Abstract:
A filter circuit extracts a desired signal in the presence of interference by using a variable gain circuit whose input is the frequency converted received signal and whose output is fed through an active filter to produce the desired signal. The signal levels before and after the active filter are detected and the higher level is used to control the gain of the variable gain circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a filter circuit suitable for extraction of a signal of the desired frequency band and particularly to a filter circuit which is useful for selection of a particular channel signal in a mobile radio system. 
     2. Description of the Related Art 
     In a hand-held transceiver, for example, a mobile radio or a portable type telephone set, etc. which is operated as a terminal of mobile communication system, an audio data is modulated by many sub-carriers frequency-divided for the high frequency (GHz) carrier frequency assigned to such radio system and moreover modulated to the channel of a particular frequency band in which each sub-carrier frequency is divided into the time slots of several tens of channels for the purpose of transmission and reception thereof. 
     In such a mobile radio system, an input receiving signal level received by a terminal changes to a large extent depending on the operating environment and it must be assumed that such input receiving signal level changes as much as 80 dB or more, for example, depending on distance between the terminal and base station and level changes caused by fading. 
     Moreover, it can also be thought that such a bad environment that the signal level of the target channel is low and there is an intensive interference wave for the adjacent channels exists. 
     Therefore, a high frequency circuit of the double-super receiving system is generally employed in the transmitting and receiving system of such mobile radio but as a filter circuit to accurately extract the target frequency, a filter using a bulk element (saw filter, piezoelectric element, etc.) is mainly used to cover the high sharpness and wide dynamic range. 
     However, since the hand-held terminal is required to be small in size and light weight, it is considered, for example, to use an active filter formed of an IC circuit as a filter circuit provided between intermediate frequency stages, etc. 
     FIG. 1 shows an example of the receiving high frequency circuit of a terminal. 
     The radio wave transmitted from the base station is received by an antenna  100 , it is then amplified by a high frequency amplifier  101  consisting of a low noise GaAs FET element, etc., for example, and this signal is converted to a first intermediate frequency through the mixing with a signal of a first local oscillator  103  being supplied to a first mixer  102 . The predetermined frequency band of this converted first intermediate frequency is extracted by a saw filter  104  and it is supplied to a second mixer  106  after it is amplified by an intermediate amplifier  105 . 
     A signal is also supplied to the second mixer circuit  106  from a second local oscillator  107  for selection of the desired channel and thereby the signal of the frequency band of the target channel can be obtained as the second intermediate frequency. 
     Therefore, an active filter circuit  109  is provided to select the signal of the second intermediate frequency band. However, since the active filter circuit  109  for selecting the channel of the target frequency band has a lower dynamic range, it is required, as explained above, to provide a gain varying amplifier  108  in the preceding stage of this active filter circuit  109  and the gain of the gain varying amplifier  108  is controlled by the signal fed back from the level detecting circuit  110  for detecting output signal level of the active filter. 
     The signal of the target frequency band extracted by the active filter circuit  109  is waveform-shaped to have a constant amplitude by a multi-stage connected limiter circuit  111  and is then input to a signal processing circuit  120  for extracting data. 
     However, for example, an active filter circuit  109  consisting of an inductance element, etc. formed by combining a resistance element and a capacitance element or active element (transistor, amplifier) is effective for reduction in size and weight of the hand-held terminal and also for reduction in power but in the IC circuit, a gm-C circuit utilizing transconductance of a pair of differential transistors is often used to realize a filter having the high precision frequency characteristic. Therefore, such filter circuit is generally limited to operate in the stable filter characteristic only for the range in which the amplifying element operates linearly. 
     In addition, when the active filter is designed to have higher sharpness and to assure lower power consumption by the low voltage drive, the following problems are generated. 
     The first problem is a dynamic range of the feedback loop by the feedback control. 
     Since the active filter circuit  109  has the frequency characteristic allowing the predetermined frequency to pass by forming inside the complicated feedback loop, this filter can generally be considered as a linear element and therefore the feedback control itself generates a problem when this circuit is provided within the loop of the feedback circuit. 
     For example, when non-linear behavior of the active filter circuit  109  does not increase monotonously, polarity of the feedback signal is inverted and normal feedback operation is not properly maintained. 
