Abstract:
A phase rotator for a Cartesian feedback power amplifier in a transmitter final stage contains an integrated voltage controlled tunable resonant circuit accomplishing band-pass filtering at a center frequency selected by local oscillator (LO) coarse trim control signals. The voltage controlled tunable resonant circuit attenuates input signal harmonic levels at large fractional bandwidths for the downconverter in the feedback LO path without setting a large number of poles in the band-pass filter. The binary-weighted course trim value for controlling the gain of the LO sets a bank of voltage-variable capacitors (VVC) in the voltage controlled tunable resonant circuit to control the center frequency in each of two 2-pole band-pass filters, creating a composite 4-pole band-pass filter at the input of a poly-phase quadrature generation circuit in the feedback LO path.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention generally relates to radio transmitters, and in particular relates to an apparatus and method for band-pass filtering in the transmitter feedback loop.  
         [0003]     2. Description of the Related Art  
         [0004]     Standards relating to radio communications such as TETRA (Terrestrial Trunked RAdio), UMTS (Universal Mobile Telecommunications System) and EDGE (Enhanced Data Rates for GSM Evolution) generally require a high degree of linearity in transmitter equipment to reduce noise between closely spaced radio channels. Linearization of power amplifiers in transmitter equipment has been extensively researched and many techniques such as Cartesian loop, Polar loop, Envelope Elimination and Restoration, LINC (LInear amplification using Nonlinear Components) and CALLUM (combined analog locked loop universal modulator) have been produced.  
         [0005]     Linearity and bandwidth are traded off in these techniques, with high linearity being possible over narrow bandwidth and moderate linearity over a broader bandwidth. Most techniques also trade linearity for efficiency. Power amplifiers used in radio transmitters are more efficient when operated at higher power but then have lower linearity, particularly near their peak power ratings. These techniques are less satisfactory for mobile communications that require both high linearity and also high efficiency for longer battery life and lower weight.  
         [0006]     The Cartesian loop technique involves negative feedback applied to a baseband input signal having in-phase and quadrature components. The feedback signal is a measure of distortion introduced in the forward path of the loop, primarily by the amplifier, and is subtracted from the input signal in real time. This modifies the input signal with an error signal that tends to cancel the distortion at the output of the amplifier.  
         [0007]     In order to obtain a stable feedback system, it is required that the feedback quadrature signals are approximately 180 degrees shifted relative to the incoming quadrature signals when the feedback loop is closed. Due to the unknown and variable phase shift generated by the feedback loop under antenna loading, this condition is not always fulfilled. The incoming and feedback quadrature signals are usually brought into the required relative phase with each other with the aid of a compensating phase rotation in the feedback loop. A common method to determine the phase rotation generated by the feedback loop is to open the loop and to measure the incoming in-phase and quadrature signals (I/Q), and the feedback quadrature signals. The measured values are analog-to-digital converted, and the phase error is calculated. Thereafter, a voltage-controlled phase rotator is regulated by the converted digital values to apply a phase shift in Cartesian loop systems to counter radio frequency (RF) delays around the feedback loop.  
         [0008]     To ensure the accuracy of the phase rotation, existing Cartesian loop circuits provide noise filtering to remove the additional noise and distortion introduced by the Cartesian loop. However, Cartesian feedback loop band-pass filter designs can be complicated because quadrature generation circuits require strong filtering of input signal harmonic levels to preserve quadrature balance. Moreover, different channels usually require different phase shifts and different optimum settings, further complicating the filter designs. To achieve adequate attenuation of the input signal harmonic levels, while still supporting the large fractional bandwidths required in such systems, band-pass filters for phase rotators have traditionally increasing the number of polls and, correspondingly, the complexity and cost of the band-pass filters. What is needed is a band-pass filter integrated into the Cartesian feedback loop that filters the input signal harmonic levels at large fractional bandwidths, while remaining a low-complexity filter design. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced, and in which like numbers represent the same or similar elements, as follows:  
         [0010]      FIG. 1  generally depicts a Cartesian feedback transmitter in accordance with a preferred embodiment.  
         [0011]      FIG. 2  shows a more detailed block diagram of selected components of the Cartesian feedback transmitter, in accordance with a preferred embodiment of the present invention.  
         [0012]      FIG. 3  shows a more detailed block diagram of selected components of the phase rotator for a Cartesian feedback transmitter, in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0013]     With reference now to the figures, and in particular with reference to  FIG. 1 , a Cartesian feedback transmitter in accordance with a preferred embodiment can be seen as generally depicted by the reference numeral  10 . The transmitter  10  includes generally a first and second information signal path  11  and  12 , a combined information signal path  13 , first and second feedback paths  14  and  15 , and a phase adjustment unit  16 .  
         [0014]     The first and second information signal paths  11  and  12  are identical to one another in this embodiment. They differ only in that the first path  11  receives an in-phase base-band information input signal  17 , whereas the second path  12  receives a quadrature base-band information input signal. Therefore, only the first path  11  will be described in detail.  
