Abstract:
The invention relates to a phase comparison circuit including a reference input and a feedback input, and an output provided with a controlled oscillator which is connected thereto. A frequency divider with an adjustable division ratio resides in the feedback path of the phase-lock loop. The phase-lock loop includes a frequency adjusting arrangement having a first and a second control input, a first output which is connected to the frequency divider and is used to generate a digital regulating signal, and a second output which is coupled to the controlled oscillator and is used to generate an analog control signal. The first control input is embodied in such a way as to adjust a frequency of the phase-lock loop, and the second control input is embodied in such a way as to switch the frequency of the phase-lock loop according to a frequency offset. The phase-lock loop can advantageously be used to compensate frequency offsets of received signals.

Description:
REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT/DE2005/000361 filed Mar. 3, 2005 which was not published in English, that claims the benefit of the priority date of German Patent Application No. DE 10 2004 010 365.8, filed on Mar. 3, 2004, the contents of which both are herein incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to a phase-lock loop for fast frequency switching and a method for fast frequency switching in a phase-lock loop. The invention further relates to a use of a phase-lock loop.  
       BACKGROUND OF THE INVENTION  
       [0003]     Phase-lock loops are used particularly in receivers to generate an oscillator signal with which a received signal is converted to an intermediate frequency. This mixing process is known as an intermediate frequency conversion. An intermediate frequency conversion to a particular intermediate frequency of 0 is especially advantageous, since a signal thus converted can be further processed especially easily. An example of a possible problem in a frequency conversion can be explained based on a conversion of OFDM signals (orthogonal frequency division multiplex signals). OFDM signals are pulsed signals, and comprise several individual subcarriers of different frequencies. Each subcarrier transmits one symbol per pulse, which is given by the amplitude and the phase position of the subcarrier during the pulse. The totality of all subcarriers per pulse is called the OFDM symbol.  
         [0004]     The individual subcarriers are distinguished by the fact that the cross correlation between any two subcarriers ideally results in a value of 0. In other words, in the frequency space, a zero crossing of a subcarrier is always within the maximum of an adjacent carrier. The frequencies of the individual subcarriers differ by n times a difference frequency.  
         [0005]     For receivers that receive OFDM signals, there is the problem of a DC signal component in the signal path of the receivers, which is called the DC offset. This is important primarily in the case of a slight frequency offset of the received signal with respect to the ideal receiving frequency. With the conversion in the receiver, the DC signal component is converted to the center frequency normally not used by OFDM signals. The later signal processing can compensate for this, however. In case of an additional frequency offset in the received OFDM signal, all subcarriers of the OFDM signal shift by the value of the frequency offset. This can cause a subcarrier of the OFDM signal to overlap with the DC signal component. The overlap can no longer be compensated for by the subsequent signal processing and is expressed in an erroneous demodulation of the subcarrier overlapped by the DC signal component.  
         [0006]     The problem of a DC signal component with simultaneous frequency offset of the carrier can be easily corrected with OFDM receivers that convert to an intermediate frequency that is not equal to 0. On the other hand a frequency conversion to the intermediate frequency 0 is desirable, particularly for send mode and for various modulation types, since such building elements can be used for both sending and receiving mode.  
         [0007]     OFDM transceivers in which the sending and receiving unit are implemented together thus use switching for the intermediate frequency generation to solve the above problem of a shifted DC signal component. The sending operation and the receiving operation thus use different local oscillator signals so that in the receiving operation the intermediate frequency is no longer  0  but at least half the signal bandwidth. A DC signal component in the receiver path then no longer falls within the useful signal band.  
         [0008]     Since such switching must take place within microseconds, it is hardly possible to switch a normal phase-lock loop as a local oscillator using corresponding control. Rather, an additional mixer is used, which converts the local oscillator signal with an auxiliary signal of a suitable frequency and thus generates the desired frequency-shifted signal. A mixer generates interference signals, however, such as harmonics of the intermediate frequency or image signals. Due to this, the problem of undesirable signal portions on the intermediate frequency remains.  
