KR20020020187A - Auto calibration circuit for radio frequency synthesizer circuits - Google Patents

Abstract

PURPOSE: An automatic calibration system for a phase locked loop is provided to operate in a multiplexing band for an overall operational frequency range and to utilize a common voltage controlled oscillator(VCO) for preventing a result from changing during a manufacturing process. CONSTITUTION: An automatic calibration system for a phase locked loop includes a voltage controlled oscillator(VCO)(12) of the phase locked loop, a comparator circuit(26), a counter(27) and a divider(35). The automatic calibration circuit selects an operational frequency range of the VCO(12) of the phase locked loop. After tuning an operational frequency of the VCO(12) at one end of a tuned frequency at every changing of a frequency, the automatic calibration system converts the range to digital. When a control voltage at a control input terminal(13) of the VCO(12) is changed from the present value, an operation to select a range is disabled. At that time, it is allowed to synchronize by using a reference signal of the loop. The automatic calibration system becomes available for a quick selection of a channel frequency even in an application such as a cellular phone necessary to change a frequency within several microseconds.

Description

AUTO CALIBRATION CIRCUIT FOR RADIO FREQUENCY SYNTHESIZER CIRCUITS}

The present invention relates to improvements in frequency synthesizers in the field of radio frequency signal processing. In particular, an auto calibration circuit is provided that allows stable frequency operation in multiple frequency bands.

Frequency synthesizers are used to generate stable carrier frequency signals in many wireless telephone applications. As the user moves from cell to cell, a change in transmission frequency is required because the next cell may operate at a different frequency, requiring a very rapid change in transmission frequency. The new transmission frequency can vary greatly, and in order for the user to be unaffected from moving from one cell to another, the transmission frequency must be reestablished in microseconds as the new channel frequency.

To quickly change the synthesizer frequency, the phase-lock loop must quickly reset the phase lock for the new channel frequency. Synthesizer design is complex because there are few voltage controlled oscillators (VCOs) that can operate across the cellular telephone bandwidth. Therefore, the VCO must be operable in multiple frequency bands, or multiple muteple VCOs can be provided in which the VCO can cover multiple frequency bands.

However, using multiple VCOs is expensive and difficult to implement.

An additional problem in implementing VCOs with wide tuning bandwidths is that process changes in the semiconductor fabrication process can result in changes in operating frequency such that there can be a 10-15% difference between the VCO frequency tuning ranges. The present invention provides a synthesizer that uses a common VCO that operates in multiple bands over the entire spectrum of interest and prevents the consequences of process variations during manufacturing.

A system is disclosed that uses a single VCO in a frequency synthesizer that can operate over a wide signal bandwidth. The VCO of the phase locked loop is digitally controlled to allow the VCO to tune to different ranges of frequencies. When the operating frequency for the synthesizer is about to be changed, a reference voltage source is connected to the voltage controlled oscillator control input to set the frequency of the voltage controlled oscillator at one end of its tuning range. The operating frequency band for the VCO is selectively switched while monitoring the voltage controlled oscillator control input. When the comparing circuit determines that the tuning voltage on the voltage controlled oscillator control input is reduced below the reference value, switching of the voltage controlled oscillator operating frequency band is suppressed, and the VCO acquires a phase locked state in the phase locked loop. The process of selecting each band and establishing phase synchronization within the band containing the new channel frequency can be performed in a few microseconds. In the GSM telephone operating mode, the frequencies for the 128 channels can be accessed using 16 operating bands.

1 shows a frequency synthesizer having a phase locked loop operating in multiple frequency bands,

2 shows a tuning range for each of a plurality of operating frequency bands for a voltage controlled oscillator;

3 is a block diagram of an automatic calibration circuit in accordance with a preferred embodiment of the present invention for selecting each band of operation for a VCO;

4 illustrates a binary search algorithm used to locate a band of operation with respect to a VCO;

5 is a block diagram of one embodiment of the present invention using a binary search algorithm for locating the VCO band of operation;

6 shows a process executed by the embodiment of FIG. 4;

Explanation of symbols for the main parts of the drawings

12: voltage controlled oscillator 13: voltage controlled oscillator control input

24: control input 26: comparison circuit

27: Counter 35: Divider

1, a block diagram of a frequency synthesizer used in an application such as a portable radiotelephone is shown. The frequency synthesizer provides up to 128 channel frequencies at approximately 200 KHz intervals in GSM portable telephone applications. The frequency synthesizer includes a VCO 12 having an output frequency F _out under the control of a control voltage applied to the control input 13 of VC0 12. The operating frequency for the VCO 12 is typically set effectively by the divider 14 in the frequency synthesizer. The phase locked loop is formed of a VCO 12, a divider 14, a phase / frequency detector 16 and a loop filter 17. A very stable reference frequency is applied to the reference input 19, and the phase difference between the reference input signal and the divided VCO 12 output signal is detected with an error signal of the loop filter 17. The loop filter 17 is designed according to the phase locked loop technique such that a loop bandwidth is obtained which allows to synchronize F _out as the F _ref input reference signal at the time allotted to change the frequency.

