fig1 represents a local oscillator ( lo ) embodiment of the present invention , and is referred to herein by the general reference numeral 100 . the lo 100 is typically used in wireless device applications that operate in the 5 . 0 + ghz spectrum , e . g ., the phy - layer in ieee - 802 . 11a wireless local area networks . embodiments of the present invention all use voltage controlled oscillators ( vco &# 39 ; s ) that switch banks of fixed capacitors to extend and stabilize the vco tuning range . an uncertainty results in what range of vco tuning frequencies will result for each selected fixed capacitor in parallel with the relatively small - value continually variable varactor . embodiments of the present invention all build calibration tables off - line using a clone charge pump and frequency measurement . such replica charge pump duplicates the performance of the main - service charge pump because it is constructed the same way on the same chip , and is subjected to the same conditions of current , voltage , and temperature , and to the same semiconductor process and variations . the lo 100 comprises a phase detector 102 that compares a reference frequency ( f ref ) to a feedback sample frequency . any differences in phase or frequency generate a digital correction signal that is sent to a main - service charge pump 104 . the charge pump converts this to an analog control signal , e . g ., a current . such current is converted to a voltage and filtered by a loop filter 106 . a replica charge pump 108 is provided for calibration . a variable - voltage control signal is applied by both charge pumps to a voltage - controlled oscillator ( vco ) 110 . a bank of fixed - value capacitors are included in a range control 112 and will dictate the basic frequency operating range of the vco 110 in several steps . within each such step , the vco output frequency can be varied by the input control voltage from the charge pumps . in fact , the result is several overlapping steps that give continuous coverage over the entire frequency - output operating range of vco 110 . the basic problem solved by all embodiments of the present invention is the determination of which fixed - value capacitors included in range control 112 should be selected to obtain a particular local oscillator operating frequency . integrated capacitor , process , temperature , operating voltage , and other device variables can make the points the steps transition from one frequency to the next unknown . such points may also change over time and not be consistent within manufacturing runs from unit to unit . the main charge pump 104 is typically connected between power - supply rails vdd and vss . such power supply voltages limit the minimum and maximum output swings that can be generated by main charge pump 104 at the input of vco 110 . the electronic construction and semiconductor process variations of main charge pump 104 also affect the minimum and maximum output swings that can be generated by main charge pump 104 . the charge pump output current is programmable , so the minimum and maximum charge pump voltages achieveable are dependent upon the selected charge pump current due to finite fet on - resistance . it is therefore desirable that the calibration be carried out with the same replica charge pump current setting as is used in the main charge pump . the operating temperature will also have some effect . but it is desirable to know the minimum and maximums , at least indirectly , because they affect the operational points the vco range steps transition from one frequency to the next . it was decided that including controls on the main charge pump 104 that would artificially drive it to its minimum and maximum output values would be impractical and not particularly effective . a much simpler solution is to try to duplicate the basic charge pump circuitry with a partial clone , e . g ., replica charge pump 108 . the clone can then be relied on to mimic the operational behavior of the main charge pump , at least at the extreme control points . since in one embodiment , only the minimum and maximum output values are of interest , the replica charge pump 108 does not require a phase detector or reference frequencies . a mode control simply causes either the minimum or maximum to be output . an output enable allows the replica charge pump 108 to be tri - stated from the vco input , e . g ., so its capacitance doesn &# 39 ; t load the output of the loop filter 106 after calibration . the vco 110 has a frequency output that depends on both the analog control voltage and a fixed - capacitor set selection . a voltage - variable - capacitance varactor in the vco 110 is combined with a number of fixed capacitors to continuously bridge a tuning gap between successive fixed capacitor selections . the actual continuous tuning range effect of the charge pump 104 on the vco 110 has some uncertainties , and which particular fixed - capacitor set that needs to be selected in range unit 112 for a particular radio channel frequency is consequently uncertain too . the vco 10 produces a synthesized local oscillator frequency output that is stabilized to the reference frequency by a phase locked loop ( pll ) 114 . such pll 114 comprises a programmable digital counter . a variety of synthesized frequencies can be output because as the division number “ n ” is changed , the vco 110 will slew in frequency so the two inputs at the phase detector 102 will match . in other words , “ n ” dictates what vco output frequency divides down to the reference frequency exactly . during calibration , the replica charge pump 108 is enabled and a frequency measurement unit 116 samples the output of vco 110 . these measurements are taken for