Filters serve as one of the pillars of the modern information age due to their ability to discern signals of a given frequency from ambient noise and other signals that occupy the same frequency at the same time. As such, filters are a key component of all wireless communication systems. Filters are also an essential component of numerous modern electrical systems beyond communication applications. Indeed, nearly every signal processing system needs some kind of filtering apparatus in order to select a desired signal out of the environment in which it operates.
Band pass, low pass, high pass, and band reject filters are designed to be selective to signals of a particular frequency range. A function that describes the nature of this selectivity is called the transfer function of the filter. Although a filter may have an effect on both the phase and magnitude of signals that pass through it, the transfer function of a filter is often best understood by considering these effects separately. FIG. 1 includes chart 100 which shows the magnitude portion of the transfer function of a band pass filter. The abscissa of chart 100 is frequency provided in units of hertz, and the ordinate is the magnitude of the transfer function of the filter. The band pass characteristic of the filter is illustrated by the fact that the peak of the transfer function 101 is at a frequency fx, and the transfer function falls off rapidly in either direction. As a result, input signals at frequency fx will be emphasized by the filter relative to input signals at other frequencies. The range of signals that are emphasized by the filter are referred to as being in the pass band of the filter. In this single pass band filter, the frequency fx can be referred to as the center frequency of the filter.
The abscissa of chart 100 includes a reference indicator at frequency fx and a second reference indicator at a target frequency fT. Target frequency fT is the desired center frequency for the filter. Non-idealities in the components, manufacturing process, and assembly process used to produce a filter will cause deleterious shifts in the transfer function of the filter. These non-idealities are nearly impossible to avoid even with the use of expensive low variance devices. However, filters can be tuned to adjust for these non-idealities. As illustrated in FIG. 1, the trimming procedure could result in a shift of the transfer function as indicated by arrow 102 to cause the pass band of the filter to shift from fx to fT.
FIG. 1 also includes a basic block diagram of a trimming circuit. The block diagram includes filter 103, filter input 104, filter output 105, and trimming circuit 106. The transfer function or center frequency may be tuned by changing the electrical characteristics of one or more of the filter components. Filter 103 is tuned to maximize the ratio of the wanted signals to the unwanted signals such that the output at 105 has a much higher signal-to-noise ratio than that at the input 104. This process is usually conducted by “trimming” the electrical characteristic of a component of the filter such as its capacitance in order to alter the transfer function of the filter.
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