Patent Publication Number: US-8532590-B2

Title: Digital phase feedback for determining phase distortion

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to, and is a divisional of, U.S. patent application Ser. No. 12/251,169, filed on Oct. 14, 2008, the entire disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Polar loop transmitters have applications in many fields, such as radio, cellular radio, telecommunications, and the like. The term “polar loop” refers to a polar modulation transmitter architecture that applies closed-loop feedback control to both the phase and amplitude of a transmitted signal by using closed-loop control of the transmitted phase as well as the amplitude modulation. In general, an important issue in polar transmitter architectures is the measurement of amplitude and phase distortion in the transmit path (for example, in the power amplifier). In order to compensate for any amplitude and/or phase distortions, adaptive predistortion compensation can be applied to the modulation signal. However, dynamically compensating for distortions by using adaptive predistortion compensation requires feedback from the transmit signal so as to be able to dynamically measure and compensate for the transmit distortions. Due to the fact that the modulation signal is applied to the modulator in polar coordinates, it can be advantageous to have the feedback signal also in polar coordinates. Therefore, a phase feedback receiver and an amplitude feedback receiver may be used to determine the polar feedback signals for compensating for phase and amplitude distortions, respectively. 
     With respect to the phase feedback signal determination, a Cartesian feedback receiver may be used to convert the radio frequency (RF) feedback signal down to an analog baseband signal, and then successively convert the analog baseband signal to a digital signal using an analog-to-digital converter (ADC). Afterwards, in the digital domain, a Cartesian-to-Polar conversion can be performed. However, the use of a Cartesian receiver, in addition to requiring conversion to Polar coordinates, can be a cumbersome approach to extracting a phase signal. Another disadvantage of the use of a Cartesian receiver for down-conversion is the typically high current consumption of the receiver due to the high signal quality requirements of the Cartesian receiver. Furthermore, the requirements on the ADC can be significant, as well as the fact that two ADCs are required (i.e., for I &amp; Q paths) to extract phase when using the Cartesian receiver approach to phase extraction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawing figures, in conjunction with the general description given above, and the detailed description of the implementations given below, serve to illustrate and explain the principles of the implementations of the best mode presently contemplated. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. In the drawings, like numerals describe substantially similar features and components throughout the several views. 
         FIG. 1  is a circuit diagram illustrating an exemplary polar transmit architecture and phase feedback receiver implementation. 
         FIG. 2  is a flowchart illustrating an exemplary method for phase distortion determination in accordance with an exemplary implementation. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part of the disclosure, and in which are shown by way of illustration, and not of limitation, exemplary implementations. Further, it should be noted that while the detailed description provides various exemplary implementations, as described below and as illustrated in the drawings, this patent is not limited to the implementations described and illustrated herein, but can extend to other implementations, as would be known or as would become known to those skilled in the art. Reference in the specification to “one implementation”, “this implementation” or “these implementations” means that a particular feature, structure, or characteristic described in connection with the implementations is included in at least one implementation, and the appearances of these phrases in various places in the specification are not necessarily all referring to the same implementation. Additionally, in the following detailed description, numerous specific details are set forth in order to provide a thorough disclosure. However, it will be apparent to one of ordinary skill in the art that these specific details may not all be needed. In other circumstances, well-known structures, materials, circuits, processes and interfaces have not been described in detail, and/or may be illustrated in block diagram form, so as to not unnecessarily obscure the disclosure. 
     This disclosure includes various arrangements and techniques for determining digital phase feedback in a polar transmitter or other systems in which determining phase distortion information is useful. In particular, the techniques included involve implementing a circuit providing phase distortion extraction which can then be used, for example, to dynamically compensate for phase distortions in a signal. A disclosed exemplary circuit can be implemented in a variety of electronic or communication devices or other systems that may require phase distortion compensation. Devices that can benefit from the circuit include, but are not limited to, polar transmitters including mobile phone transmitters, such as GSM (Global System for Mobile communications) or UMTS (Universal Mobile Telecommunications System). Further, while the following systems and methods are described with reference to a polar transmitter, such as for use in a mobile communication device, it will be appreciated that the disclosed techniques and circuits can be implemented generally in any similar electronic/communication system. 
