Abstract:
A signal strength detecting device of a communication system is disclosed. The signal strength detecting device is coupled to a frequency down mixer of the communication system and the frequency down mixer is used for receiving and converting a first signal to a second signal whose frequencies are lower than frequencies of the first signal. The signal strength detecting device comprises a frequency up converter for receiving and converting the second signal to a third signal whose frequencies are higher than the frequencies of the second signal and a detecting unit for detecting strength of the third signal and generating a signal strength indicator to the communication system according to a detecting result corresponding to the strength of the third signal, wherein the signal strength indicator represents the strength of the first signal received by the frequency down mixer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a signal detecting device and related method, more particularly, to a device of a communication system for detecting signal strength and related method. 
         [0003]    2. Description of the Prior Art 
         [0004]    Generally, the automatic gain control (AGC) has been used in a radio frequency (RF) receiver to adjust strength of received signals at the analog front-end so that the received signals do not suffer much distortion at a stage of converting into back-end digital signals. Before adjusting gains, the receiver must measure the strength of received signals in order to get the appropriate range for signal adjustment. 
         [0005]    The RF receiver normally includes one or several amplifiers with variable gains and analog to digital converters with a range of linear transformation. The RF receiver adjusts amplifier gains according to strength of detected signals to maintain the signals received by analog to digital converters within the range of the transformation to avoid distortion. 
         [0006]    The typical method of signal strength detection is to detect strength of signals whose frequencies have been downed to an intermediate frequency band or near baseband frequency band and then to adjust corresponding amplifier gains according to the detected strengths. U.S. Pat. No. 7,212,798 discloses, in FIG. 2, a signal strength detecting device detecting strengths of the received signals at three stages from the intermediate to low frequency stages in the signal receiving path, i.e. the analog RSSI detectors 227 and 217 and the digital measuring logic circuit 231. However, detecting signal strength at the low frequency (e.g. 200 KHz) requires long signal settling time and ripple effect is more severe in the low frequency signals. In addition, the signal strength detecting device usually demands capacitors with larger capacitance to reduce ripples, thereby occupying larger circuit area. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore an objective of the present invention to provide a signal strength detecting device and related methods in order to increase efficiency and accuracy of strength detection and also to reduce the circuit area. 
         [0008]    The present invention mainly decreases frequencies of a received signal of a communication system, raises the frequencies and then executes the strength detection for the received signal. 
         [0009]    The present invention discloses a signal strength detecting device of a communication system. The signal strength detecting device is coupled to a frequency down mixer of the communication system is disclosed. The frequency down mixer is used for receiving and converting a first signal to a second signal whose frequencies are lower than frequencies of the first signal. The signal strength detecting device comprises a frequency up converter for receiving and converting the second signal to a third signal whose frequencies are higher than the frequencies of the second signal and a detecting unit for detecting strength of the third signal and generating a signal strength indicator to the communication system according to a detecting result corresponding to the strength of the third signal, wherein the signal strength indicator represents the strength of the first signal received by the frequency down mixer. 
         [0010]    The present invention further discloses a signal strength detecting method for a signal strength detecting device of a communication system. The signal strength detecting device is coupled to a frequency down mixer for receiving and converting a first signal to a second signal whose frequencies are lower than frequencies of the first signal. The signal strength detection method comprises receiving and converting the second signal to a third signal whose frequencies are higher than frequencies of the second signal,
       detecting strength of the third signal and generating a signal strength indicator to the communication system according to a detecting result corresponding to the strength of the third signal, wherein the signal strength indicator represents strength of the first signal received by the frequency down mixer.       
 
         [0012]    The present invention further discloses a communication system comprising a signal reception circuit and a signal transmission circuit. The communication system comprises a frequency down mixer installed in the signal transmission circuit, for receiving and converting a first signal to a second signal whose frequencies are lower than frequencies of the first signal and a frequency up mixer installed in the signal transmission circuit and coupled to the frequency down mixer, for receiving and converting the second signal to a third signal whose frequencies are higher than frequencies of the second signal when the communication system does not transmit any signal and a detecting unit coupled to the frequency up mixer, for detecting strength of the third signal and generating a signal strength indicator according to a detecting result corresponding to the strength of the third signal, wherein the signal strength indicator represents a strength of the first signal received from the frequency down mixer. 
