Analog filter circuit and adjustment method thereof

Abstract

An analog filter circuit in which filter characteristic deviation can be adjusted with simple circuitry and its adjustment method can be provided. The analog filter circuit includes a low pass filter and a high pass filter and output signals of both filters are input to a comparison and adjustment section from which an adjustment signal is fed back to the low pass and high pass filters and also input to a predetermined-band pass filter having predetermined correlation to the low pass and high pass filters. The low pass and high pass filters are tuned so that the frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while that line of the high pass filter linearly rises, both the lines crossing at a reference frequency. When filter characteristic deviation occurs, it is adjusted to compensate gain difference between the output signals produced by filtering the reference signal with a reference frequency and, accordingly, the predetermined-band pass filter is adjusted to its desired filter characteristic.

1 . An analog filter circuit, comprising: 
 a predetermined-band pass filter;    a low pass filter and a high pass filter having predetermined correlation to the predetermined-band pass filter in filter characteristics and characterized in that the frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while that line of the high pass filter linearly rises, both the lines crossing at a reference frequency; and    a comparison and adjustment section which compares output signals from the low pass filter and the high pass filter filtering an input signal with the reference frequency, when gain difference between the output signals is detected, feeds back a signal to the low pass filter and the high pass filter to shift back the crossing point of the characteristic lines toward a direction to compensate the gain difference, and, accordingly, adjusts the predetermined-band pass filter characteristic.    
   
   
       2 . The analog filter circuit according to  claim 1 , wherein, in each of the predetermined-band pass filter, the low pass filter, and the high pass filter, at least one of component elements determining a filter characteristic has the same structure and characteristics as corresponding elements in other filters, the filter characteristic depends on a component element's parameter value, equally among the filters, and the component element's parameter value is variably adjusted by the comparison and adjustment section.  
   
   
       3 . The analog filter circuit according to  claim 2 , wherein the predetermined-band pass filter, the low pass filter, and the high pass filter are passive filters, each including a variable capacitor element having a variable capacitance value as the component element's parameter value.  
   
   
       4 . The analog filter circuit according to  claim 2 , wherein the predetermined-band pass filter, the low pass filter, and the high pass filter are active filters, each including a variable capacitor element having a variable capacitance value as the component element's parameter value and/or a trans-conductance amplifier having a variable mutual conductance as the component element's parameter value.  
   
   
       5 . The analog filter circuit according to  claim 1 , wherein the comparison and adjustment section comprises: 
 DC converters to smooth output signals from the low pass filter and the high pass filter; and    a comparator which outputs an adjustment signal, based on difference between the levels of the output signals smoothed.    
   
   
       6 . The analog filter circuit according to  claim 5 , wherein, in each of the predetermined-band pass filter, the low pass filter, and the high pass filter, at least one of component elements determining a filter characteristic has the same structure and characteristics as corresponding elements in other filters and has a predetermined parameter ratio and a component element's parameter value is variably adjusted, depending on the adjustment signal.  
   
   
       7 . The analog filter circuit according to  claim 6 , 
 wherein the component element's parameter value which is variably adjusted is a capacitance value and/or a mutual conductance value, and    wherein the component element whose parameter value is adjusted, depending on the adjustment signal, is a variable capacitor element and/or a trans-conductance amplifier.    
   
   
       8 . A method for adjusting an analog filter circuit comprising a predetermined-band pass filter and, a low pass filter and a high pass filter having predetermined correlation to the predetermined-band pass filter in filter characteristics and in which the frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while that line of the high pass filter linearly rises, both the lines crossing at a reference frequency, the method comprising the steps of: 
 inputting an input signal with the reference frequency to the low pass filter and the high pass filter;    comparing output signals from the low pass filter and the high pass filter;    feeding back a signal to the low pass filter and the high pass filter to shift back the crossing point of the characteristic lines toward a direction to compensate gain difference; and    adjusting the predetermined-band pass filter in accordance with the feeding back.