Abstract:
An Automatic Frequency Control (AFC) circuit for a mobile terminal employs a fractional-N Phase Locked Loop (PLL) to directly reduce errors in the synthesized frequency, such as due to component tolerances, temperature drift, and the like. The frequency error is detected by the average speed of rotation of the I,Q constellation. A corresponding offset is added to the tuning frequency selection word prior to encoding, such as in a ΔΣ modulator, to generate an effective non-integer PLL frequency division factor over a specified duration. The ΔΣ modulator may include dithering the different integer values by a pseudo-random number to minimize noise in the output frequency spectrum introduced by the fractional-N division. Component and parameter selection allow a high degree of resolution in frequency control of the fractional-N PLL. By directly controlling for the frequency error, a DAC and XTAL oscillator tuning circuit may be eliminated from the AFC circuit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to the field of wireless communications and specifically to an improved automatic frequency control circuit and method.  
           [0002]    Automatic frequency control (AFC) is a fundamental process for radio communication devices. AFC typically includes one or more phase-locked loop (PLL) circuit(s), which generate a high frequency periodic signal from a low frequency reference signal. AFC is used to perform a wide variety of tasks in wireless communication system components, such as frequency synthesis, AM and FM detection, frequency multiplication, tone decoding, pulse synchronization of signals from noisy sources, and the like, with high frequency accuracy.  
           [0003]    One conventional, representative application of AFC, described herein to explicate the present invention, is in conjunction with the downconversion of a received Radio Frequency (RF) signal, such as that produced by the transmitter at a base station, to the baseband frequency in a receiver, such as a wireless communication mobile terminal, for demodulation of the RF signal. The downconversion is accomplished by multiplying (mixing) the RF signal with a high frequency signal (often referred to in the art as a Local Oscillator (LO) frequency signal) to produce sum and difference components. The resulting analog I and Q signals of the difference component (representing the In-phase and Quadrature-phase components, respectively) may be digitized and processed by a baseband processor, such as an appropriately programmed Digital Signal Processor (DSP), to extract information symbols from the signal, as is well known in the art. The LO signal is typically the output of a phase locked loop, which typically uses a high quality crystal (XTAL) oscillator, the frequency of which is multiplied by a programmable factor by the phase locked loop (PLL) circuit.  
           [0004]    The residual frequency offset between the actual RF signal and the desired RF frequency (due to the XTAL oscillator frequency error) is an error that should be eliminated or minimized to within an allowable tolerance. This frequency error can be determined in the baseband processor from the speed of rotation of the (I,Q) constellation. This frequency error (in digital representation) is typically converted to an analog voltage by a Digital-to-Analog Converter (DAC) and applied to a frequency control circuit of the XTAL oscillator generating the PLL&#39;s reference frequency. In particular, the output voltage of the DAC may be applied to a Variable Capacitance diode (Varicap) through a series of decoupling resistors and capacitors to change the capacitance in the overall resonant circuit, and thereby change the frequency of oscillation slightly. This slight change in the XTAL oscillator circuit output frequency is multiplied by the PLL, resulting in a corresponding change in the local oscillator frequency. The difference between this altered LO frequency and the RF signal is again detected at the baseband processor, and the XTAL oscillator reference frequency again adjusted, in a closed-loop feedback manner. This frequency control loop is commonly referred to as Automatic Frequency Control (AFC).  
           [0005]    The voltage level fed back to the XTAL oscillator tuning circuit is not, in general, a direct function of the detected frequency error. Each element in the XTAL oscillator tuning circuit introduces errors due to component tolerance, aging, reflow soldering effects, and the like; furthermore, many of these errors change non-linearly with temperature. Typically, the XTAL oscillator tuning circuit is extensively characterized at the factory, and control voltages to effect various frequency shifts are stored in look-up tables in the baseband processor. Additionally, the baseband processor may dynamically alter or add to the look-up table entries to reflect XTAL oscillator tuning circuit control voltages and associated temperatures and/or other operating characteristics, to build up a more robust error frequency control model over time.  
           [0006]    The XTAL oscillator tuning circuit components and DAC add cost to the mobile terminal, consume printed circuit board space, and decrease reliability by providing additional failure points. The extensive characterization of the XTAL oscillator tuning circuit to create initial look-up table entries is time consuming and costly. Finally, the dynamic adaptation of the look-up table entries consumes limited processing resources and adds complexity to the radio control software.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention relates to a method of automatically controlling an output frequency. The phase difference between a reference frequency and a divided output frequency is detected. The output frequency is generated in response to the phase difference, and error in the output frequency is detected. The output frequency is divided by at least two distinct integer values over a specified duration, the integer values selected in response to the error in the output frequency, to generate the divided output frequency. In this manner, the output frequency is controlled to reduce the error.  
