System and method for an electronically tunable frequency filter having constant bandwidth and temperature compensation for center frequency, bandwidth and insertion loss

A novel system and method for a frequency tunable filter is described. The inventive system and method may be used, for example, in multi-band cellular phones and/or transceivers in wireless networks. The inventive system and method avoids the creation of undesired spurs thus minimizing design difficulties and obviating the need for complex multiple frequency conversions and the associated deleterious effects on performance. The inventive system and method provides at an output terminal an output signal at one of a plurality of predetermined frequencies. The inventive system and method comprises input circuitry for providing an input signal that is at least in part a function of a dynamic parameter, a processor for providing a control signal as a function of said input signal, and at least one adjustable resonance circuit capable of providing a filter signal at one of a plurality of predetermined frequencies in response to said control signal, wherein said filter signal is applied to said output terminal to thereby provide said output signal at one of a plurality of predetermined frequencies. The present inventive system and method also provides for reducing spurious signals from a composite signal created from mixing a plurality of signals by filtering the composite signal.

BACKGROUND OF THE INVENTION

Current frequency plans for frequency converters can be very complex due to the desired avoidance of creating interfering signals or spurs. One of the most important aspects in any radio frequency transceiver design is a properly chosen frequency plan. The basis for frequency planning is drawn from the well-known formula describing the creation of a spurious signal as follows:
fsp=mfrf±nflo
where fspis the spurious output frequency or spur, frfis the frequency of the RF signal and flois the local oscillator frequency.

Frequency plans have become quite complicated for modern communication systems such as multi-band cellular phones and transceivers in wireless networks. The design of these communication devices involve finding optimum solutions to several issues. These issues include defining the proper intermediate frequency to eliminate in band spurious signals, minimizing the number of IF filters, and determining the frequency swing for the voltage controlled oscillator. As a result there is a need for a frequency tunable filter for avoiding the creation of these undesired spurs, thus eliminating design difficulties, obviating the need for complex multiple frequency conversions and the associated deleterious effects on performance and cost ineffectiveness that accompanies them.

Furthermore, with limited spectrum resources serving greater and greater numbers of users the need for precise control of bandwidth and center frequency become even more important in the crowded airwaves. Bandpass, band reject and other types of filters composed of electronic components experience bandwidth variance as well as center frequency creep as a result of, among other things, temperature changes and the associated effect on the electrical characteristics of the electronic components. Typical transceivers deployed with these types of filters either operate with the aforementioned performance degradation or employ complicated and expensive compensation circuitry to combat this problem. Thus there is a need for a frequency tunable filter with temperature compensation to maintain a constant bandwidth and steadfast center frequency without sacrificing performance or cost effectiveness.

Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art to provide a novel system and method for a frequency tunable filter to provide at an output terminal an output signal at one of a plurality of predetermined frequencies comprising, input circuitry for providing an input signal that is at least in part a function of a dynamic parameter, a processor for providing a control signal as a function of said input signal; and at least one adjustable resonance circuit capable of providing a filter signal at one of a plurality of predetermined frequencies in response to said control signal, wherein said filter signal is applied to said output terminal to thereby provide said output signal at one of a plurality of predetermined frequencies.

It is also an object of the present invention to provide a novel improved method for reducing spurious signals from a composite signal created from mixing a plurality of signals by filtering the composite signal. An improvement in one embodiment can include the steps of selecting a control signal from a set of predetermined control signals, tuning the frequency of a filter based on the selected control signals and filtering the composite signal with the tuned filter.

These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a schematic of a frequency tunable bandpass filter100according to an embodiment of the present invention, although a band pass filter is shown, the subject matter of the present invention is applicable to other type of filters such as, but not limited to, band reject or slot filters. The tunable filter100is composed of one or more adjustable resonance circuits110a-ein parallel that define the center frequency of the filter and one or more of bandwidth control circuits120a-ethat define the bandwidth of the filter.