The auto calibration circuit 21 is under the control of the serial interface 220 to the synthesizer. In the case of a cellular telephone, the serial interface may generate a command to the auto calibration circuit 21 to reset the band in which the VCO operates, so that the phase locked loop may be synchronized at the frequency set by the divider 14. In cellular telephone applications, the divider 14 may function as a frequency modulator as is known in cellular telephone technology.

VCO 12 has an operating frequency range under the control of automatic calibration circuit 21. The multiple switched frequency control elements of the VCO 12 set one of the 16 bands of frequency operation. During the frequency change, the auto calibration circuit 21 sets the frequency of the operation of the VCO 12 so that the control voltage appearing at the control input 24 is within a range where the frequency generated by the VCO 12 achieves phase synchronization. Change continuously until is displayed.

The various frequency bands in which the VCO 12 can be tuned are shown in more detail than in FIG. 2. Within each band n, VCO 12 will establish phase synchronization with the input reference frequency signal. As will be apparent from FIG. 3, the VCO band is digitally selected by switching individual tuning elements, such as capacitors, in parallel with the continuously tuned varactors of the VCO 12, The applied net tuning capacitance corresponds to one of the frequency bands shown in FIG. The n frequency band shown has a nominal tuning range that varies from VCO to VCO due to manufacturing process variations. Skew for the tuning range is effectively modified by the present invention when the operating band for the VCO is selected.

In accordance with the process implemented by the preferred embodiment, the selection of the operating frequency band begins at the lowest band 1 and proceeds to the highest frequency band n while the VCO 12 searches for the frequency band to which to establish phase synchronization to the reference frequency. . During this search, the VCO is reset to the high end of its tuning range by charging the control input 13 to the reference voltage VDS. When each band is selected, the VCO control voltage will remain at the reference value until the band containing the frequency selected by the divider 14 is selected and the VCO starts phase synchronization with the reference frequency signal. The phase locked condition is maintained by monitoring the voltage potential on the control input 13 of the VCO 12. When the control voltage 13 moves from its predetermined reference value toward the phase synchronization condition, the auto calibration circuit 21 prohibits any other band selection, thus establishing the VCO 12 frequency band.

A more detailed block diagram of the automatic calibration circuit is shown in FIG. The band select component for the VCO 12 includes a capacitor bank 12B connected to the tuning varactor 12A of the VC0 12 via the switches S1-S4. The connection of the tuning capacitor is under the control of a binary 4-bit signal from the counter 27 that can select up to 16 operating bands for the VCO 12. By selectively connecting or disconnecting each of the tuning capacitors 12B in parallel with the varactor tuning capacitors 12A, a different operating band for the VCO 12 is selected.

When a change in frequency is selected by the serial interface 22 of the frequency synthesizer of FIG. 1, the divider 14 is set, and the automatic calibration circuit is reset by the serial interface 22 via the terminal 30. . The reference voltage source 31 is briefly connected to the control input 13 of the VCO 12 via the switch 29. Capacitor 32 and inductor 33 retain the charge for presetting the VCO 12 output frequency at one end of its frequency range, which in the preferred embodiment is the upper frequency tuning limit for VCO 12. .

Frequency band selection is under the control of the counter 27. Counter 27 counters the divided clock pulses generated from programmable frequency divider 35 and generates a change in the 4-bit binary counter 27 output, resulting in a new value and a new tuning bandwidth of capacitance 12C. Is generated. The pulse applied to the counter 27 has a sufficient width so that the phase locked loop can obtain synchronization when the appropriate band is selected.

The control voltage on the control input 13 is continuously monitored by the comparator 26 when the frequency band is being selected by the counter 27. When the comparator 26 indicates that the voltage on the control input 13 of the VCO 12 has decreased from its preset value stored on the capacitor 32 by an amount equal to the comparator threshold, the comparator 26 counters (27) switches to prohibit any further indexing of the VCO 12 operating frequency band.