     In this case, here rise the problems, for example, that intensive signal is momentarily input and after the filter circuit  109  is deeply saturated, the intrinsic level control operation does not start. 
     Next, the second problem is effective use of dynamic range for the receiving signal. 
     As shown in FIG. 1, when the gain control is conducted with a signal used to detect an output signal level of the active filter circuit  109  by arranging the gain varying amplifier  108  to the preceding stage of the active filter circuit  109 , here rise the problems that the control level La of the desired wave of the frequency band passing through the active filter circuit  109  is different from the control level Lb of the interference wave of the frequency band attenuated by the filter circuit as shown in FIG.  2  and this level differences is uniquely determined by the attenuation characteristic of the active filter circuit  109 . 
     In general, due to characteristic difference of each node voltage forming the active filter circuit  109 , the upper limit of receiving range a of the desired wave by saturation of the desired wave itself in the passing frequency band must be set to the level different from that of the upper limit of the mixing range b of the interference wave restricted by masking the desired wave with the saturated interference wave within the rejection band. Moreover, in order to effectively use the dynamic range of the filter, the control level La of the desired wave is preferably set to the upper limit of the receiving range a of the desired wave, while the control level Lb of the interference wave is preferably set based on the upper limit of the mixing range b of the interference wave. 
     However, if a level difference between the control level La of the desired wave and the control level Lb of the interference wave is uniquely determined only by the attenuation characteristic of the active filter circuit  109 , here rises a problem that if it is attempted to control only one level desired most effectively, the other level is not always controlled to the optimum level and the control level for effectively utilizing the operation range of the filter cannot be set. 
     SUMMARY OF THE INVENTION 
     In order to solve the problems explained above, the filter circuit of the present invention is provided with: 
     filter means having different gain characteristics for the first frequency band and second frequency band; 
     gain control means for controlling an input signal including the signals of at least first and second frequency bands and supplying the level-controlled output signal to the filter means; 
     first level detecting means for detecting the level of output signal of the filter means; 
     second level detecting means for detecting the input level of the filter means; and 
     switching means for outputting the detecting signal of higher level of the detecting signals of the first or second level detecting means. 
     The gain of the gain control means is controlled by the signal output from this switching means. 
     The filter means may be structured by the controllably connected first and second filter circuits. In this case, control is conducted by detecting an output level of the first filter circuit with the first level detecting means. 
     Moreover, it is also possible to conduct gain control of the desired wave by supplying an output of the gain control means to the first filter circuit having the desired frequency characteristic and the second filter circuit having the characteristic similar to that of the first filter circuit and then detecting an output signal of the second filter circuit with the first detecting means. 
     Since the filter circuit of the present invention is designed, as explained above, to control the level of the signal including interference wave to the predetermined value with the second level detecting means to detect the signal to be input to the active filter circuit and to set the control level for the desired wave with the first level detecting means to detect the signal level of the desired wave output from the active filter circuit, the stable level control can be realized even when the filter circuit has the non-linear characteristic and the dynamic range can be used effectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an example for use of active filter by setting an adequate dynamic range. 
     FIG. 2 is a diagram for explaining the signal level input to the active filter circuit. 
     FIG. 3 is a block diagram showing an example of the filter circuit of the present invention. 
     FIG.  4 A and FIG. 4B are diagrams for explaining the characteristic of the clip circuit and signal level of each input frequency band. 
     FIG. 5 is a block diagram of a filter circuit of another embodiment of the present invention. 
     FIG. 6 is a block diagram of a filter circuit of another embodiment of the present invention. 
     FIG. 7 is a circuit example for realizing a gain control system with an analog circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the filter circuit of the present invention will be explained with reference to FIG.  3 . 
     In this FIG., the antenna, high frequency amplifier, and first and second frequency converters are designated by the reference numerals like those in FIG.  1 . 
     A signal of the desired frequency band extracted by the filter circuit and a signal of the frequency band of interference wave are input to the gain variable circuit  11  from the terminal  10 , supplied to the active filter circuit  12  structured by active element and the output signal from the terminal  13  of the active filter circuit  12  is waveform-shaped by the limiter circuit and is then supplied to the demodulating circuit for signal data. 
