receiving apparatus is abnormal. Once detected, a defective portion can be located and corrected. 
     However, in the folding-back test shown in FIG. 2A, the local station antenna  2  and the frequency shifter  14  must be manually mounted and dismounted. The folding-back test cannot be easily performed because of is manual nature. In addition, this type of test interrupts the transceiver operation. 
     Alternatively, in a folding-back test circuit for a radio transmitting/receiving apparatus is shown in FIG. 2B, a frequency shifter  14  is disposed in the local station transmitting/receiving apparatus  1 . A transmission side directional coupler  15  is disposed between a local station transmitter  3  and a local station branching coupler  5 . In addition, a reception side directional coupler  16  is disposed between a local station transmitter  3  and a local station branching filter  5 . A signal is folded back from the local station transmitter  3  to the local station receiver  4  through the directional couplers  15  and  16 . In this structure, it is not necessary to mount and dismount the local station antenna  2  and the frequency shifter  14 . In addition, the same radio frequency signal folding-back test circuit (not shown) may be disposed in the opposite station radio transmitting/receiving apparatus  6  shown in FIG.  2 B. 
     However, in the structure shown in FIG. 2B, since a transmission signal of the opposite station radio transmitting/receiving apparatus  6  is always received, the output signal of the opposite station transmitter  8  should be turned off. In other words, to perform a radio frequency signal folding-back test for the local station radio transmitting/receiving apparatus  1 , a transmission signal of the opposite station should be manually turned off. A manual turnoff is prescribed in case the opposite station receiver  9  becomes defective. If an automatic shutoff signal apparatus  6  gets defective, even if a signal that causes the off is transmitted from the local station radio transmitting/receiving apparatus  1  to the opposite radio transmitting/receiving apparatus  6  with a defective receiver  9 , the output signal of the opposite station transmitter  8  cannot be turned off. 
     As an application of the folding-back test circuit shown in FIG. 2B, an attenuator is disposed between the local station branching filter  5  of the folding-back test circuit and the receiver side directional coupler  16 . When a radio frequency signal folding-back test is performed, the attenuation amount of the attenuator is set to maximum value. Thus, the transmission signal of the opposite station radio transmitting/receiving apparatus  6  is not transmitted to the local station receiver  4 . 
     In the radio frequency signal folding-back test method, since the insertion loss of the attenuator is large, noise figure (NF) as a performance index of the receiver deteriorates. When the attenuator is manually disposed, even if the insertion loss can be suppressed, the manual operation is required. 
     SUMMARY OF THE INVENTION 
     An abject of the present invention is to provide a radio frequency signal folding-back test apparatus for to perform frequency signal folding-back control testing signal without using the need to manually turn off a transmission signal of the opposite station. 
     In addition, since a reception signal is attenuated by a downstream circuit of the reception signal first converter, NF of the reception signal converter does not deteriorate. Even if a radio frequency signal folding-back function is added, the performance of the reception signal converter does not deteriorate. 
     A first aspect of the present invention is a radio frequency signal folding-back transmitting/receiving circuit for use with a radio transmitting/receiving apparatus, comprising a signal folding-back means for folding back a transmission signal of a local station to a radio frequency signal line of a receiver of the local station through a directional coupler, a first band-pass filter for allowing a reception signal that is output from an opposite station to pass, a second band-pass filter, connected to the first band-pass filter in parallel, for allowing a transmission signal whose frequency is different from the frequency of the reception signal to pass, and a selecting means for selecting the second band-pass filter when a radio frequency signal folding-back test is performed. 
     A second aspect of the present invention is a radio transmitting/receiving apparatus for transmitting/receiving a signal through a microwave circuit, comprising a signal folding-back means for inputting a transmission signal of a local station to a radio frequency line of a receiver of the local station through a directional coupler, a first band-pass filter and a second band-pass filter, connected in parallel, for allowing a reception signal received from a radio transmitting/receiving apparatus of an opposite station and a transmission signal whose frequency is different from the frequency of the reception signal, and a selecting means for selecting the first band-pass filter in normal state and for selecting the second band-pass filter in radio frequency signal folding-back test state. 
     According to the present invention, a frequency shifter disposed in a radio frequency signal folding-back transmitting/receiving circuit shifts the frequency of a transmission output signal of the local station by two channels or more in the channel allocation of the transmitting/receiving apparatus so as to treat a signal received from the opposite station as an out-of-band signal. Thus, a radio frequency signal folding-back test for the local station radio transmitting/receiving apparatus can be easily performed without need to manually turn off the output signal of the transmitter of the opposite station radio transmitting/receiving apparatus. 