The above-described implementation of the present invention makes the tuning range of the VCO larger by operating the digital tuning of the VCO 13 to different frequency bands, and by tuning the VCO 12 frequencies within each band to the tuning varactors 12a. You can increase it considerably. In addition, because conventional semiconductor processing techniques produce components with wide tolerances, VCO circuits tend to have large variations in tuning range due to manufacturing process variations. Therefore, if the VCO frequency cannot be tuned within its nominal tuning range, the next highest adjacent frequency band can be used to generate the channel frequency of interest.

Each time channel frequency selection is performed by the synthesizer, the counter 27 is reset and charging on the capacitor 32 is reestablished by the momentary closure of the switch 29. The comparator 26 threshold is any large in the process or temperature change that would erroneously trigger the comparator 26 so that the frequency synchronization can be reliably inhibited when a phase locked condition is detected on the control input of the VCO 12. The offset may also be set to a value that compensates.

A preferred embodiment of the present invention begins to search for a band containing the channel frequency of interest in the lowest frequency band. However, other embodiments of the invention may begin from other locations. For example, it is possible to provide a counter that starts its count from band number 8 and indexes into a lower frequency band or a higher frequency band depending on the state of the control voltage on VCO 12.

4 illustrates a binary search tree that may be implemented in one embodiment of the present invention to locate a frequency band of interest. The search algorithm starts assuming that the number of bands to be selected is 16 bands, 0-15, according to the preceding embodiment. The search starts from intermediate band 8. For example, if band 3 has the selected synthesizer frequency of interest, the search algorithm will first locate band 3 by determining whether the control voltage on VCO 12 is greater or less than the reference voltage. Since band 3 lies below band 8, the search process selects an intermediate band between bands 8 and 0 and sets the VCO to operate in band 4. The reference voltage will then be reestablished on the VCO control input, and a similar determination is made as to whether the VCO control voltage is above or below the set reference voltage. Since band 3 lies below band 4, the counter is set to halfway between band 4 and band 0, that is, band 2.

Once the VCO is operating in band 2, the control voltage input to the VCO is recharged and the control input voltage is compared to the voltage reference. Since band 3 lies above band 2, the system will recognize that the control voltage is now above or equal to the reference voltage. The location of the band is known as band 3 or band 2 because the location of the transition where the VCO control voltage changes its relative magnitude with respect to the reference voltage is known. Once the system is tuned into band 3, it will acquire synchronization with the signal calibrating the frequency synthesizer.

The binary search algorithm described above locates any band including the signal frequency by examining only four of the 16 bands.

Figure 5 shows the above embodiment of the present invention in which the correct VCO 12 frequency band is retrieved using a binary search algorithm. Using a binary search algorithm to locate the appropriate frequency band for the VCO 12 provides a fairly large speed in selecting a new synthesizer output signal frequency.

5 includes the same basic structure as in the previous embodiment. However, the embodiment of FIG. 5 starts searching from band number 8, which is halfway between the lowest and highest frequency bands of operation, using 16 bands of frequency operation for VC0 12. At the beginning of the search, a successive approximation register (SAR) 27 supplies binary 1000 as the output to select band number 8 for the VCO 12. At the same time, capacitor 32 is charged to V _ref via switch 29 and reference voltage source 31. Comparator 26 monitors the control voltage on VCO 12, and when the voltage decreases from the precharged voltage level, comparator 26 causes the frequency band containing the signal to be phase locked to be below frequency band number 8. Switch to indicate that it is on. Alternatively, if the control voltage on VCO 12 is maintained at its preset value, comparator 26 indicates to SAR 27 that the band of interest is above band 8. Therefore, the SAR 27 switches the band to a higher frequency or a lower frequency band and will continue to search for the frequency band containing the frequency of interest. The VCO control voltage input is recharged each time the band is switched by the enable signal applied by the counter 27, which in this embodiment is a continuous approximation register.

The search for frequency bands that create locked conditions is shown more specifically in FIG. 6. When the reset signal is received from the serial interface 22, the process begins at 40, indicating that the search will begin for a new operating frequency band. Since sixteen frequency bands can be identified by the four-bit output signal n from the SAR 27, the index i is set equal to N, i.e. four bits, representing sixteen different bands. Thus, the intermediate band for starting the search is also set as 1000 (band 8) by the SAR 27 which enables the switch 29 to preset the control voltage of the VCO 12 at the high end of its tuning band. do.