         [0009]     Another solution to compensate for a DC signal component in the case of a frequency offset of an OFDM signal meeting WLAN standard 802.11a can be found in a publication by A. Behzad et. al. Entitled “120.4 Direct conversion CMOS Transceiver with Automatic Frequency control for 802.11a Wireless LANs”, ISSCC 2003, Session 20, Paper 20.4. The method described there is very cumbersome, however.  
       SUMMARY OF THE INVENTION  
       [0010]     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.  
         [0011]     The present invention is directed to a phase-lock loop, which enables very fast frequency switching without a prolonged settling of the phase-lock loop. The invention also comprises a method for fast frequency switching in a phase-lock loop.  
         [0012]     According to one embodiment the invention, a phase-lock loop comprises a phase comparison circuit with a reference frequency input, a feedback input and an output. The phase comparison circuit is connected to a controlled oscillator. A frequency divider with an adjustable division ratio in a feedback path of the phase-lock loop is connected via its signal input to an output of the controlled oscillator and via a signal output to the feedback input of the phase comparison circuit. The phase-lock loop further comprises a frequency adjustment arrangement with a first control input to adjust a frequency of the oscillator, a second control input for a signal to shift the frequency of the oscillator and with a first output to provide a digital control signal and a second output to provide an analog control signal. The first output is connected to the frequency divider, while the second output is coupled to the controlled oscillator.  
         [0013]     The first control input advantageously serves to adjust the frequency of the phase-lock loop by adjusting the division ratio, while the second control input is used only for signals to shift the frequency and thus for a frequency offset. The signal for frequency shifting in one embodiment is fed to the digital frequency divider on the one hand and on the other hand applied directly to the controlled oscillator for a frequency shift. Because of this, the controlled oscillator generates an output signal with the shifted frequency very quickly while the phase-lock loop gains time for its regulating adjustment. If the controlled oscillator continues to exhibit a slight deviation from the desired frequency adjusted at the first and second control input by applying the second control signal, this is corrected by the phase-lock loop. The desired frequency is nevertheless achieved faster due to the application of the second control signal.  
         [0014]     The output signal of the phase-lock loop, now frequency-shifted by a specific amount, can in one example be used as a local oscillator signal in case of frequency conversion to compensate for a frequency offset of a signal to be switched and thus to correct the frequency offset in the received signal. A DC signal component occurring in a receiver path can thus be easily detected and corrected.  
         [0015]     In another embodiment, a method for frequency adjustment with frequency offset in a phase-lock loop comprises generating a phase-lock loop and applying a first control signal to an adjustable frequency divider of the phase-lock loop for nominal frequency adjustment of an output signal of the phase-lock loop. The method further comprises applying a second control signal to a controlled oscillator of the phase-lock loop to adjust a frequency offset of the output signal of the phase-lock loop.  
         [0016]     It is useful in one embodiment if the first control signal is generated from a channel adjustment signal and the second control signal before application. The channel adjustment signal in one example corresponds to the nominally desired output frequency of the phase-lock loop without frequency offset.  
         [0017]     With the method according to the invention, the output frequency of a phase-lock loop can thus very quickly be shifted by a value determined by the second control signal. The actual phase-lock loop, with its phase detector and its frequency divider in the feedback path of the lock loop, thus gains additional time to regulate and to adjust precisely to the desired frequency.  
         [0018]     In one embodiment, for use in WLAN receivers (wireless local area network receivers) which use OFDM signals for data transmission, it is advantageous to perform a frequency shift in the local oscillator signal at high speed and to thereby ensure a conversion to the intermediate frequency 0 without errors.  
         [0019]     An advantageous further embodiment of the method is provided where the second control signal applied to the controlled oscillator is sent through a digital-analog converter with a readjusted transmission factor. This is advantageous if a drift still exists between the adjusted frequency in the frequency divider of the phase-lock loop and the second control signal. This drift can be compensated for through the additional re-adjustable transmission factor.  