     In addition, an attenuating circuit is disposed after a reception signal converter rather than directly to an immediate receiver section, station branching filter  22  that most attenuates NF (noise), NF can be prevented from deteriorating. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a circuit diagram showing opposite radio transmitting/receiving apparatuses of a communication system according to a conventional reference; 
     FIGS. 2A and 2B are a circuit diagrams showing opposite radio transmitting/receiving apparatuses of the communication system according to a conventional reference; 
     FIG. 3 is a circuit diagram showing a radio transmitting/receiving apparatus of the communication system according to the present invention; and 
     FIG. 4 is a circuit diagram showing a radio transmitting/receiving apparatus of the communication system according to the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     (Structure of First Embodiment) 
     Referring now to FIG. 3, a radio frequency signal folding-back circuit according to a first embodiment of the present invention will be described. 
     A transmitting/receiving panel has ones transmission signal converter and two reception signal converters. A transmission signal converter is a transmission signal converter  18  converts a transmission intermediate frequency signal that is input to a transmission intermediate frequency signal input terminal  17  into a radio frequency signal. The two reception signal converters are a reception signal first converter  24  and a reception signal second converter  28 . The reception signal first converter  24  converts a radio frequency signal that is input from an opposite station into an intermediate frequency signal. 
     The transmitting/receiving panel also has a common first local oscillator  25 , a reception signal second local oscillator  27 , and a branching filter  22  to convert the frequency of a signal into another frequency. The common first local oscillator  25  is used in common with the transmission signal converter  18  and the reception signal first converter  24 . The reception signal second local oscillator  27  is used for the reception signal second converter  28 . The branching filter  22  prevents an unnecessary frequency signal from being transmitted. In addition, the branching filter  22  extracts a desired frequency signal from a transmitting/receiving panel radio frequency signal input/output terminal  23  connected to an antenna (not shown). 
     In addition, the transmitting/receiving panel has a transmission side directional coupler  15 , a frequency shifter  14 , and a reception side directional coupler  16  so as to perform a radio frequency signal folding-back test. The transmission side directional coupler  15 , the frequency shifter  14 , and the reception side directional coupler  16  fold back a transmission signal of the local station as a reception signal to the local station reception signal converters  24  and  28 . 
     In addition, the transmitting/receiving panel has an opposite transmission signal attenuating circuit BPF  31  that attenuates an opposite station transmission signal received as a reception signal of the local station. The opposite station transmission signal attenuating circuit BPF  31  is disposed in parallel with the reception signal second converter. Only when a radio wave folding-back test is performed, radio frequency signal switches  32  disposed on both sides of the BPF  31  cause the BPF  31  to pass a reception signal. 
     In the normal state, a transmitting portion of the local station and a receiving portion of the local station operate independently. In the transmitting portion, a transmission intermediate frequency signal that has been modulated with a transmission signal is supplied to the transmission intermediate frequency signal input terminal  17 . The transmission signal converter  18  mixes the transmission intermediate frequency signal with a local oscillation frequency signal oscillated by the common first local oscillator  25 . The resultant signal is amplified through a band-pass filter. The amplified transmission signal is output from the transmitting/receiving panel radio frequency signal input/output terminal  23  connected to an antenna system through an isolator  19 , the transmission side directional coupler  15 , and the branching filter  22 . The isolator  19  remove a disturbance signal that is input from the, antenna system. The local oscillation frequency signal oscillated by the common first local oscillator  25  is obtained by a PLL circuit that synchronizes with a reference oscillation frequency. A transmission frequency component that is inductively coupled by the transmission side directional coupler  15  is not used because the radio frequency signal folding-back operation switch is turned off. 
     On the other hand, in the receiving portion, a radio frequency reception signal that is input to the transmitting/receiving panel radio frequency signal input/output terminal  23  through the antenna system is input to the reception signal first converter  24  through the branching filter  22 , the reception side directional coupler  16 , and the isolator  19  that removes a radio frequency transmission signal that is input from the transmitting portion. The reception signal first converter  24  mixes the radio wave reception signal with a local oscillation frequency signal oscillated by the common first local oscillator  25  and obtains a reception first intermediate frequency component. The reception first intermediate frequency component is input to the reception signal second converter  28 . 
     In the reception signal second converter  28 , the reception first intermediate frequency signal is supplied to a mixer  33  through a buffer, radio frequency signal switches  32 , and a band-pass filter  26 . The radio frequency signal switches  32  selects the band-pass filter  26  or the band-pass filter  31 . The mixer  32  mixes the reception first intermediate frequency signal with the local oscillation frequency signal oscillated by the reception signal second local oscillator  27  as the PLL circuit in synchronization with the reference oscillation frequency and obtains the reception second intermediate frequency component. The reception second intermediate frequency component is supplied to an intermediate frequency amplifying circuit. The intermediate frequency amplifying circuit amplifies the reception second intermediate frequency signal. An Automatic Gain Control (AGC) circuit maintains the level of the amplified signal. An output signal of the AGC circuit is supplied to a reception intermediate frequency signal output terminal  30 . 