The control voltage to VCO 12 is checked at decision block 42. If the control voltage is less than the predetermined reference value, the content 1000 of the SAR 27 is shifted one bit to the right and the most significant bit b ₃ is set to zero in step 43. At this time, the switch 29 retracts the OR gate from the SAR 27 in step 46 to recharge the capacitor 32 to the reference voltage V _ref to place the VCO 12 at the high end of its tuning range. Activated by the enable signal. The number of bits i is reduced in step 48 and the control voltage is again checked against the reference voltage in step 42.

Upon determining that the VCO control voltage is less than V _ref , indicating that the frequency band of interest is above band 8, the process shifts bits b _3- b ₀ of the starting binary 1000 to the right and the most significant bit of the step ( By setting it to 1 in 44), it will continue.

The process is based on the contents of the register 27, depending on whether the frequency band is above or below the band 8, the intermediate band between any one of the lowest frequency band 0 or the highest frequency band 15 and the band 8, i.e. By selecting the next band as band 4, it continues. Steps 42-48 are executed a total of i times. Each time steps 42-48 are calculated, the location of the node is identified in the search tree of FIG. When i decreases to zero, the location of the band containing the frequency of interest is known.

The foregoing execution sequence of FIG. 6 may be described with respect to the embodiment of FIG. 4 in which the frequency synthesizer determines the location of the signal in band 3. FIG. At the beginning of process 40, i is set equal to 4, the total bit width represented by b ₃ -b ₀ . The control voltage 4 of the VCO 12 was previously set to V _ref , and in step 42 a determination is made whether the control voltage is less than, greater than or equal to V _ref . Since band 3 is below band 8, the contents of SAR register 27 are shifted to the right, and B3 is set to zero in step 43. The control voltage input to the VCO 12 is recharged again by closing the switch 29 momentarily in step 46. Index i is decreased by 1 in step 48. The contents of register 27 now identify band 4 corresponding to the node in the search tree of FIG. The control voltage is again determined by the comparator 36 to be below the reference voltage, bits B ₂ -B ₀ are shifted to the right and B ₂ is set to zero. The contents of register 27 are identified by 0010, that is, band 2 of the search tree. Index i again decreases from 2 to 1 in step 48. The control voltage is determined to be greater than or equal to V _ref in band 2. Thus, this indication is that the band is likely to lie above band 2. Therefore, the contents of the register 27 are shifted to the right and B ₁ is set to 1, so that the band 3 (the contents of the register 27 is 0011) is established.

As each bit position B ₃ , B ₂ , B ₁ and B ₀ is shifted through steps 42-49 of FIG. 6, the resulting content of the SAR 27 identifies the band containing the signal frequency of interest. . Once the search tree is traversed, the phase synchronization condition establishes itself. Thus, the aforementioned system for quickly locating the frequency band of interest starts searching in the middle of a range of frequency bands, determines the relationship between the control voltage on the VCO 12 with respect to the reference voltage, Obviously, it depends on selecting the band above or below the previous band. After n comparisons equal to the number of output bits from the SAR register 27, the last level of the search tree of FIG. 4 will be revealed by the SAR 27, and the comparator output indicates the direction of the search to locate the band of interest. Will change. The SAR 27 will identify two possible frequency bands that include the frequency of interest, and the selection between the two is performed by the relative state of the comparator 27 for each of the two bands.

The foregoing description of the invention illustrates and describes the invention. In addition, the present disclosure merely illustrates and describes only preferred embodiments of the invention, and as mentioned above, the invention can be used in a variety of other combinations, modifications, environments, and within the scope of the inventive concepts disclosed herein. Changes or variations may be made in accordance with the above teachings and / or skills or knowledge of the related art. The above described embodiments describe the highest mode of practicing the present invention and are intended to be used by those skilled in the art in such or other embodiments, with other modifications necessary for a particular application or use of the present invention. Thus, the technology is not intended to limit the invention to the form disclosed herein. Also, the appended claims should be construed to include other embodiments.

The present invention provides a synthesizer that uses a common VCO that operates in multiple bands over the entire spectrum of interest and prevents the consequences of process variations during manufacturing.