         [0020]     It is thus practical in this example if the frequency adjustment arrangement comprises a digital-analog converter, whose output is connected to the second input of the controlled oscillator and whose input is connected to the second control input. In an advantageous further embodiment of the invention, the digital-analog converter exhibits a readjustment input to change an amplitude of the analog control signal. This makes it possible to readjust an inexact control. Alternatively, a digital-analog converter with a series-connected digital multiplier can also be used.  
         [0021]     In another further embodiment of the invention, the frequency adjustment arrangement comprises a sigma-delta modulator, whose input is coupled with the first and second control input and whose output is coupled with the adjustable frequency divider. The output signal of the sigma-delta modulator thus makes up the fraction of the divider value for the frequency divider.  
         [0022]     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The invention is described in detail below based on an exemplary embodiment with the aid of drawings. Shown are:  
         [0024]      FIG. 1  is a block diagram illustrating exemplary embodiment of the invention;  
         [0025]      FIG. 2  is a flow chart illustrating an exemplary embodiment of a method according to the invention; and  
         [0026]      FIG. 3  is a block diagram illustrating an exemplary embodiment of an application in a receiver path. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]      FIG. 1  shows a phase-lock loop according to the invention with a first control input  1 , a second control input  2 , a reference input  3  and a signal output  4 . The reference input  3  is configured to receive a reference signal with frequency fref to a phase comparator  5  whose second input is connected to a feedback path. An output  52  of the phase comparator  5  is connected via a loop filter  7  to a first control input  81  of a voltage-controlled oscillator  8  to adjust its output frequency.  
         [0028]     The phase comparator  5  compares the phase of the signals at the feedback input  51  and at the input  3  and generates a regulating signal from this, which will be issued at the output  52 . The regulating signal is filtered through the loop filter  7  and fed to the control input  81  to adjust the frequency of output signal of the voltage-controlled oscillator  8 .  
         [0029]     The output of the voltage-controlled oscillator  8  makes up the output  4  of the phase-lock loop according to the invention and is also connected via the frequency divider  9  in the feedback path of the phase-lock loop with the input  51  of the phase detector  5 . The division ratio of the frequency divider  9  can be adjusted to a regulating input by means of a divider control signal.  
         [0030]     The output signal at output  4  of the phase-lock loop is kept stable by the regulating signal of the phase detector  5 . Deviations or fluctuations, the so-called phase jitters of the oscillator, are settled in that the phase detector  5  calculates the deviation, issues the corresponding regulating signal at the output and feeds this via the loop filter  7  to the control input of the oscillator. In order to achieve another output frequency of the voltage-controlled oscillator and thus the phase-lock loop, a regulating signal for a frequency divider must be adjusted with another division ratio.  
         [0031]     Digital control signals are provided at the inputs  1  and  2 . The control inputs  1  and  2  lead to an adder. The adder  10  adds the digital signals available at the inputs  1  and  2 . The digital output signal corresponds to the division ratio and contains a whole-number component N as well as a fractional component DN. Take a division ratio of 3.521 as an example. In that case, the whole-number component N=3, while the fraction DN=521. The division ratio is calculated from the two control signals and can consist of just a whole number, a fraction only or a combination of the two.  
         [0032]     The adder  10  feeds the whole-number component N via another adder directly to the control input of the frequency divider  9 . The first adder  10  is also connected to a sigma-delta modulator  11 . Through this connection, the adder  10  feeds the fractional component DN to the sigma-delta modulator  11 , which is configured to generate averages and from this generates the fractional component ΔN suitable for the frequency divider. The output of the sigma-delta modulator  11  is likewise connected via the second adder to the control input of the frequency divider  9 . Any average division ratio can thus be adjusted at the frequency divider  9  via the whole-number component N and the fractional component ΔN. It must be noted that the division ratio and the control signals at the inputs  1  and  2  are dependent on the frequency of the reference signal applied to the reference input. If this frequency is adjusted to a fixed 10 MHz, for example, then, for a desired output frequency of 25 MHz, the division ratio must equal to 2.5.  