           [0008]    In another aspect, the present invention relates to a fractional-N phase locked loop. The fractional-N phase locked loop includes a phase-frequency detector generating a control voltage in response to a reference frequency and a divided output frequency. It also includes a voltage-controlled oscillator generating an output frequency in response to the control voltage. The fractional-N phase locked loop also includes a fractional-N frequency divider generating the divided output frequency from the output frequency, the divided output frequency differing from the output frequency by a non-integer factor over a specified duration. The fractional-N phase locked loop additionally includes an error detector to detect errors in the output frequency and to generate a control signal to control the fractional-N frequency divider to compensate for the error. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    [0009]FIG. 1 is a functional circuit diagram of a prior art AFC circuit; and  
         [0010]    [0010]FIG. 2 is a functional circuit diagram of an AFC circuit with direct, fractional-N PLL control of frequency errors. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    Modern mobile communication terminals use Phase Locked loop (PLL) frequency synthesizers to control the receive and transmit channel frequencies. The general architecture of a PLL-based Automatic Frequency Control (AFC) circuit for downconverting a received RF signal to baseband is depicted in FIG. 1, indicated generally at  10 . A transmitted RF signal at frequency f RF  is received at antenna  12 , amplified by RF low-noise amplifier  14 , and mixed with frequency f OUT  at mixer  16  (multiple conversion (&gt;1) hetrodyne receivers may include additional downconversion mixers). The frequency f OUT  is generated by a Phase Locked loop  20 , based on a reference frequency f REF  generated by a crystal (XTAL) oscillator  30  and its associated control circuit  38 . The mixer  16  outputs sum and difference components (f RF +f LO ) and (f RF −f LO ), which are digitized by Analog to Digital Converters (ADC)  32 , and processed by a baseband processor  34 . The baseband processor  34  detects and extracts information symbols, and detects frequency errors in the baseband signal frequency through the average speed of rotation of symbols in the I,Q constellation, as is known in the art.  
         [0012]    The operation of PLL  20  is well understood by those skilled in the art, and is only briefly described herein. The PLL  20  comprises a phase-frequency detector (PFD)  22  that includes a charge pump (CP)  23 , a loop filter  24 , a Voltage Controlled Oscillator (VCO)  26 , and a frequency divider  28 . In steady-state condition, the PLL  20  maintains a constant (in most cases zero) average phase difference at the input of the PFD  22 . If a phase difference exists between the reference frequency input from the XTAL oscillator  30  (f REF ) and divided VCO output frequency input (f OUT /N), the charge pump  23  issues net charge pulses at its output that get filtered by the loop filter  24 . The resulting voltage is applied to the control voltage node of the VCO  26  to generate the desired output frequency (f OUT ). The frequency divider  28  ensures that in the phase-locked condition:  
           f   OUT   =N*f   REF .   (1)  
         [0013]    As such, drifts in the reference frequency f REF  translate directly to errors in the output frequency f OUT . For example, a typical reference frequency f REF  at the XTAL oscillator  30  may be 20 MHz, with an accuracy of ±20 parts per million (ppm), or ±400 Hz. If the division ratio N of frequency divider  28  is 100, the resulting AFC output frequency f OUT  is 2 GHz. However, the error of 400 Hz at the XTAL  30 , is also multiplied by a factor of 100 in the PLL  20 , resulting in an error in the AFC output frequency f OUT  of up to ±40 kHz. The GSM protocol (one wireless communication standard), specifies a channel width of 200 kHz, with an allowed error of ±0.1 ppm, or ±200 Hz. In transmit mode, therefore, the system  10  of FIG. 1 is unable, without error control, to generate a transmit frequency signal that complies with the relevant protocol.  
         [0014]    One common mode of error control is to compensate for the error by altering the reference frequency f REF  of the XTAL oscillator  30  by closing the AFC loop through the DAC  36  and control circuit  38 . As described above, the baseband processor  34 , upon detecting a frequency error through the average speed of rotation of symbols in the I,Q constellation, may access a look-up table to obtain a digital representation of a control voltage V C . This digital value is output to a Digital-to-Analog Converter (DAC)  36  for conversion to an analog control voltage. The control voltage V C  is applied to a reference frequency correction circuit  38  (including one or more varicaps  40 ) that alters the effective capacitance seen by the XTAL oscillator  30 , correspondingly altering the reference frequency f REF . The altered f REF  will cause a phase error at PFD  22 , and the PLL  20  will adjust the output frequency f OUT  accordingly, as described above.  