         [0015]     The first information path  11  couples its input  17  to a differential summer  19 . The remaining input to this differential summer  19  couples to the first feedback path  14 . The summer output couples to a variable gain base-band amplifier  21  that itself couples through a low-pass filter  22  to a summer  23 . Variable gain base-band amplifier  21  provides gain control of the information path  11  by gain control signal  43 . The remaining input to this summer  23  couples to the input  17  to support open loop operation. The output of the summer  23  passes through another amplification stage  24  to a mixer  26 , which up-converts the incoming base-band signal to a predetermined carrier frequency of choice. The injection signal for the mixer  26  is provided by a quadrature generator  28  modulated by local oscillator (LO)  27 , with the second information path  12  receiving an injection signal that has been phase shifted by  90  degrees by quadrature generator  28 .  
         [0016]     The outputs of both information paths  11  and  12  couple to the inputs of a summer  29  that represents the input to the combined information signal path  13 . The output of the summer  29  couples to the input of an exciter  31  and then through a power amplifier (PA)  32  to an appropriate output element  33 .  
         [0017]     A coupler  34  responsive to the output of the PA  32  provides a feedback signal to both the first and second feedback paths  14  and  15 . The up-converted signal as obtained from the PA output is first down-converted through appropriate RF feedback downconverters  36  and  37 , and then provided to the subtractive inputs of the first and second information signal path differential summers  19  as mentioned above. The down-conversion injection signals for the RF feedback downconverters  36  and  37  are provided in quadrature by quadrature generator  38  under the provision of an appropriate phase shift by a phase shift unit  16 .  
         [0018]     The phase shift unit  16  provides comparators  39  and  44  to detect phase differences between the two inputs  17  and  18  and the two feedback paths  14  and  15 , and to provide any differential information to a control unit  41  that in turn controls a phase rotator  42  that couples between the quadrature generator  28  and the quadrature generator  38  to provide a phase shift to the quadrature signals received from quadrature generator  28  and then applied to the inputs of quadrature generator  38 , which generates the injection inputs for the radio frequency (RF) feedback downconverters  36  and  37 .  
         [0019]     With reference now to  FIG. 2 , there is shown a more detailed block diagram of phase rotator  42 , quadrature generator  38 , RF feedback downconverters  36  and  37 , and local oscillator  30 , in accordance with a preferred embodiment of the present invention. Local oscillator  30  generates the local oscillation (LO) signals used as injection signals into up-mixers  26  and phase rotator  42 . An oscillation signal is generated by VCO  27  at four-times (4×) the system operating frequency set by Coarse Trim Bits at input  90  to VCO  27 . The Coarse Trim Bits represent a 4-bit value specifying a desired center frequency for the output transmissions of transmitter  10  on element  33 . The VCO oscillation signal is output by VCO  27 , amplified by amplification stage  91 , and received at quadrature generator  64 , which is a divide-by-four quadrature generator to generate quadrature components (I/Q) at one-fourth (¼) the frequency of the VCO oscillation signal. As will be appreciated, local oscillator  30  can be set to any desired operating frequency by tuning VCO  27  to a corresponding frequency. As an example, VCO  27  generates a four gigahertz (4 GHz) oscillation signal that is divided down to one gigahertz (1 GHz) quadrature LO signals (I/Q) by quadrature generator  64 . Quadrature generator  64  generates a cosine synthesizer signal (Cosine LO) and a sine synthesizer signal (Sine LO) that is buffered by amplifier stages  92 ,  93 , respectively, and thereafter coupled to mixer  60 ,  62 , respectively. Quadrature generator  64  also outputs Cosine LO and Sine LO to up-mixers  26  through amplifier stages  94  and  95 .  
         [0020]     Quadrature modulator  50  receives a cosine phase rotation value  51  and the sine phase rotation value  53  from a lookup function in control circuitry  41 . Cosine value  51  and sine value  53  are coupled to digital-to-analog converters (DACs)  52  and  54 , respectively, which are in turn coupled through buffers  56 ,  58  to mixers  60 ,  62 , respectively. Mixers  60 ,  62  up-convert the incoming phase rotation values  51 ,  53  to a 45 degree angle to the predetermined carrier frequency of choice as a function of the injection signals Cosine LO and Sine LO provided by local oscillator  30 .  
         [0021]     The quadrature generated Cosine and Sine LO signals are mixed with the cosine and sine lookup signals  51 ,  53  at mixers  60 ,  62  and their outputs summed at summer  66  following amplification at buffers  61 ,  63 , respectively. Summer  66  sums the input from buffer  61  as a negative value and the input from buffer  63  as a positive value to generate a differential output  67  from quadrature modulator  50 . The differential output  67  from quadrature modulator  50  is coupled to a buffer  68  generating a RF differential pair output on differential connection  70 , which is further amplified by buffer  77  as an output from phase rotator  42  on connection  72 .  