     Exemplary implementations, as will be described in greater detail below, convert the phase feedback signal directly to the digital domain. Thus, in the exemplary implementations, no additional Cartesian-to-Polar conversion is necessary. Additionally, exemplary implementations include direct extraction of phase distortion information for measuring the phase distortion, which information may then be used for determining phase predistortion compensation in a polar transmitter or other system. For example, in some implementations, the phase distortion information may be sent to a processing block or other device which is able to use the information to determine coefficients for adjusting phase modulation transmit characteristics, such as for reducing phase distortion in a polar transmitter or other system. 
       FIG. 1  illustrates a circuit diagram of a section of a polar transmitter including circuitry for phase distortion extraction according to exemplary implementations.  FIG. 2  illustrates a block diagram for a method of carrying out phase distortion extraction according to exemplary implementations. It should be noted that the order in which the blocks are described is not intended to be construed as a limitation, and any number of the described system blocks can be combined in any order to implement the system and method, or an alternate system and method. Additionally, individual blocks may be deleted without departing from the spirit and scope of the subject matter described herein. Furthermore, the system and method can be implemented in any suitable hardware, firmware, or a combination thereof. 
     Example Circuit 
       FIG. 1  illustrates an exemplary structure of a polar transmitter including a phase feedback receiver according to one possible implementation. Exemplary circuit  100  is meant to explain concepts related to isolation and measurement of phase distortion at a basic level and the number of components shown does not limit the actual implementation of circuit  100 . Exemplary circuit  100  includes a transmit signal path  101  and a phase feedback path  103 . In the transmit path  101 , a reference clock  102  connected to an RF phase modulator  106  passes a reference clock signal  104  to RF phase modulator  106 . This reference clock signal  104  is also delivered to a time-to-digital converter (TDC)  108  to serve as a reference signal r REF    110 , as discussed further below. 
     RF phase modulator  106  receives the reference clock signal  104  and also receives a frequency modulation signal  112  from a frequency modulation signal block  114 , and generates a phase-modulated RF carrier or transmit signal  116 . Thus, the frequency modulation signal block  114  applies frequency modulation signal  112  to RF phase modulator  106 . Phase modulator  106  receives frequency modulation  112  signal and reference clock signal  104  and produces phase-modulated transmit signal  116 . In some implementations, the phase-modulated transmit signal  116  is a modulated high frequency oscillating signal having an instantaneous frequency that is equal to the reference clock signal frequency multiplied by the frequency of the frequency modulation signal  112 . The modulation frequency signal  112  is applied digitally, so the RF phase modulator  106  generates the phase-modulated high frequency transmit signal  116  which is modulated according to digital modulation frequency signal  112  and, in some implementations, output to a mixer  118 . The modulated transmit signal  116  is then multiplied at mixer  118  with an amplitude modulation signal  120  received from an amplitude modulation signal block  122  to generate a multiplied transmit signal S TX    124 . Mixer  118  may be a multiplier, such as a Gilbert cell, or other device that carries out the same function. Alternatively, in some implementations, the modulated transmit signal  116  can be amplified directly by a power amplifier in place of mixer  118  and amplitude modulation signal  122  in order to generate the transmit signal  124 . The proposed architecture should measure any phase distortions produced by the described mixer, power amplifier, or any other non-ideal element in the transmit path. Phase distortions are often related to amplitude level and therefore can be named AMPM distortions (i.e., Amplitude Modulation to Phase Modulation distortions) 
     In exemplary implementations, a purpose of the phase feedback path  103  is to detect any phase distortion of the modulated transmit signal S TX    124  after multiplication by the mixer  118  or a power amplifier, i.e., phase distortion caused by the transmit path  101 . A coupler  126  therefore traces the signal output from the mixer  118  or power amplifier. The coupler  126  may be a directional coupler, which is used to send a signal in the forward direction, and which also provides a traced feedback signal S FB    128 , which is derived from the transmit signal S TX    124 , and which may be used in the feedback path  103  for determining phase distortion. In order to be able to detect the phase of feedback signal S FB    128 , a phase restoration block  130  is used to remove the amplitude information from feedback signal S FB    128 . Phase restoration block  130  may be, for example, a signal limiter or other device or arrangement configured to remove the amplitude information from feedback signal S FB    128 . 