each minimum and maximum for each fixed - value capacitor selection possible in range unit 112 . a calibrator 118 controls the range selections , the manipulations of replica charge pump 108 , and stores the data in a correction table 120 . after calibration , a channel selection request indexes a lookup table in correction tables 120 and drives an appropriate value of “ n ” to the pll 114 and the proper capacitor combination selection to range unit 112 . in general , embodiments of the present invention include a selectable bank of fixed capacitors that is operated to change the vco range . a set of correction tables is generated for converting particular radio channel selections into a correct fixed - capacitor set choice . for example , some radio channel selections could be at points near the frequency break between two successive fixed - capacitor set choices . in one instance , the varactor would be operated near its high - end voltage limit , and in the other instance of fixed - capacitor selection the varactor would be run down to near its minimum voltage limit . fig2 represents a replica charge pump 200 that could be used as replica charge pump 108 in fig1 . the replica charge pump 200 includes a charge pump device 202 that is electronically constructed the same as main charge pump 104 in fig1 . it also is subjected to the same operating conditions , e . g ., by being integrated on the same chip and powered by the same supply rails . a pair of load fet &# 39 ; s 204 and 206 mimic the dc loads placed on the output of main charge pump 104 when the voltage output ( vo ) is high . another pair of load fet &# 39 ; s 208 and 210 mimic the dc loads placed on the output of main charge pump 104 when the voltage output ( vo ) is low . a tri - state output switch 212 allows the replica charge pump 200 to be off loaded from the external output load . fig3 diagrams a voltage - controlled oscillator ( vco ) calibrator embodiment of the present invention , and is referred to herein by the general reference numeral 300 . the vco calibrator 300 comprises a 4 - 5 ghz oscillator 302 that outputs a frequency dependent on a digitally controlled fixed - capacitor bank 304 and a varactor diode 306 . a vco control voltage 308 , e . g ., from a charge pump , and a 5 - bit range control 310 dictate what frequency “ fo ” will be output . a divider 312 provides a sample signal output 314 to the phase detector and to a digital gate 316 . a time base 318 allows a gating interval number of clock pulses that can pass through in a base measurement time period . these pulses are counted and stored in a digital memory 320 . a number of measurements are taken at a variety of capacitor combinations of range control 310 and the minimum and maximum vco control voltages 308 . such measurements are stored in a lookup table and made available post - calibration in a signal 322 . the operational assumption is that the output frequency of will repeat its measured value that was taken during calibration , and that restoring the corresponding 5 - bit range control word will provide an initial fixed value of capacitance from capacitor bank 304 that will allow frequency lock within range of the vco control voltage 308 . sixty - four such measurements and corresponding table lookup entries are preferred . a translation of radio - channels , for example , to measured frequency table values is preferably executed in software during operation . e . g ., in the wireless communication system , described in u . s . patent application ser . no . 60 / 283 , 609 filed apr . 13 , 2001 and assigned to the assignee of the present invention . such patent application is incorporated herein by reference . the translation , is then available for use as 5 - bit range control signal 310 . in general , the 5 - bit range control value associated with the vco control voltage 308 at minimum is preferred over the corresponding one at maximum . in alternative embodiments of the present invention , the calibration measurements are taken at a variety of temperatures as monitored by an on - chip temperature sensor . the translation software then preferably finds the set that has the closest calibration temperature for current operating conditions . in an alternative embodiment of the present invention , a replica charge pump is used to find the center frequency of the range afforded by each fixed capacitor . such center frequency is measured , and then stored in a look up table together with a calibration value that represents current conditions , e . g ., temperature . otherwise , the value stored could be normalized for the current conditions . during normal operation , the selecting of a channel / frequency includes looking up the capacitor that is appropriate to use . the tuning response and voltage / temperature effects from a normalized table can affect the capacitor choice . if there is sufficient overlap between frequency ranges , and some safety factor is included , an accurate selection can be made based on all process and environmental factors . such a method would never have to measure the frequencies resulting from the minima and maxima control voltages . in general , embodiments of the present invention log predetermined points in the tuning range using the replica charge pump . on - the - fly frequency calculations can be made from previously measured data , and result in an accurate fixed - capacitor choice . although particular embodiments of the present invention have been described and illustrated , such is not intended to limit the invention . modifications and changes will no doubt become apparent to those skilled in the art , and it is intended that the invention only be limited by the scope of the appended claims .