     When a radio frequency signal folding-back test is performed, a high frequency signal folding-back control signal  21  causes a radio frequency signal folding-back operation switch  20  to be turned on. Thus, the frequency shifter  14  shifts the frequency of the transmission signal supplied from the transmission side directional coupler  15  by for example two channels. The reception side directional coupler  16  induces the resultant signal to the reception system. Thus, the resultant signal is input to the reception signal first converter  24  through the isolator  19 . In the above-described normal state, the radio frequency signal folding-back operation switch  20  is turned off. Unless the radio frequency signal folding-back test is performed, a transmission signal of the local station is not input to the reception signal first converter  24 . 
     The reception signal first converter  24  down-converts both a transmission signal that is input to the reception signal first converter  24  and a reception signal as a transmission signal of the opposite station into first intermediate frequency signals. 
     The radio frequency signal folding-back control signal  21  causes the radio frequency signal switches  32  to connect the, reception signal first converter  24  and the BPF  31 . The center frequency of the BPF  31  is apart from the center frequency of the BPF  26  by two channels. Thus, a folded signal of the local station that passes through the BPF  31  does not attenuate. On the other hand, in the normal state rather than the folding-back test state, a folded signal of the local station that passes through the BPF  26  of the reception signal second converter  28  does not attenuate. In the folding-back test state, the BPF  31  attenuates a transmission signal of the opposite station (not shown). 
     To cause a folded reception signal whose frequency is apart by two channels from the frequency of a transmission signal of the local station to be output from the reception intermediate frequency signal output terminal  20  as a predetermined reception second intermediate frequency signal, the radio frequency signal folding-back control signal  21  causes the frequency of the reception signal second local oscillator  27  to be shifted by two channels. Thus, the mixer  33  of the reception signal second converter  28  converts a folded intermediate frequency signal that has been shifted by two channels as a transmission signal of the local station into a predetermined reception intermediate frequency signal. 
     When a radio frequency signal folding-back test is performed, a transmission signal of the local station that is input from the transmission intermediate frequency signal input terminal  17  is transmitted as a predetermined reception intermediate frequency signal from the reception intermediate frequency signal output terminal  30 . The frequency of a reception signal of the local station as a transmission signal of the opposite station is shifted by two channels from the frequency of a transmission signal of the local station. In addition, the level of the reception signal of the local station is lower than the level of the folded signal as the transmission signal of the local station. Thus, the radio frequency signal folding-back test can be performed without deterioration of transmission characteristics. 
     In the embodiment, the difference between the transmission frequency and the reception frequency is two channels. However, the difference between the transmission frequency and the reception frequency is not limited to two channels as long as frequencies are separated by the band-pass filters  26  and  31 . Particularly, in a radio frequency signal folding-back test, when a narrow-band test radio frequency signal is used for a transmission signal, with a band-pass filter that passes the transmission signal, the radio transmitting/receiving apparatus can be easily tested. However, such a structure is not applied when a wide-band transmission signal is tested. 
     In the embodiment, input/output signals of the local station radio transmitting/receiving apparatus are intermediate frequency signals. However, an original signal shown in FIG. 1 may be used. When the original signal is composed with a folded signal thereof in a radio frequency signal folding test, each circuit of the radio transmitting/receiving apparatus can be tested. 
     Referring now to FIG. 4, in a radio frequency signal folding-back test, as a means for attenuating a reception signal of the local station as a transmission signal of the opposite apparatus, a f 0  variable Band-Pass filter (BPf)  42  that varies f 0  with an electric signal is used. The above arrangement results in a simplified structure. The variable BPF  32  is composed of a radio frequency operational amplifier, which is formed with a pin diode and capacitors disposed both sides of the pin diode. By vary the resistance of the pin diode with a bias voltage, the pass band can be varied. However, the present invention is not limited to the f 0  variable BPF  42 . In other words, another variable BPF can be used. 
     Referring to FIG. 4, in the normal state, the variable BPF  42  of the reception signal second converter causes a radio frequency reception signal of the opposite transmitter to pass. In the radio frequency signal folding-back test, the variable BPF  42  causes a down-converted transmission frequency component to pass. The variable BPF  42  is composed of a radio frequency operational amplifier. By varying the resistance of the pin diode with a bias voltage, the pass band can be varied. 
     In a radio frequency signal folding-back test, the radio frequency signal folding-back circuit shown in FIG. 4 compares a transmission intermediate frequency component that is input from the transmission intermediate frequency signal input terminal  17  with the output signal of the reception intermediate frequency signal output terminal of the reception signal second converter  28  and determines whether or not there is an error, whether or not a delay time period is proper, and whether or not the output level is proper. 
     According to the present invention, a radio frequency signal folding-back test for the radio transmitting/receiving apparatus of the local station can be easily performed by the radio transmitting/receiving apparatus of the local station using a radio frequency signal folding-back control signal without need to manually turn off a transmission signal of the opposite station. 
     In addition, since a reception signal corresponding to a transmission signal of the opposite station is attenuated by a downstream circuit of the reception signal first converter, NF of the reception signal converter does not deteriorate. Even if a radio frequency signal folding-back function is added, the performance of the reception signal converter does not deteriorate. 
     Although the present invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.