     After passing through phase restoration block  130 , phase-restored feedback signal S PHI,FB    132  is still a high frequency signal, but now contains only phase information, including both the phase modulation and any phase distortion. In order to remove the phase modulation from signal S PHI,FB    132 , a multi-modulus divider (MMDIV)  134  may be used. MMDIV  134  divides the high frequency signal down to a lower frequency and at the same time removes the phase modulation. A divider ratio sequence used by MMDIV  134  for dividing the high frequency signal is generated by a Sigma-Delta modulator  136  as an input signal  138 . By causing the input signal  138  of the Sigma-Delta modulator  136  to be based on the frequency modulation signal  112 , the division carried out by MMDIV  134  is able to cancel out the original phase modulation of the transmit signal if a delay of the modulation signal  112  is matched to the delay of the feedback path  103 . Accordingly, in these implementations, a delayed frequency modulation signal  140  that is input to the sigma delta modulator  136  is exactly the same as the frequency modulation signal  112  which was used with the reference signal  104  in the RF phase modulator  106  to generate the transmit signal  116 . The only difference is that delayed modulation signal  140  is delayed by ΔT compared to the original frequency modulation signal  112  so as to match a delay in the feedback signal reaching MMDIV  134 . 
     A delay block delays the original frequency modulation signal  112  to compensate for the delay which is accumulated from the modulation signal input of the phase modulator  106  to signal S PHI,FB    132  that is input to MMDIV  134 . In exemplary implementations, delay block  142  is implemented as an all-pass filter, and the actual delay can be programmable. However, this function can be implemented by any other technique or device known in the art so that the delay achieved by the delay block matches the delay in the feedback signal S PHI,FB    132  in reaching MMDIV  134 , whereby the output signal  138  of the Sigma-Delta modulator  136  reaches MMDIV  134  at the same time as the feedback signal S PHI,FB    132  for matching the frequency modulation signal. 
     First, it may be assumed that the transmit path is ideal, i.e., does not add any phase distortion. The delayed frequency modulation signal  140  is received by Sigma-Delta modulator  136 , which outputs to the MMDIV  134  a divider ratio  138  (i.e., a divisor) corresponding to the ratio between the instantaneous carrier frequency and the reference frequency, and which also corresponds to ratio between the instantaneous frequency of the feedback signal S PHI,FB    132  and reference frequency, so that the carrier frequency is reduced to reference frequency and original phase modulation added by RF phase modulator  106  is removed. In exemplary implementations, Sigma Delta modulator  136  outputs an instantaneous digital integer value corresponding to the delayed instantaneous frequency modulation signal  140 , and this integer value is used as a divisor by MMDIV  134  to reduce the frequency of the feedback signal S PHI,FB    132  for removing the original phase modulation. Accordingly, after compensating for the delay in signal S PHI,FB    132  through use of the delay block  142 , the original phase modulation portion of the feedback signal S PHI,FB    132  is removed by MMDIV  134 , resulting in an output signal r PHI,FB    144 . The divider ratio changes with a rate which is a number of times higher than the bandwidth of the original modulation (oversampled) so that on average the division tracks the original modulation. The mean value of the chosen divisor is chosen to divide the carrier down to the reference clock rate. 