         [0033]     The control input  2  is additionally connected to a digital-analog converter  12 , which converts the signal available at the control input  2  into an analog signal and feeds this to a second regulating input  82  of the voltage-controlled oscillator  8  to adjust its output frequency. With the second control signal at the control input  2 , the voltage-controlled oscillator is thus directly controlled and its frequency changed. In the exemplary embodiment, the voltage-controlled oscillator contains two regulating inputs  81  and  82 . It is possible, however, to combine the two regulating inputs into one and to feed the regulating signal of the phase comparator  5  and the analog signal through an adder to the regulating input.  
         [0034]     The digital-analog converter  12  is additionally designed for a variable gain at the output side of readable signals. In addition, it has a control input, at which a signal can be fed to adjust its gain. An amplitude of the analog output signal of the converter  12  can thus be adjusted depending on the fed control signal.  
         [0035]     In a normal operating mode, the phase-lock loop according to the invention is fed a signal for its nominal frequency adjustment of the output signal via the control input  1 . In the case of a use in an OFDM receiver, this is for example a control signal for the sender frequency of the channel to be used. The adder  10  generates from this a rational division ratio with a whole number value N and a fractional value DN, which is converted via the sigma-delta modulator  11  into the fractional component ΔN. The two signal components are fed as the division ratio to the frequency divider  9 . After a lock-in of the phase-lock loop, the voltage-controlled oscillator  8  generates a signal at the output with the frequency adjusted by that at the control input  1 . For the compensation of a frequency offset in the OFDM receiver path, the phase-lock loop according to the invention must change its output frequency as fast as possible.  
         [0036]     A readjustment of the phase-lock loop by a change in the division ratio at the frequency divider  9  is, however, not fast enough for this. For this reason, the frequency offset to be adjusted is fed using the control signal to the control input  2  and to the adder  10  as well as the digital-analog converter  12 . This converts the signal into an analog control signal and applies it directly to the second control input  82  of the voltage-controlled oscillator  8 . The output frequency of the oscillator changes according to the available control signal. At the same time, the adder  10  generates a new rational division ratio from the channel adjustment signal now available at the control input  1  and the frequency offset signal at the control input  2 . The division ratios N and ΔN generated by the adder  10  and the one generated with a sigma-delta modulator  11  are added and fed to the frequency divider  9  as the new frequency division ratio.  
         [0037]     By means of the analog control signal at the second control input of the voltage-controlled oscillator  8 , the latter already generates a new output signal, which is fed in the feedback path to the frequency divider  9 . This now divides it according to the new division ratio and feeds it in turn to the phase detector for comparison with the reference signal. If the output frequency of the new output signal adjusted by the analog control signal is the desired frequency, then the phase detector  5  generates no further regulating signal with the newly adjusted frequency division ratio. The phase-lock loop is locked in at the new frequency.  
         [0038]     If there is a deviation between the output signal at the output  4  and the signal adjusted by the new division ratio, then the phase detector  5  generates a control signal. The phase-lock loop regulates towards the signal adjusted by the frequency divider  9 . This results in a slight drift after applying the analog control signal. This drift is measured using appropriate means. An additional control signal is generated from this and fed to the digital-analog converter  12  as a gain factor to compensate for the incorrect offset adjustment.  
         [0039]      FIG. 2  shows an exemplary embodiment of the method according to the invention as it is used, for example, in WLAN-OFDM receivers with an intermediate frequency conversion to the intermediate frequency 0. The phase-lock loop according to the invention is used here as a local oscillator to generate a local oscillator signal for the frequency conversion to the intermediate frequency 0.  