         [0015]    Since the frequency divider  28  and PFD  22  are typically implemented using digital circuitry, the output frequency f OUT  can only assume integer multiples of the reference frequency f REF  (see equation 1). This in turn limits the minimum frequency step at the VCO  26  output frequency to f REF . For a variety of reasons, including locking times, reference spur amplitudes and loop stability, f REF  should be generally be chosen as high as possible. A common choice in integer-N synthesizers is therefore f REF =channel spacing.  
         [0016]    In order to increase the frequency resolution of the PLL  20  to frequency increments below f REF , various methods for the implementation of fractional-N frequency synthesizers have been proposed. Generally, these methods make use of the fact that fast changes in the feedback division ratio N (such as once every period of f REF ) can be averaged by the lowpass response of the PLL  20  itself. As an example, over a duration comprising ten reference cycles, if the division ratio is (N) during seven cycles, and (N+1) during the remaining three cycles, the average non-integer division ratio is (N+0.3). Traditionally, the periodic nature of the changes in the division ratio between N and (N+1) have led to spurious tones in the output spectrum of the PLL  20  that are difficult to suppress. Additionally, the smaller the output frequency step in the fractional-N PLL  20 , the higher the number of spurs in the output spectrum. Recently, a number of innovative, all-digital methods have been proposed to implement the timing sequences of changes to the feedback division ratio N in such a way that the impact on the spurious response and phase noise behavior is minimized. One such method is the use of a digital ΔΣ modulator (indicated at  42  in FIG. 1) for the division ratio control.  
         [0017]    Generally, the digital AY modulator  42  functions as an encoder that encodes a fixed or relatively slowly varying number F (the desired tuning frequency, represented by n-bits) into a fast stream of output values Δ(z). Typically Δ(z) is represented only by a small number of output bits of the quantizer  44  (in the extreme case only 1-bit). The transfer function 46 of H(z) inside the oversampling ΔΣ loop is chosen such that quantization and dithering noise is shaped towards high frequencies as much as possible. Typically, the quantization noise power spectrum has a maximum at ½ the sampling frequency, which in the PLL  20  application may be conveniently chosen to be equal to the reference frequency f REF , although this is not necessary for proper operation. Dithering by a value generated from pseudo-random number generator  48  is generally necessary in cases where the input to the modulator is not moving sufficiently fast to suppress limit cycles in the quantizer output sequence. This is the case in the PLL  20  application, as the input F corresponds to a particular channel number (or frequency) that does not change often. The ΔΣ modulator  42  is designed for a certain modulus Q that can either be programmable or predetermined by design. In essence, the feedback amplifier  50  with a gain of Q provides a means of scaling the output sequence Δ(z) such that its average assumes the value F/Q. An explanation of the detailed operation of the ΔΣ modulator  42  can be found in J. C. Candy &amp; G. C. Temes,  Oversampling Methods for AID and DIA Conversion,  in  Oversampling Delta-Sigma Data Converters: Theory, Design and Simulation,  pp. 1-29, IEEE Press, J. Wiley &amp; Sons, 1992, incorporated herein in its entirety.  
         [0018]    The output sequence of the ΔΣ modulator  42  is summed together with the integer portion of the desired fractional division ratio N, supplied by the baseband processor  34 , to form a sequence of momentary integer division ratios N(z)=N+Δ(z) that are applied to the feedback divider  28  in the PLL  20 . In a similar fashion to an integer PLL  20  as described above, the fractional-N PLL  20  will stabilize at an average zero phase error at the input to the PFD  22  and thereby generate the VCO  26  output frequency:  
           f   OUT   =f   REF *( N+F/Q ).   (2)  
         [0019]    In general, f REF =f XTAL /R, where f XTAL  is the output frequency of the XTAL oscillator  30 . However, in most cases R=1 is chosen for the benefit of improved noise shaping. Since the modulus Q of the ΔΣ modulator is an un-restricted design parameter, the frequency step size at the VCO output (f REF /Q) can be chosen by design or programmed dynamically.  