     Now the case will be considered in which the transmit path produces some phase distortion due to non-ideal analog components. The output signal r PHI,FB    144  output by the MMDIV  134  has a mean frequency that is equal to the frequency of the reference signal r REF    110  output by the reference clock  102 . Also included in the MMDIV output  144  is the phase distortion of the transmit path without the original phase modulation due to removal by MMDIV  134 . Accordingly, the phase of the output signal r PHI,FB    144  relates to the reference clock phase, including any constant phase shift, plus any phase distortion added by the transmit path  101 , i.e., the phase modulator  106 , the mixer  118 , and by the phase feedback path  103 . By comparing the output signal r PHI,FB  with the reference signal r REF    110 , the phase distortion caused by the phase modulator  106 , the mixer  118  (i.e., the transmit path  101 ) and phase feedback path  103  can be determined. Consequently, to be able to measure only phase distortion of the transmit path, any phase distortion caused by the phase feedback path  103  needs to be small in relation to the phase distortion caused by the transmit path elements, such as the mixer  118  or power amplifier, in order to not further distort the measurement result. The time difference of the MMDIV output signal r PHI,FB  with respect to the reference clock  110  can be quantized and converted to the digital domain by Time-to-digital Converter (TDC)  108  and output as a digital signal  148 . The TDC operates by comparing, for example, the rising edge of reference clock r REF    110  with the rising edge of r PHI,FB , producing an output which is a digital quantized number relating to time difference between rising edges. If there is no distortion in the feedback path, then the output of the TDC will be a constant number relating to any constant phase shift between reference and feedback signals. If, however, the transmit path includes some distortion, then this distortion will be present at the output of the TDC  108  in the form of a quantized time delta. 
     Additionally, Sigma-Delta modulator  136  may introduce additional noise to the phase of the output signal r PHI,FB    144 . Most of the energy of this noise however is at high frequencies due to the Sigma-Delta  136  characteristic, so the noise can be attenuated by a digital lowpass filter  150 . Accordingly, following this attenuation, the feedback path  103  generates as output a digital signal PHI FB    152 , which is proportional to the phase distortion of the transmit path  101  (except for a constant offset), as long as the additional distortion coming from the phase feedback path  103  is small. This difference phase signal PHI FB  can then be sent back to the RF phase modulator  106  to dynamically correct the phase error in the transmit signal (polar loop transmitter), or the signal can be used to calculate phase pre-distortion characteristics (predistortion) for use in compensating a transmit signal in a non-dynamic way. Accordingly, implementations are appropriate for applications including polar modulators, polar loop transmitters, pre-distortion systems, or any transmitter system where it is desirable to improve or have knowledge of the phase characteristics of the transmitted signal. 
     Example Method 
       FIG. 2  is a flowchart illustrating an exemplary method  200  for determining phase distortion in a polar loop transmitter, pre-distortion system, or the like. The method introduced may, but is not required to, be implemented at least partially in architectures such as illustrated in  FIG. 1 . The order in which the method below is described is not intended to be construed as a limitation, and any number of the described method blocks can be combined in any order to implement the method, or an alternate method. Thus, it is to be appreciated that certain acts in the method need not be performed in the order described, may be modified, and/or may be omitted entirely. Additionally, individual blocks may be deleted from the method without departing from the spirit and scope of the subject matter described herein. Furthermore, the method can be implemented in any suitable hardware, firmware, or a combination thereof. 
     At block  202 , a reference clock signal and a frequency modulation signal are used to generate a transmit signal. In an implementation, the transmit signal is generated by an RF phase modulator referenced to a reference clock, where the RF phase modulator generates the modulated transmit signal based upon the received frequency modulation signal and the reference clock signal. Optionally, the transmit signal may also be multiplied or amplified following modulation. 
     At block  204 , a trace signal is derived from the modulated transmit signal for use as a feedback signal. In an exemplary implementation, a coupler is used to derive the trace signal for use in a feedback loop. 
     At block  206 , phase restoration of the feedback single is carried out if the transmit signal was mixed or amplified following generation of the transmit signal and prior to derivation of the feedback signal. In an exemplary implementation, the transmit signal may pass through an amplifier or multiplier prior to derivation of the feedback signal from the transmit signal. In these implementations, a signal limiter may be used to remove amplitude information from the feedback signal. Block  206  is only necessary if the amplitude of the signal has been affected, such as through use of a multiplier or amplifier on the modulated transmit signal, or in any case where amplitude modulation is used to convey information. 