         [0040]     In a first step  100 , the local oscillator signal, which is given by the signal at the output  4 , is adjusted to a sender frequency using a channel adjustment signal, which indicates a specific division ratio. The sender frequency of the frequency channel results here from the standard. A conversion of a received OFDM signal is thus initially accomplished with this local oscillator signal. The converted signal is prepared, digitalized and fed to further signal processing circuits.  
         [0041]     The W-LAN Standard 802.11a further specifies that OFDM receivers, upon receiving a signal, detect a frequency offset between the frequency specified by the standard and adjusted by the channel adjustment signal and the frequency of the received signal. Such a frequency offset can be caused by various external effects, such as the Doppler effect, but also by an offset in the sending frequency of the sender.  
         [0042]     A detection and correction of the offset occurs at the beginning of a signal burst of an 802.11a data package, for which the standard specifies a maximum of 16 μs. An existing frequency offset is detected in step  200  and from this, a frequency offset for compensation is calculated. This offset is fed as a control signal. A new division ratio results from the channel adjusting signal and the control signal.  
         [0043]     At  300 , the control signal is applied to a digital-analog converter, which generates from it an analog control signal for frequency adjustment of a voltage-controlled oscillator of the phase-lock loop. At the same time, an adder generates a new division ratio from the channel adjusting signal and the control signal, which represents the frequency offset. The new division ratio is used to adjust the phase-lock loop and fed to a frequency divider in a reverse path. In step  400 , the simultaneous application of the analog control signal and a new division ratio generates a new local oscillator signal, which is shifted by the measured frequency offset. The deviation between the now adjusted frequency and the desired frequency, which is specified by the division ratio, is calculated. At  500 , a control signal is generated from this to compensate for this deviation and is connected to a regulating input of the digital-analog converter.  
         [0044]     The speed of the adjustment of the phase-lock loop to the new frequency by the direct application of the control signal representing a frequency offset is so high here that the frequency offset can be compensated for in the received signal before the beginning of a useful signal. Data losses in the later useful signal with the conversion to the intermediate frequency 0 can thus be prevented.  
         [0045]     A frequency offset in the received signal is compensated for by the frequency shift of the local oscillator signal. A DC current component in the receiver path can thus be suppressed without problems using a fast Fourier transformation. In addition to the use in OFDM receivers, the phase-lock loop according to the invention can be used for local oscillators, with which the frequency switching of the output signal should be accomplished as quickly as possible. A frequency offset in the output signal can be achieved especially fast by the direct feed of the second control signal to the oscillator.  
         [0046]      FIG. 3  shows a section of the receiver path of an OFDM receiver with the phase-lock loop according to one embodiment of the invention. The receiving path  95  comprises a device  94  for detecting a frequency offset, which is connected with a regulating output  942  with the second control input  2  of the phase-lock loop  91  according to the invention. A signal applied to the input  951  of the receiving path is converted to the intermediate frequency 0 in a conversion device  92 . For this, the conversion device  92  uses a local oscillator signal, which was generated by the phase-lock loop  91  according to the invention at its output  4 . The frequency of the local oscillator signal results from the channel adjustment signal at input  1 . If the signal converted to the intermediate frequency 0 exhibits a frequency offset, this is detected in a detection device  94 . This is connected at its input to the output of the conversion device  92 . The detection device  94  generates a control signal from the frequency offset. This is fed to the conversion device  92  at the second control input.  
         [0047]     The phase-lock loop according to the invention adjusts the new frequency very fast and thus compensates for the frequency offset that thus occurs. The received signal is now converted using the frequency-shifted local oscillator signal such that a DC signal component occurring in a receiver path lies at the unused center frequency 0. The converted signal can then be further processed in the demodulation device  93 , also connected with the output of the conversion device.  
         [0048]     Although the invention has been illustrated and described with respect to one or more implementations, alterations and/or modifications may be made to the illustrated examples without departing from the spirit and scope of the appended claims. In particular regard to the various functions performed by the above described components or structures (blocks, units, assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.