Abstract:
A bandpass filter which has a tunable passband frequency and independently controllable Q and passband gain, where the filter employs a separate passband gain control which is summed with a gain loop controlling the Q of the filter.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to bandpass filters, and more particularly relates to passband frequency tunable bandpass filters with independent control of the passband tune frequency, Q and gain. 
   BACKGROUND OF THE INVENTION 
   In past years, tunable bandpass filters have been available which provide high Qs. Typically, these filters will have a large gain associated with maintaining the high Q. 
   While these filters have been widely understood, their application is at least somewhat limited. For example, in certain tunable bandpass filters, it has been necessary to attenuate the incoming signal because of the high gain that is normally associated with maintaining high Q. This attenuation of the incoming signal creates problems of its own, such as a less desirable signal to noise ratio. 
   Consequently, there exists a need for improved methods and systems for tuning a variable high Q bandpass filter while maintaining desirable gain levels and S/N ratios in an efficient manner. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a system and method for tuning a frequency of a bandpass filter with independently variable Q and gain. 
   It is a feature of the present invention to utilize separate and independent controls for the Q of the filter and for gain in the filter passband. 
   It is an advantage of the present invention to achieve improved performance with tunable bandpass filters. 
   It is another advantage of the present invention to achieve tunability of the filter without the need for changing more than one transfer function. 
   The present invention is an apparatus and method for controlling tunable bandpass filters, designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features, and achieve the already articulated advantages. The present invention is carried out in an “unwanted passband gain-less” manner in a sense that the passband gain can be independently controlled so as not to be linked to a variable Q of the bandpass filter. 
   Accordingly, the present invention is a tunable bandpass filter system and method including independent controls for passband gain, Q, and passband frequency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention, in conjunction with the appended drawings wherein: 
       FIG. 1  is a block diagram view of a filter topology of a filter system of the present invention. 
   

   DETAILED DESCRIPTION 
   Now referring to  FIG. 1 , there is shown a passband frequency tunable bandpass filter system of the present invention generally designated  100 , which shows three main transfer function blocks  106 ,  108  and  110 . 
   Passband frequency tuning block  108  is a frequency tunable RC all pass structure which preferably uses an R control input with 10 bits to tune the resistance R and a C control input with 8 bits to tune the capacitance C. The controls for R and C, respectively, control B in the transfer function:
 
(S−B)/(S+B)
 
   In a preferred embodiment of the present invention, block  108  provides the ability to have a bandpass filter with a passband which is tunable across a wide range of frequencies, such as from 2 MHz to 450 MHz. 
   Loop Kb sets the Q of the filter  100 . Loop Kb consists of a gain summation block  104 , transfer function block  106  (S−A)/(S+A), gain summation block  112 , and passband frequency tuning block  108 . In a preferred embodiment of the present invention, the Q of the filter  100  could be controlled from 5 to 300. The Q of the filter  100  is preferably controlled by a Q control, which is a 10-bit gain control for loop Kb. 
   Loop Ka, which consists of gain summation block  102  (which in a preferred embodiment may be identical to block  104 ), transfer function  110  (S−A)/(S+A), and gain summation block  112 , controls the gain of the filter  100  in the passband tuned by block  108 . The control is accomplished by passband gain control, which may be a 10-bit control of the gain for loop Ka. In a preferred embodiment of the present invention, the bandpass gain may be controllable from 0 dB to 60 dB. 
   The present invention may be best implemented using the following formula: 
       Y   =       [         K   3     -       K   1     ⁢     K   2           1   +       K   4     ⁢     K   2           ]     [       [       S   ⁡     (     B   -   A     )           S   2     +       S   ⁡     (     A   +   B     )       ⁡     [       1   -       K   4     ⁢     K   2           1   +       K   4     ⁢     K   2           ]       +     (     A   ·   B     )         ]     +           [           ⁢         S   2     -     (     A   ·   B     )           S   2     +       S   ⁡     (     A   +   B     )       ⁡     [       1   -       K   4     ⁢     K   2           1   +       K   4     ⁢     K   2           ]       +     A   ·   B         ]     ]             
 
Where:
     S=jω   K 1  is the gain from Node A to the output of the blocks G 1  and S 1 .   K 4  is the gain from Node F to the output of the block S 1 , Node C.   K 3  is the gain from Node D to the output of the block S 1 , the Output.   K 2  is the gain from Node E to the output of the block S 1 , the Output.
 
In the block diagram, bock S 1  represents the summation of the voltages at Node A with gain K 1  and Node F with the gain of K 4 . Block S 1  represents the summation of the voltages at Node E with the gain of −K 2  and Node D with the gain K 3 . Y 1 , Y 2  are allpass filters, with the transfer functions of (S−A)/(S+A). Y 3  is an allpass filter, with the transfer functions of (S−B)/(S+B). The value for A and B represent a value that is not dependent on the frequency. In many cases, the values for A and B will be determined by the function 1/RC, which is based upon the topology of the allpass filter. The topology of the allpass filter structure can take on many different forms to realize the same transfer function. The block S 1  could also be represented as gain blocks for K 1  and K 4  followed by a summation network. Block S 2  could also be represented as gain blocks for K 3  and −1*K 2  followed by a summation network. The gain and summation functions can be achieved with many different circuit topologies.
   

   Throughout this description, no reference is made to the hardware implementation of this bandpass filter, because it is believed that the beneficial aspects of the present invention would be achievable with various implementation schemes. For example, the entire filter  100  could be performed by an ASIC; a circuit card with discrete components could be used and preferred in certain applications; it is also conceivable that a very high power general-purpose digital computer could execute software which performs the filter functions on a digital signal. 
   It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps, and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.