       14   a  designates a first coefficient circuit for multiplying the predetermined coefficient to the output signal level of the active filter circuit  12 ;  14   b  designates a second coefficient circuit for multiplying the predetermined coefficient to the input signal level of the active filter circuit  12 ;  15   a ,  15   b  designate respectively a first and a second level detecting circuit for detecting output levels of the first and second coefficient circuits  14   a ,  14   b.    
     Moreover,  16  designates a switch circuit for relatively detecting and outputting the signal of the higher level of the detected signals of the first and second level detecting circuits  15   a  and  15   b . For example, this switch circuit may be structured by an analog circuit which preferentially outputs the higher level signal utilizing, for example, the differential circuit. 
     An output of the switch circuit  16  is input to a comparator (differential amplifier)  17  for comparison with an output of the reference signal source  18 . An output controls the gain of the gain variable circuit  11  via a clip circuit  19 . 
     The clip circuit  19  used has the input/output characteristic, for example, as shown in FIG.  4 A and can adjust degree of gain control by adjusting the output characteristic. 
     Since the present invention has a structure explained above, when an interference signal element Sb in the level higher than the signal element Sa of the desired signal in the selected channel exists in the adjacent channels of the input signal, for example, as shown in FIG. 4B, the level of the interference signal having passed the gain variable circuit  11  is adequately converted by the coefficient circuit  14   b  and it is then detected by the second level detecting circuit  15   b.    
     In the switch circuit  16 , the signal from a level detector  15   b  is selected and output to control the gain of a gain variable circuit  11  via the comparator  17  and clip circuit  19 . As a result, the gain control is conducted first so that the input signal level including the interference wave is output under the controlled condition. 
     Namely, the interference wave of the frequency band to be rejected by the active filter circuit  12  is level-controlled by the gain variable circuit  11  so that the active filter circuit  12  is never saturated. 
     As explained above, when the level of desired frequency signal being input simultaneously is lower than the interference wave level, the detected signal of the interference wave element Sb output from the second level detecting circuit  15   b  controls first the gain to suppress the input level of interference wave element Sb. After the level of the interference wave is suppressed, the level of the desired wave signal element Sa with less attenuation amount through the pass-band area from the output side of the active filter circuit  12  is superior to the level of the interference wave and thereby an output of the first level detecting circuit  15   a  controls the gain of the gain variable circuit  11 . 
     As explained, the filter circuit of the present invention is provided with the function to suppress the interference wave in the input side of the active filter circuit  12  even when an intensive interference wave of several tens of kHz, for example, exists in the adjacent channels together with the desired wave. Therefore, even when a signal having a wide dynamic range which does not monotonously increase the gain characteristic of the active filter circuit  12  is input, the feedback circuit no longer becomes unstable and the active filter circuit  12  can be prevented to enter the lock condition which disables to realize its functions and to become unstable because it is saturated, even under the bad condition that the intensive interference wave is input simultaneously during the reception of signal. 
     Moreover, as shown in FIG. 2, the control range b of the frequency band of the interference wave and the control level range a of the desired wave can be uniquely determined by adjusting input/output characteristics of the clip circuit  19  which can control the coefficient values of the coefficient circuits  14   a ,  14   b  before the level detection and the gain of the control loop as a whole. 
     FIG. 5 shows another embodiment of the present invention and the elements like those in FIG. 3 are designated by the like reference numerals. 
     In this embodiment, the active filter circuit  12  is designed as a multi-stage filter circuit by controllably-connecting two or more filter circuits  12 P,  12 Q. 
     The first level detecting circuit  15   a  for adjusting the gain of the desired wave detects an output of one filter of the multi-stage connected intermediate filter circuit, for example,  12 P. 
     The circuit of this embodiment includes the active filter circuit  12  which is formed of the multi-stage circuit and is formed of the filter providing a complicated feedback loop and many stages of filter. Accordingly, the feedback control resulting in less amount of delay can be structured and assures improvement of stability by obtaining the feedback signal from the filter circuit of the preceding stage having a comparatively linear input/output characteristic and a wide dynamic range to conduct the gain control for the desired wave. 
     FIG. 6 shows another embodiment of the present invention which conducts the gain control in the same manner as FIG. 3, except for the structure that the active filter circuit  12  is formed by connecting in parallel two filter circuits having almost the same characteristic. 