Claims (24)

In an automatic calibration system for a frequency synthesizer phase locked loop, A voltage tuning element for generating a signal in a multiple frequency band operative to change the operating frequency band in response to a digital signal, and for changing the operating frequency within each of the frequency band in response to a control voltage A voltage controlled oscillator having: A counter for supplying a tuning control digital signal for selecting each of the frequency bands during the selection of the frequency band of the voltage controlled oscillator; A reference voltage source providing a voltage for setting the frequency of said voltage controlled oscillator operation to one end of its tuning range; A switching circuit for charging the control input of the voltage controlled oscillator to the reference voltage at the start of a band changing operation for the voltage controlled oscillator; A clock signal generator for providing the counter with a clock signal that indexes the counter to select a different operating band for the voltage controlled oscillator; A comparison circuit connected to sense a tuning voltage on the voltage controlled oscillator control input for each band selected by the counter and to prohibit the counter from further switching bands when the control input decreases from the charged level. Automatic calibration system comprising a. The method of claim 1, The counter is reset at the beginning of each calibration interval to select the same band each time a new signal frequency is generated by the synthesizer. The method of claim 1, The voltage controlled oscillator is biased at the high end of its tuning range. The method of claim 2, The switching circuit is enabled to charge the voltage controlled oscillator control input to the reference voltage whenever a new signal frequency is generated by the synthesizer. The method of claim 1, And said control voltage comprises a capacitor for storing said reference voltage applied from said switching circuit. An automatic calibration system for selecting an operating frequency band of a frequency synthesizer, To generate an output signal to the synthesizer in response to a digital signal to select one of a plurality of operating frequency bands, and to change an operating frequency within the one frequency band in response to a control voltage received on a control voltage input. A voltage controlled oscillator having a voltage tuning element, A reference voltage source for providing the tuning element with a voltage for setting the frequency of operation of the voltage controlled oscillator at one end of its tuning range; A switching circuit for charging the control voltage input of the voltage controlled oscillator to the reference voltage at the beginning of a calibration interval; A comparison circuit for comparing the voltage on the voltage controlled oscillator input with the reference voltage; A counter providing a tuning control digital signal for selecting each of the frequency bands during calibration of the frequency synthesizer—the counter sets a beginning band of operation for the voltage controlled oscillator, the comparison circuit Continuously switches to another operating frequency band until a predetermined relationship indicating that a phase locked loop of the synthesizer has achieved a frequency synchronization condition with a selected frequency exists between the control voltage and the reference voltage. Including automatic calibration system. The method of claim 6, And the counter sets the starting operation band to be the lowest of the frequency bands. A method for automatically calibrating a voltage controlled oscillator operating in a phase locked loop within a plurality of discrete frequency bands, the method comprising: Precharging the control voltage on the voltage controlled oscillator to a reference voltage that places the voltage controlled oscillator at one end of its frequency range; Varying the operating frequency of the voltage controlled oscillator in discrete steps, Monitoring the control voltage of the voltage controlled oscillator; Inhibiting further variation in the voltage controlled oscillator operating frequency when the control voltage changes from the reference value. The method of claim 8, Precharging the voltage controlled oscillator control input prior to each calibration interval. The method of claim 8, Resetting a frequency band of operation for the voltage controlled oscillator to a predetermined one of the frequency bands before each calibration interval. The method of claim 8, The precharging step places the voltage controlled oscillator at the high frequency end of its range, the frequency band change starting from the lowest operating frequency band. The method of claim 9, The further change in frequency of operation is inhibited when the control voltage is less than the reference voltage. The method of claim 8, The step of changing the operating frequency band discretely Generating a clock signal having a duration that allows the voltage controlled oscillator to achieve a synchronization condition with a reference signal; Inducing a step for varying the operating frequency band by counting the clock signal to set a lock condition for the phase locked loop when the frequency band where the control voltage comprises the reference signal is selected Automatic calibration method comprising the steps of. The method of claim 8, The frequency band is stepped in a direction depending on the relationship between the reference voltage and the control voltage. The method of claim 11, And initially selecting an operating frequency band existing between the lowest operating frequency band and the highest operating frequency band. An automatic calibration system for selecting an operating frequency band of a frequency synthesizer, Generating an output signal to the synthesizer and having a plurality of fixed tuning elements selectively operated to select one of a plurality of operating frequency bands in response to a digital signal, the one frequency band A voltage controlled oscillator having a voltage tuning element for varying operating frequency within A reference voltage source for providing a voltage for setting an operating frequency of said voltage controlled oscillator at one end of its tuning range; A switching circuit for charging the control input of the voltage