         [0020]    The AFC circuit  10  of FIG. 1 provides adequate performance, and in particular is able to correct for errors in the AFC output frequency f OUT  due to drifts or inaccuracies of the XTAL oscillator  30  via the frequency correction feedback path comprising the DAC  36  and reference frequency correction circuit  38 . However, as discussed above, these components add cost, consume board space, reduce reliability, and add computational complexity to the baseband processor  34  by requiring it to dynamically build and maintain a look-up table storing the complex relationship between environmental conditions and the proper frequency control voltage V C  to apply to the reference frequency correction circuit  38 .  
         [0021]    According to the present invention, the frequency correction feedback path comprising the DAC  36  and reference frequency correction circuit  38  may be omitted entirely, and frequency error control corrected directly by the fractional-N PLL  20 . The present invention exploits the fact that the frequency resolution f REF /Q of modern fractional-N PLLs  20  can be chosen almost arbitrarily low by providing a large enough fractional modulus Q, without compromising either the switching speed or the spurious performance of the AFC output signal.  
         [0022]    One implementation of the present invention is depicted in FIG. 2. In the AFC circuit  11 , similar to the prior art implementation shown in FIG. 1, the RF input signal at f RF  is received at antenna  12  and amplified at low-noise amplifier  14 . The signal is downconverted to the baseband by mixing with the output f OUT  of the fractional-N PLL  20 . The result is converted into the digital domain via ADC  32 , and received symbols are extracted from the data stream in the baseband processor  34 . The speed of rotation of the I,Q constellation is detected, and a digital frequency-error word ΔF is made available that represents the frequency error in f OUT  due to errors in f REF . Rather than utilize the detected frequency error to retrieve a control voltage value from a look-up table, covert it to an analog voltage V C , and use V C  to control the output frequency of the XTAL oscillator  30  f XTAL , the present invention controls the error directly, utilizing the fractional-N PLL  20  described above.  
         [0023]    The baseband processor  34  dynamically generates the parameters (N, F, ΔF). In the AFC circuit  11  of FIG. 2, the output frequency f OUT  is given by:  
           f   OUT   =f   XTAL *( N+F/Q+ΔF/Q ),  
         [0024]    or  
               f   OUT     =       f   XTAL     *     (     N   +       F   +     Δ                 F       Q       )               (   3   )                               
 
         [0025]    The value ΔF, which is used for direct AFC, is calculated from f ERR , the frequency difference word available from the speed of rotation detection in the baseband processor  34 . f ERR  is translated into the PLL  20  frequency correction word ΔF, such that preferably, the rotational frequency of the I,Q constellation vanishes to within the achievable resolution, by:  
         Δ F=Q *( f   ERR   /f   XTAL ).   (4)  
         [0026]    Since the fractional modulus Q of the PLL  20  is known by design, the uncompensated frequency error of the XTAL oscillator  30  is limited to the ppm-level and the PLL  20  frequency error is known from the measurement of the I,Q rotational speed, a one-step AFC correction may be achieved by applying the correction word ΔF to the PLL  20 . No tuning of the XTAL oscillator  30  is necessary.  
         [0027]    One embodiment of the present invention as outlined above implements equation 4 by use of an adder  52 . The adder  52  takes the sum of the channel-setting fractional word (F) and the error-correcting AFC word (ΔF) and applies it to the input of the ΔΣ modulator  42 . One of skill in the art will readily recognize, however, that the sum F+ΔF could alternatively be calculated within the baseband processor  34 . Additionally, the use of the ΔΣ modulator  42  as a separate component, and the external summation of N+Δ(z) are particular design choices that do not limit the scope of the present invention.  
         [0028]    As a quantitative example of the present invention, consider a mobile terminal transmission at 2 GHz, using a XTAL oscillator  30  with a frequency of 20 MHz ±20 ppm, or 400 Hz, as in the example above. Compliance with the GSM protocol dictates a maximum frequency error of 0.1 ppm, or 200 Hz for a 2 GHz transmission. To achieve this frequency from the 20 MHz XTAL oscillator  30 , the fractional division ratio of the frequency divider  28  in the PLL  20  must be ≈100. In this example, for convenience of implementation of the ΔΣ modulator  42 , Q may be chosen as 2 20 =1,048,576. The minimum uncorrected frequency increment at the PLL  20  output is then 20 MHz/2 20 =19.07348 Hz. Assuming the GSM channel spacing of 200 kHz, the increment in the fractional portion of the division ratio F from one channel to the next is ROUND (200 kHz/19.07348 Hz)=10,486. The frequency error correction word ΔF as determined from the AFC algorithm is then added to this number and updated as necessary to minimize the frequency error.  
         [0029]    Although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.