     In this case, the active filter circuit  12 S is formed of a filter having the desired frequency characteristic, while the filter circuit  12 R is formed of a dummy circuit similar to the filter circuit  12 S. 
     The dummy circuit is formed of a circuit which is similar in characteristic to the filter characteristic for selecting the desired signal wave and can easily be designed and the feedback circuit is designed to operate in a stable condition considering phase margin and gain margin, etc. 
     In this case, since the actual signal path is different from the control signal path, the second filter circuit  12 R can be used by adjusting level to the signal level having sufficient S/N. 
     FIG. 7 shows a circuit example in the case of forming the control circuit to conduct the gain control of the active filter circuit with an analog circuit. 
     In this FIG., the first level detecting section DA 1  is formed of a first coefficient device for detecting an output signal In 1  of the active filter, transistors Q 1 , Q 2  and output transistors Q 3 , Q 4  in which the first level detector forms a differential circuit. 
     Moreover, the second level detecting section DA 2  is formed of a second coefficient device for detecting an input signal In 2  of the active filter circuit, transistors Q 5 , Q 6  and output transistors Q 7 , Q 8  in which the second level detector forms a differential circuit. The differential input is applied to these circuits but unbalanced input can also be applied. 
     A ratio R 1 /R 2  of the resistors R 1  and R 2  and a ratio R 3 /R 4  of resistors R 3  and R 4  respectively set the first and second coefficient values and these resistance values can determine the control level of the interference wave and that of the desired wave. 
     The signal of the desired level multiplied by a coefficient by the output transistors Q 3 , Q 4  and Q 7 , Q 8  is detected as the positive polarity signal and the currents I 1  and I 2  output from the output transistors of these level detecting sections DA 1 , DA 2  are connected in common and are then absorbed by a constant current source IO. 
     A part of the higher level signal current supplied from the output transistors Q 3 , Q 4  and Q 7 , Q 8  is supplied to the differential type transistor Q 9  forming the next integral circuit section DA 3 , and it is then compared with the reference voltage Vref 1  supplied to the other transistor Q 19 . Thereby, a difference of these voltages charges a capacitor C. 
     The capacitor C receives a current from the transistors Q 11 , Q 12  forming a current Miller circuit when a signal higher than the predetermined reference voltage Vref 1  is input and raises its potential. 
     The DC element signal charged in the capacitor C is further compared with the reference voltage Vref 2  in the differential transistors Q 13 , Q 14  forming a current voltage converting section DA 4  and the current corresponding to the difference signal element controls the gain of the gain variable circuit  11  so that the input signal level is set to the predetermined level. 
     Here, when the current sources I 1 , I 2  provided in the current voltage converting section DA 4  are set to the identical values, the control is performed so that an output current becomes equal to a constant value I 1  if the input signal voltage Vc of capacitor C is lower than the reference voltage Vref 2  and a current of I 1 -I 2  is output if the input signal voltage Vc of capacitor C becomes high. Namely, the adjustment is conducted in matching with the control characteristic of the gain variable circuit  11  of which the gain is controlled by a current. 
     The circuit corresponding to the switch circuit is formed by connecting in common the emitter to the constant current source I 0  and a larger one of the full-wave rectifying output of transistors Q 3 , Q 4  and the full-wave rectifying output of transistors Q 7 , Q 8  gives an inverse bias to the other detecting circuit and the larger rectifying output is preferentially input to the next integral circuit section DA 3 . 
     As explained above, since the filter circuit of the present invention provides a gain variable circuit in the former stage and the gain of this gain variable circuit is controlled by the input side signal level and output side signal level of the active filter circuit, there is provided the effect that even if there is a large difference between the frequency band passing the active filter and the signal element of the rejected frequency band, the desired control level can be given to each frequency band and even if the active filter circuit has the non-linear characteristic, the desired frequency can be extracted stably. 
     Moreover, even when the active filter circuit has the characteristic which does not increase monotonously owing to the complicated feedback circuit, the function to select the frequency depending on the filter characteristic can be maintained and particularly the practical effect can be attained at the time of using the active filter circuit of the present invention to a mobile radio system which is required to realize reduction in size and weight.