     At block  208 , the frequency modulation signal is delayed to match the delay of the feedback signal. In an exemplary implementation, an allpass filter is used to delay the frequency modulation signal. 
     At block  210 , a divisor is derived from the delayed frequency modulation signal. In an exemplary implementation, a Sigma-Delta modulator is used to determine a divider ratio to use as a divisor for reducing the frequency of the feedback signal, whilst removing the original frequency modulation. 
     At block  212 , the frequency of the feedback signal is divided by the divisor determined in block  210  to remove the modulation from the feedback single so as to obtain a feedback signal having only the phase distortions plus the reference phase. In an exemplary implementation, a multi modulus divider receives the divider ratio information from the Sigma Delta modulator, and the multi-modulus divider uses the divisor to divide the higher frequency single down to a lower frequency. 
     At block  214 , the feedback signal is compared to a reference signal to determine a time difference representing the phase distortion portion, and the phase distortion portion is output as a digital signal in the form of a quantized time delta. In an exemplary implementation, the feedback signal output from the multi-modulus divider is delivered to a time-to-digital converter which uses the reference clock to extract the time delta from the feedback signal and convert the time delta portion to a digital output signal. Thus, the output signal from the multi-modulus divider is compared with a reference signal from the reference clock to determine the phase distortion of the modulation path. Optionally, in some implementations, the digital signal representing the phase distortion is then passed through a low pass filter to remove any noise added by the Sigma-Delta modulator. Accordingly, the method results in a digital difference phase signal that represents the phase distortion produced by the transmit path, assuming that any phase distortion produced by the feedback path is small compared to the overall phase distortion. 
     Exemplary implementations provide advantages that include that the phase feedback receiver does not measure the absolute phase of the transmit signal but only the difference from an ideal phase modulation signal. This greatly reduces the resolution requirements of the time-to-digital conversion. Due to the fact, that the ideal phase modulation signal is exactly known, because it is the same signal as that which is applied at the RF phase modulator  106 , the generated difference signal very closely equals the actual phase distortion. Furthermore, exemplary implementations do not require a dedicated ADC, let alone the two ADCs that are required when using a Cartesian demodulation approach to extract phase. Instead, in exemplary implementations, only a single TDC is used. 
     As will be apparent from the foregoing disclosure, implementations provide for a phase feedback path which determines the phase distortion of a transmit signal by using a phase restoration block, a multi modulus divider whose divider ratio is switched by a Sigma-Delta modulator, and a time to digital converter. The known phase modulation is removed from the feedback signal by the multi modulus divider, so that the TDC measures only the time difference between the feedback signal and the ideal reference signal generated from the reference clock, therefore producing a measure of phase distortion. This determined time difference representing the phase distortions added primarily by the transmit path can be used for adaptive predistortion compensation calculations in order to compensate for the phase distortions of the transmit path (e.g., from the modulator and multiplier or power amplifier). 
     Further, it should be noted that the system configuration illustrated in  FIG. 1  is purely exemplary of systems in which the implementations may be provided, and the implementations are not limited to a particular hardware configuration. In the description, numerous details are set forth for purposes of explanation in order to provide a thorough understanding of the disclosure. However, it will be apparent to one skilled in the art that not all of these specific details are required. 
     From the foregoing, it will be apparent that methods and apparatuses for determining the phase distortion of a transmit signal are provided. Additionally, while specific implementations have been illustrated and described in this specification, those of ordinary skill in the art appreciate that any arrangement that is calculated to achieve the same purpose may be substituted for the specific implementations disclosed. This disclosure is intended to cover any and all adaptations or variations of the disclosed implementations, and it is to be understood that the terms used in the following claims should not be construed to limit this patent to the specific implementations disclosed in the specification. Rather, the scope of this patent is to be determined entirely by the following claims, which are to be construed in accordance with the established doctrines of claim interpretation, along with the full range of equivalents to which such claims are entitled.