controlled oscillator to the reference voltage whenever a new operating band is selected for the voltage controlled oscillator; A comparison circuit for comparing the voltage on the voltage controlled oscillator input with the reference voltage; A register for providing a tuning control digital signal for selecting each of the frequency bands during calibration of the frequency synthesizer, wherein the register sets a starting operating band for the voltage controlled oscillator, wherein the comparison circuit sets the frequency at which the synthesizer is selected And continuously increasing and decreasing the operating frequency band until a frequency synchronization condition with is achieved. The method of claim 16, The register is programmed to select an initial operating frequency band for the voltage controlled oscillator that exists between the upper and lower operating frequency bands for the voltage controlled oscillator and compares the control voltage with the reference voltage while searching for the selected frequency signal. And an automatic calibration system for continuously changing the frequency band to a higher or lower operating frequency band as a result. The method of claim 17, The register is a continuous approximation register that shifts its contents from the initial count identifying the initial operating band to the right in response to a clock pulse and changes the value of its most significant bit in response to the comparison of the control voltage and the reference voltage. Automatic calibration system (a successive approximation register). The method of claim 17, The register selects a band halfway between the lower operating frequency band and the higher operating frequency band, and, as a result of the comparison of the control voltage and the reference voltage, during the higher frequency band and the lower operating frequency band. Automatic calibration system for selecting the next band between any one of said intermediate bands. A method for automatically calibrating a voltage controlled oscillator operating in a phase locked loop within a plurality of discrete frequency bands, the method comprising: Precharging the control voltage on the voltage controlled oscillator with a reference voltage that places the voltage controlled oscillator at one end of its frequency range; Monitoring the control voltage of the voltage controlled oscillator by comparing the control voltage with the reference voltage; Changing the operating frequency of the voltage controlled oscillator to a higher or lower operating frequency band as a result of the comparison of the control voltage and the reference voltage in discrete steps; Recharging the control voltage on the voltage controlled oscillator to the reference voltage whenever the operating frequency band of the voltage controlled oscillator changes; Inhibiting further change in operating frequency of the voltage controlled oscillator when the control voltage changes from the reference value. The method of claim 20, The step of varying the operating frequency of the voltage controlled oscillator is continuous with an operating frequency band existing between any one of the highest or lowest operating frequency band and the currently tuned frequency band according to the relationship between the control voltage and the reference voltage. Automatic calibration method to select. The method of claim 21, The currently tuned operating frequency band is initially selected midway between the lowest and highest operating frequency bands at the beginning of a calibration interval. A method of automatically calibrating a voltage controlled oscillator operating in a phase locked loop within multiple discrete frequency bands, Precharging the control voltage on the voltage controlled oscillator to a reference voltage that places the voltage controlled oscillator at one end of its frequency range; Comparing the control voltage of the voltage controlled oscillator with a reference voltage; The operating frequency band of the voltage controlled oscillator is higher or lower in the currently tuned frequency band, that is, the current tuned frequency band and the upper or lower portion, depending on a result of the comparison of the control voltage and the reference voltage. Continuously changing to an intermediate band between any one of the operating frequency bands, Whenever the operating frequency band of the voltage controlled oscillator is changed, recharging the control voltage on the voltage controlled oscillator to a reference voltage; Determining, from a result of the change in the continuous frequency band and the comparison of the control voltage and the predetermined voltage, a frequency band of operation comprising a signal causing the phase locked loop to achieve a synchronization condition; Automatic calibration method. A method of automatically calibrating a voltage controlled oscillator operating in a phase locked loop within multiple discrete frequency bands, Precharging the control voltage on the voltage controlled oscillator to a reference voltage that places the voltage controlled oscillator at one end of its frequency range; Storing a binary number representing an initial operating frequency band for the voltage controlled oscillator into a continuous approximation register; Selecting an operating band based on the contents of the continuous approximation register; Comparing the control voltage and the reference voltage of the voltage controlled oscillator (a) if the reference voltage exceeds the control voltage, shifting the register contents to the right to set the most significant bit of the shifted contents to zero; (b) if the reference voltage is less than the control voltage, shifting the register contents to the right to set the most significant bit of the shifted contents to 1, Recharging the voltage controlled oscillator input to the reference voltage each time the register contents are shifted; If N is the number of bits of the binary number, shift the contents of the register until the register is shifted N times, and set the most significant bit of the shifted content according to the comparison of the voltage on the voltage controlled oscillator input, Continuing to recharge the voltage controlled oscillator input to thereby identify the operating frequency band at which the register content achieves a phase locked condition.