         [0013]    The present invention further discloses a signal strength detecting method for a communication system. The signal strength detecting method comprises receiving and converting a first signal to a second signal whose frequencies are lower than frequencies of the first signal, in a signal reception circuit of the communication system, receiving and converting the second signal to a third signal whose frequencies are higher than frequencies of the second signal when the communication system does not transmit any signal, in a signal transmission circuit of the communication system, detecting strength of the third signal and generating a signal strength indicator according to a detecting result corresponding to the strength of the third signal, wherein the signal strength indicator represents strength of the first signal received by a frequency down mixer of the signal transmission circuit. 
         [0014]    The present invention discloses a signal strength detecting device of a communication system. The communication system comprises a signal reception circuit including a frequency down mixer and a signal transmission circuit including a frequency up mixer. The signal strength detecting device is coupled to the frequency down mixer for receiving and converting a first signal to a second signal whose frequencies are lower than frequencies of the first signal. The signal strength detecting device comprises a frequency up mixer for receiving and converting the second signal to a third signal whose frequencies are higher than frequencies of the second signal and a detecting unit for detecting strength of the third signal and generating a signal strength indicator to the communication system according a detecting result corresponding to the strength of the third signal, wherein the strength of the third signal represents strength of the first signal received by the frequency down mixer. 
         [0015]    The present invention discloses the signal strength detecting device applicable for both time division multiple access (TDMA) and frequency division multiple access (FDMA) reception system. When the TDMA system is adopted in the signal strength detecting device, the signal strength detecting device can perform the strength detection of the received signal without extra circuit by using existed circuit of the receiver and the transmitter. 
         [0016]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a schematic diagram of a RF transceiver according to an example. 
           [0018]      FIG. 2  is a schematic diagram of detecting signal strength according to an example. 
           [0019]      FIG. 3  is a flowchart of processes of the signal strength detection according to examples. 
           [0020]      FIG. 4  is a schematic diagram of a receiver of the concept of the signal strength detection. 
           [0021]      FIG. 5  is a schematic diagram of a peak value detecting device of the detection unit according to  FIG. 4 . 
           [0022]      FIG. 6  is a schematic diagram of a RF transceiver according to an example. 
           [0023]      FIG. 7  is a schematic diagram of a receiver according to an example. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Please refer to  FIG. 1 , which illustrates a schematic diagram of a radio frequency (RF) transceiver  10  adopting the concept of the signal strength detection according to an example of the present invention. The RF transceiver  10  comprises an antenna  12 , a band pass filter  13 , a switch  14 , a receiver  15 , a transmitter  16 , a detection unit  17  and an adjusting unit  19  and is applicable for a time division duplex (TDD) communication system. The RF transceiver  10  utilizes the switch  14  to receive signals by the receiver  15  or transmit signals by the transmitter  16  to the antenna  12 . The band pass filter  13  filters a band pass signal S B  (the signal received by the RF transceiver  10 ) from radio signals captured by the antenna  12 , wherein the central frequency of the received signal maybe the carrier frequency (central frequency) f c  of the received signal. The receiver  15  comprises a Balun converter  100 , a low noise amplifier (LNA)  102 , a reception path of in-phase signals formed by a frequency mixer  110 , an intermediate frequency filter  112 , an intermediate frequency amplifier  114  and an analog to digital converter (ADC)  116  and a reception path of quadrature signals formed by a frequency mixer  120 , an intermediate frequency filter  122 , an intermediate frequency amplifier  124  and an ADC  126 . The Balun converter  100 , in short, is used to connect a balanced conducting wire at the receiver  15  side and an unbalanced coaxial cable at the antenna  12  side to reach impedance match between the receiver  15  and the antenna  12  sides and to increase the antenna bandwidth. The function of Balun  100  is a well-known in the art, so the detailed description is omitted herein. The LNA  102  is a variable gain amplifier (VGA) for adjusting the band pass signal S B . In the reception path of in-phase signals (hereinafter in-phase path) of the receiver  15 , the frequency mixers  110  and  120  are used as frequency down converter for converting the frequency of the band pass signal SB down to an intermediate frequency signal S IF  (not shown in  FIG. 1 ) divided into an in-phase intermediate frequency signal S IFI  and a quadrature intermediate frequency signal S IFQ . The frequency mixer  110  in the in-phase path is used for converting the frequency of band pass signal S B  down to the baseband or near baseband frequency of the in-phase intermediate frequency signal S IFI . After the in-phase intermediate frequency signal S IFI  is filtered by the intermediate frequency filter  112  and the signal amplitude thereof is adjusted by the intermediate frequency amplifier  114 , the ADC  116  converts the in-phase intermediate frequency signal S IFI  to the digital signal and further the in-phase intermediate frequency signal S IFI  is demodulated by a backend baseband circuit (not shown in  FIG. 1 ). Since the operation of the quadrature signal path in the receiver  15  is similar with the abovementioned operation of the in-phase signal path, the related operation of the quadrature signal path is not given herein. 