The resonance circuits110a-eshown are composed of inductance elements112and variable capacitance elements which allow for center frequency adjustments. The variable capacitance elements preferably are implemented with varactors. As is known in the art, a varactor can change its capacitance as a function of the change of capacitance of a pn junction. A pn junction's capacitance decreases with increasing reverse voltage. The use of varactors in variable capacitance application is well known in the art, and thus not further expanded upon here. Although not shown inFIG. 1, coaxial resonators with varactors or other means to tune the resonance frequency is likewise envisioned. The bandpass filter shown inFIG. 1may typically include biasing circuitry to remove circuit and DC bias as is known in the art.

Each of the bandwidth control circuits120a-eis comprised of inductors122and variable capacitance components, which may be varactors as described above with reference to the resonance circuits. In that both the bandwidth and the center frequency are governed in part by the variable capacitance components, controlling the capacitance of the variable capacitance component controls (tunes) the filter. The filter100is tuned to one of the predetermined frequency centers and bandwidths for the filter signal desired as an output signal at the output terminal102.

FIG. 2is a representation of a processor200for controlling the resonance frequency of the resonance circuits110and the bandwidth controller circuits120via the variable capacitance components. The resonance frequency of the adjustable resonance circuits allows the filter100to provide a plurality of predetermined frequencies. The processor200can also control the same by varying the inductance of variable inductance elements (adjustable frequency coaxial resonators, variable inductors, etc.). The processor200correlates a changeable or dynamic parameter to a desired output or control signal232. The dynamic parameter can be a center frequency, bandwidth or a characteristic such as temperature that can deleteriously affect the center frequency, bandwidth or other function of the filter100and therefore requires compensation to remove the deleterious effect or change filter settings.

The processor200can contains one or more look up tables to correlate the input (dynamic parameter) from the input circuitry221and control signals232to obtain the desired properties for the variable capacitance components and thus the filter100. The processor200, for illustration contains two tables, one for temperature compensation230and another for center frequency240. The processor can have numerous tables for correlating various parameters, and additionally each table preferably would have plural layers of tables, one for each varactor or variable component controlled.

In the illustration ofFIG. 2, look up table230correlates temperature to a control voltage which in turn adjusts the varactors111and121of the electronically tunable filter100. For example as shown inFIG. 2, a temperature parameter231of 72 degrees correlates to a control signal voltage232of 0.05 for one or more of the variable components to adjust for temperature induced capacitance or resonance changes. Similarly table240could correspond a new center frequency to a set of capacitance's and related control signals that adjust the variable components such that the filter100is tuned to a desired center frequency. The control signals or output of the processor can be empirically determined, theoretically derived or adaptively determined. The input to the processor may be voltages, signals or other indication at least in part that represents the value of the dynamic parameter desired or present.

For dynamic parameters related to physical properties, such as temperature, the values of the dynamic parameter can be represented to the processor200via a transducer220, such as a thermal couple, or thermal pad. The processor200may include cascades of lookup tables230to correlate input signal231related to the dynamic parameter to a series of control signals232related to the desired filter properties, one for each of the variable components. The processor200may employ mathematical operations on the inputs231to obtain the output value inclusive or exclusive of a classical look up table.

FIG. 3shows a schematic of a frequency converter with a frequency tunable filter in the L-band. The L-Band is a portion of the electromagnetic spectrum commonly used in satellite and microwave application, with frequencies in the 400 MHz to 2100 MHz, however this band is used for illustration only and the invention should not be construed to be limited to this band.

An input signal301having a frequency of 140 MHz is passed through a standard band pass filter310, and then is mixed in mixer320with a frequency supplied by a lowband VCO321or a highband VCO322, the mixing frequency being, for example between 810-1910 MHz in 1 MHz steps. The mixed or composite signal, complete with spurious and unwanted signals can be filter in an electronically tunable filter330according to an embodiment of the present invention, as illustrated a band slot filter, and further filtered by an electronically tunable bandpass filter340as taught in an embodiment of the present invention. The result of the mixing and filtering resulting in a output signal350in the L-band 950 to 2050 MHz. The frequency converter300inFIG. 3may also include other electronic components such as attenuates, switches, amplifiers etc. as commonly known in the art.