         [0022]     For optimal performance, phase rotator  42  requires rejection of the harmonics generated by the quadrature generator  64  prior to injection into poly-phase quadrature generator  38  at the inputs to RF feedback downconverters  36 ,  37 . A 4-pole band-pass filter function is fully integrated into the feedback LO path by inclusion of parallel-connected LC band-pass filters  80  and  82 . While BPF&#39;s  80 ,  82  are shown integrated into phase rotator  42  in a preferred embodiment, other embodiments have the band-pass filters integrated outside the phase rotator or as discrete components connected in parallel with the injection signals into the down-conversion mixers.  
         [0023]     Phase rotator  42  includes a set of tunable, band-pass filters (BPFs)  80 ,  82  coupled in parallel with differential connections  70  and  72 , respectively, to form an integrated, tunable 4-pole BPF, in accordance with a preferred embodiment of the present invention. Although two BPFs  80 ,  82  are shown in a preferred embodiment, the present invention is not limited to such a configuration, and may be implemented with a single BPF or any number of BPFs. In accordance with the preferred embodiment, BPFs  80 ,  82  are parallel LC filters tunable to a capacitive value providing an optimized 4-pole filtering function across a bandwidth centered at the feedback LO path output frequency.  
         [0024]     Each of band-pass filters  80 ,  82  is a 2-pole band-pass filter that is fully tunable by Variable Voltage Control at an input  78  receiving the Coarse Trim Bits used to set the operating frequency of VCO  27 . For example, if VCO  27  has its Course Trim Bits set to 1 GHz, the resulting 4 GHz VCO output signals are divided by four by quadrature generator  64  and output as Cosine LO and Sine LO centered at 1 GHz. Accordingly, band-pass filters  80  and  82  are set by Variable Voltage Control to a band-pass centered on the output frequency of quadrature modulator  50 . In the above example, Variable Voltage Control input  78  would set the center frequency of BPFs  80 ,  82  at 1 GHz.  
         [0025]     With reference now to  FIG. 3 , there is shown a more detailed block diagram of BPFs  80 ,  82  and buffers  68  and  77 , in accordance with a preferred embodiment of the present invention. Buffer  68  amplifies the input from quadrature generator  50  on a RF differential pair  75 ,  76  over connection  70 . The amplified signal is filtered by parallel connected BPF  80  and received by buffer  77 , which in turn further amplifies the quadrature signals on differential outputs over connection  72 . The quadrature signals are further filtered by parallel connected BPF  82  on the output of phase rotator  42 . Band-pass filters  80  and  82  are identically designed, so only band-pass filter  80  will now be described in detail.  
         [0026]     Within band-pass filter  80  are inductors  84 ,  86  coupled to the supply voltage (Vcc) at first ends thereof. Inductor  84  is also coupled at its second end to a first differential output  75  of connection  70 , and inductor  86  is coupled at its second end to a second differential output  76  of connection  70 . A resonant tank circuit  88  is coupled between connections  75  and  76 , and employs a variable reactive stage, such as voltage-variable capacitors (VVC), a varactor, or any other variable reactive device. Resonant tank circuit  88  is capable of receiving the binary-weighted Coarse Trim Bits at control input  78 , which is comprised of four input bits specified as VVC bits (VVC  1 , VVC  2 , VVC  3 , VVC  4 ) in a preferred embodiment. The VVC bits are inputs to separate capacitor pairs within resonant tank circuit  88 . A corresponding VVC bit enables or disables a corresponding capacitor pair within resonant tank circuit  88 . The combined settings of VVC 1 - 4  defines the reactance of the BPF  82  by enabling or disabling the capacitor pairs, and thereby establishes a response for the resonant tank circuit  88 . Resonant tank circuit  88  is designed consistent with the format of the Coarse Trim Bits such that a selected transmitter frequency on VVC 1 -VVC 4  will set the resonant tank circuit  88  to a corresponding capacitance that in turn will tune the BPF  80  to a center frequency at the desired transmitter frequency. While four capacitive elements are shown in resonant tank circuit  88 , fewer or more capacitive elements controlled by fewer or more VVC bits may be utilized to accomplish the desired band-pass filtering function.  
         [0027]     It should be noted that, while a VVC is utilized in a preferred embodiment, the variable reactive stage in BPF  80  can be tuned by varying either or both of the capacitors and the inductive coils. However, it is preferable to use a variable voltage capacitor structure due to advantages of integration. Also, the VVC is more easily tuned by simply applying the appropriate voltage signals that are already present for course tuning the VCO. Further, while the preferred embodiment has been described as embodied within a Cartesian feedback power amplifier, it will be appreciated that the present invention applies to other transmitter architectures where it is desirable to control the phase rotation of a feedback signal from a linear power amplifiers, for example in polar modulation.  
         [0028]     While the invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. Any variations, modifications, additions, and improvements to the embodiments described are possible and may fall within the scope of the invention as detailed within the following claims.