         [0025]    In addition, the transmitter  16  comprises a Balun converter  150 , a power amplifier  152 , an adder  154 , a transmission path of in-phase signals formed by a frequency mixer  160 , a low pass filter  162 , a VGA  164  and an ADC  166 , and a transmission path of quadrature signals formed by a frequency mixer  170 , a low pass filter  172 , a VGA  174  and an ADC  176 . Since the operation principle of the transmitter  16  is the inverse operation of the receiver  15 , please refer above mention for details. 
         [0026]    The main concept of the present invention is to execute signal strength detection in the high frequency band, achieved by the operation of the RF transceiver  10  described below. The high frequency band can be regarded as the frequency band far from the base band, near the RF frequency. 
         [0027]    The RF transceiver  10  can execute the auto gain control (AGC) for the received signal (e.g. S B ). The AGC adjusts amplifier gains of the receiver  15  according to the detecting signal strength so that the strength of the received signal can be well-situated when the received signal is received at the ADCs  116  and  126 . In this situation, the ADCs  116  and  126  can correctly produce a digital received signal based on the received signal. In the RF transceiver  10 , the strength detection utilizes the frequency mixers  110  and  120  in the receiver  15  to convert down the frequency of the band pass signal S B , and then utilizes the frequency mixers  160  and  170  in the transmitter  16  to convert up the frequency of the band pass signal S B  whose frequencies have been converted down. Finally, the detection unit  17  detects the strength of the in-phase baseband frequency signal S IFI  and the quadrature baseband frequency signal S IFQ  (i.e. a high frequency signal HFS), whose frequencies have been converted up. The frequency mixers  160  and  170  are used for frequency up converting of the strength detection when the transmitter  16  does not transmit signals. According to the strength detecting result, the detection unit  17  generates a signal strength indicator RSSI represented as the strength of the band pass signal S B  received by the frequency mixers  110  and  120 . The adjusting unit  19  has AGC function for adjusting gains of at least one of the LNA  102 , and the intermediate frequency amplifiers  114  and  124  according to the signal strength indicator RSSI, so that the strength of the band pass signal S B  received by frequency mixers  110  and  120  is adjusted. 
         [0028]    In order to illustrate the concept of the present invention more in details, please refer  FIG. 2 , which illustrates a schematic diagram of detecting signal strength according to an example of the present invention. After the LNA  102  adjusts the amplitude of the band pass signal S B , the frequency mixers  110  and  120  transform the band pass signal S B  to the intermediate frequency signal S IF  with lower frequency. The intermediate frequency signal S IF  can be divided into an in-phase intermediate frequency signal S IFI  and a quadrature intermediate frequency signal S IFQ . The frequency of the band pass signal S B  is usually higher far from the frequency of the intermediate frequency signal S IF . More, the frequency mixers  110  and  120  can have the single-sideband modulation (SBB) function. Then, the frequency mixers  160  and  170  transform the in-phase intermediate frequency signal S IFI  and the quadrature intermediate frequency signal S IFQ  to an in-phase high frequency signal HFS_I and a quadrature high frequency signal HFS_Q. Finally the in-phase high frequency signal HFS_I is combined with the quadrature high frequency signal HFS_Q by the adder  154  to generate a high frequency signal HFS whose central frequency is near but different from the frequency of the band pass signal S B . In other words, the frequency of the high frequency signal HFS is usually much higher than the frequency of the intermediate frequency signal S IF . The detection unit  17  detects the strength of the high frequency signal HFS and thereby generates the signal strength indicator RSSI which is the base of the gain adjustment for an amplifier. 
         [0029]    From the above, the RF transceiver  10  converts down and then up the frequency of the band pass signal S B  to execute the strength detection of the band pass signal S B  at the high frequency band. Hence, the settling time of the detected signals becomes shorter and the detected signal suffers weaker ripple effect. Besides, the RF transceiver  10  performs frequency down converting in the receiver  15  and frequency up converting in the transmitter  16 . This shows that, in the TDMA system, an existed frequency down mixers of the receiver and a frequency up mixers of the transmitter can be employed to execute the signal strength detection. Hence, this circuit sharing mechanism between the receiver and the transmitter can eliminate use of extra frequency up mixers in the receiver to save the cost. 
         [0030]    In addition, take an example based on  FIG. 2 . Assume that the frequency of the band pass signal S B  is 2450 MHz and the frequency mixers  110  and  120  transform the band pass signal S B  with a 2450 MHz central frequency f c  (may be generated by the local oscillator not shown in  FIG. 2 ) and filter out a 4902 MHz high frequency mirror signal via SBB to obtain a 2 MHz low frequency part which is the in-phase intermediate frequency signal S IFI  and the quadrature intermediate frequency signal S IFQ . Afterward, the frequency mixers  160  and  170  transform the in-phase intermediate frequency signal S IFI  and the quadrature intermediate frequency signal S IFQ  with the same central frequency f c . Then, the adder  154  combines the in-phase intermediate frequency signal S IFI  with the quadrature intermediate frequency signal S IFQ  to generate a 2448 MHz high frequency signal HFS. Besides, as can be seen from above, by arranging frequencies used in the frequency mixers, the frequencies of the high frequency signal HFS and the band pass signal S B  can be staggered, thereby avoiding affecting the received band pass signal S B . 
         [0031]    Please refer to  FIG. 3 , which is a flowchart of a signal strength detecting process  30  according to an example. The signal strength detecting process  30  maybe used for realizing the strength detection of the band pass signal S B  in the RF transceiver  10  and includes the following steps: 
         [0032]    Step  300 : Start. 
         [0033]    Step  310 : Convert down the frequency of the band pass signal S B  to the frequency of the intermediate frequency signal S IF  including the in-phase intermediate frequency signal S IFI  and the quadrature intermediate frequency signal S IFQ . 
         [0034]    Step  320 : Convert up the frequency of the in-phase intermediate frequency S IFI  and the quadrature intermediate frequency S IFQ  to the in-phase high frequency signal HFS_I and the quadrature high frequency signal HFS_Q, respectively. 
         [0035]    Step  330 : Combine the in-phase high frequency signal HFS_I with the quadrature high frequency signal HFS_Q to generate the high frequency signal HFS. 
         [0036]    Step  340 : Detect the strength of the high frequency signal HFS. 
         [0037]    Step  350 : Generate the signal strength indicator RSSI according to the detected strength. 
         [0038]    Step  360 : End. 
         [0039]    According to the signal strength detecting process  30 , the frequency of band pass signal S B  is converted down by the frequency mixers  110  and  120 , and then converted up by the frequency mixers  160  and  170 , so as to generate the high frequency signal HFS with the frequency near but different from the frequency of the band pass signal S B . Finally, the corresponding signal strength indicator RSSI is generated according to the detected strength of the high frequency signal HFS. Thus, the strength of the band pass signal S B  received by the frequency mixers  110  and  120  is adjusted to achieve the auto gain control of the receiver  15 . 
         [0040]    Please refer to  FIG. 4 , which illustrates a schematic diagram of a receiver  40  adopting the concept of the signal strength detection according to an example of the present invention. The receiver  40  is applicable for both the TDMA and FDMA systems and comprises an antenna  42 , a band pass filter  43 , a LNA  402 , frequency up mixers  460  and  470 , a detection unit  475 , and a demodulation unit  480 . In the receiver  40 , a reception path of in-phase signals is formed by a frequency down mixer  410 , an intermediate frequency filter  412 , an intermediate frequency amplifier  414  and an ADC  416 , and a reception path of quadrature signals is formed by a frequency down mixer  420 , an intermediate frequency filter  422 , an intermediate amplifier  424  and an ADC  426 . In the receiver  40 , after the antenna  42  receives radio signals, the radio signals are filtered by the band pass filter  43  to generate a band pass signal S B1 . The frequency of the band pass signal S B1  is converted down by the frequency down mixers  410  and  420  and then converted up by the frequency up mixers  460  and  470  to generate a corresponding high frequency signal HFS 1  (not shown in  FIG. 4 ) whose in-phase part is HFS 1 _I and whose quadrature part is HFS 1 _Q. Finally, the strength of the high frequency signal HFS 1  is detected by the detection unit  47 . The abovementioned operation principle of frequency down and up converting is similar with the RF transceiver  10  in  FIG. 1 , and therefore, please refer former description for details and it is not given herein. The detection unit  475  is used to detect the peak voltage value of the signal whose frequencies are converted up by the frequency up mixers  460  and  470  and to generate a signal strength indicator RSSI_ 1  according to the detected peak voltage value. In addition, the demodulation unit  480  with the auto gain control function is used to adjust gains for at least one of the LNA  402 , and the intermediate frequency amplifiers  414  and  424  according to the signal strength indicator RSSI_ 1 . Therefore, the strength of the band pass signal S B1  received at the frequency down mixers  410  and  420  can be adjusted continuously to avoid errors in the analog to digital transformation. From the above, an independent set of the frequency up mixers  460  and  470  is employed by the receiver  40 , thereby allowing frequency down/up converting and SBB for the received signals can be executed in the receiver  40 . Therefore, the receiver  40  is applicable for the TDMA or FDMA systems. 
         [0041]    Please refer to  FIG. 5 , which illustrates a schematic diagram of a peak detecting circuit  50  for detecting the peak value for the detection unit  475  according to an example. The peak detecting circuit  50  comprises an operation amplifier  472 , a transistor  474 , a current source  476 , a resistor R and a capacitor C and receives an input signal SIN by a ┌+┘ terminal of the operation amplifier  472 , and the transistor  474  conducts and the capacitor C starts to be charged when the voltage on the ┌+┘ terminal is larger than the voltage on a ┌−┘ terminal of the operation amplifier  472  by over a threshold value. Afterwards, the voltage on the ┌−┘ terminal pulls high with voltage rising of an output voltage VOUT to turn off the transistor  474 . After the transistor  474  is turned off, the capacitor C is discharged to lower the output voltage VOUT. When the voltage on the ┌−┘ terminal goes lower to a certain degree, the transistor  474  is conducted again. In order to execute the signal strength detection smoothly, the output voltage VOUT needs to be maintained at a peak value of the detected signal. In this situation, if the input signal SIN is a high frequency signal, the peak detecting circuit  50  can employ circuit with a smaller RC constant. On the contrary, if the input signal SIN is a low frequency signal, the peak detecting circuit  50  requires circuit with a larger RC constant. In the example of the present invention, the input signal SIN is the high frequency signal HFS 1  in  FIG. 4 . Therefore, the peak detecting circuit  50  can employ capacitors with smaller capacitance and less area occupancy. 
         [0042]    Please refer to  FIG. 6 , which illustrates a schematic diagram of a RF transceiver  60  according to a second example. The RF transceiver  60  is made based on modification to the RF transceiver  10 , and therefore, the same components in the RF transceiver  60  as the components in the RF transceiver  10  are marked with the same symbols. In the RF transceiver  60 , in advance, the band pass signal SB is inputted to a detection unit  617  which has the same ability as the detection unit  17  in  FIG. 1  and further can determine if the strength detection of the present invention and the follow-up amplifier gain adjustment has to be performed for the band pass signal SB, according to the detected strength of the band pass signal S B . Steps of frequency down/up converting of the band pass signal S B  can be executed when the detection unit  617  determines that the band pass signal S B  causes error in A/D transformation of the ADCs  116  and  126 . 
         [0043]    Similarly, please refer to  FIG. 7 , which illustrates a schematic diagram of a receiver  70  according to a second example. The receiver  70  is made based on modification to the receiver  40 , and therefore, the same components in receiver  70  as the components in the receiver  40  are marked with the same symbols. In the receiver  70 , in advance, the band pass signal S B1  whose frequencies have not been converted down is inputted to a peak value detector (e.g. the peak value detector  470 ) of a detection unit  475  which sends a peak value detecting result to a demodulation unit  780 . The demodulation unit  780  has the same ability as the demodulation unit  480  and further can determine if the strength detection of the present invention and the follow-up amplifier gain adjustment has to be performed for the band pass signal S B , according to the peak value detecting result. 
         [0044]    In conclusion, the present invention executes strength detection after converting the signal frequency up to a high frequency band to shorten signal stable time and weak ripple effect So that the efficiency and accuracy of the strength detection can be enhanced. The concept of the present invention is applicable for both the TDMA and FDMA systems. In the TDMA system, the concept of the present invention can employ existed frequency mixers of the receiver and the transmitter to convert up the frequency of the received signal whose frequencies have been converted down (e.g. the band pass signal SB whose frequencies have been converted down) during the time the transmitter does not transmit any signal. Moreover, the example of the present invention can avoid interference between the converted-up signals and the original received signals by appropriately choosing reference frequencies of the frequency mixers (e.g. the central frequency f c ) for frequency down/up converting. Besides, the signal strength detecting circuit (e.g. the peak value detector  470  in  FIG. 5 ) can employ capacitors with smaller capacitance to reduce the occupied area. 
         [0045]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.