Abstract:
A technique for cancelling out target IM2 components in a wireless receiver&#39;s mixer output is disclosed. A differential RF signal and a differential local oscillator (LO) signal are mixed by a mixer to demodulate the RF signal. A first common node signal is generated between a first resistor and a second resistor coupled across the mixer&#39;s differential output terminals. A second common node signal is generated between a third resistor and a fourth resistor coupled across the differential output terminals, where a capacitor is coupled between the second common node and a power supply terminal. The second common node signal provides a stable reference signal for IM2 components above a certain frequency. The two common node signals are subtracted to create a difference signal. The difference signal is scaled by a scaling factor obtained during calibration. The scaled difference signal is coupled to the mixer output to offset IM2 distortion.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a circuit in a wireless receiver&#39;s demodulator that substantially cancels second-order intermodulation distortion (IM2) in a mixer&#39;s output. 
       BACKGROUND 
       [0002]    Direct conversion RF receivers are widely used. In such a receiver, the transmitted modulated RF signal is downconverted by mixing the RF signal with a local oscillator (LO) signal having about the same frequency as the carrier. The mixer subtracts the LO signal from the RF signal, leaving the baseband signal for further processing by the receiver. When two frequencies are mixed within a circuit generating some inherent distortion, or if there is some RF interference, intermodulation (IM) products exist. Some IM products can be filtered out and others are difficult to accurately filter out. In the case of a direct conversion RF receiver, a second-order intermodulation (IM2) component frequency is likely to be very close to, or within the bandwidth of, the baseband signal, making it difficult to filter out. Hence, it is desirable to provide a non-filtering circuit to remove such IM2 distortion. 
         [0003]    Various methods have been used to remove such IM2 distortion in a differential demodulator. Such methods, however, are fairly complex and require significant silicon real estate to fabricate. The IM2 canceller of U.S. Pat. No. 8,000,676, for example, uses two feedback loops and requires two reference voltages to be generated to offset IM2 distortion of a common mode signal in a demodulator circuit of a direct conversion RF receiver. 
         [0004]    What is needed is a simpler and smaller IM2 canceller for a differential demodulator. 
       SUMMARY 
       [0005]    One embodiment of the invention is a direct conversion wireless receiver that receives a modulated RF signal. The RF signal is converted into a differential RF signal. A differential mixer has differential inputs connected to receive the RF signal and a differential LO signal of a frequency about the same as the carrier signal. The differential outputs of the mixer are connected to a power supply voltage Vcc via matched load resistors. The baseband differential output is taken across the load resistors. 
         [0006]    The mixer mixes the RF signal and the LO signal to generate a differential baseband signal. The differential baseband signal may be differential positive and negative signals, ideally relative to a stable DC value. Due to inherent distortion by the mixer, or due to RF to LO leakage interference, IM2 products are generated, which are to be desirably cancelled. The IM2 products may be present in the output as common-mode and differential signals. The common-mode IM2 signal is substantially cancelled by differential baseband amplifiers or differential analog-to-digital converters that are later stages in the signal path, but any residual differential IM2 signal is not cancelled. The IM2 products generated by the inherent distortion of the mixer cause the output common-mode signal to vary, such as with the amplitude of the RF signal, rather than be zero or a stable DC voltage. This common-mode IM2 signal can be used to cancel the differential IM2 distortion in the differential baseband signal. 
         [0007]    The invention detects the difference between the varying common-mode signal and a reference voltage and directly compensates the baseband signal based on this difference signal to effectively cancel the IM2 products of interest in the baseband signal. 
         [0008]    A first resistor and matched second resistor are connected in series across the mixer&#39;s differential outputs. The signal at the common node of the two resistors is connected to the input of a first differential amplifier. In an ideal receiver, this first common node signal would be a fixed DC common-mode signal. However, due to inherent distortion by the mixer, or due to interference, this first common node signal will vary, distorting the baseband output of the mixer. 
         [0009]    A third resistor and matched fourth resistor are also connected across the differential mixer output, and their common node is connected to Vcc via a capacitor, effectively creating an AC ground above a certain frequency. Thus, this second common node of the third and fourth resistors will be referenced to DC. This second common node signal acts as a reference voltage and is applied to the other input of the first differential amplifier. The first differential amplifier thus outputs a differential signal corresponding to the difference between the two common node signals. 
         [0010]    During initial calibration of the receiver during fabrication, the user performs a test on the receiver to derive a scaling factor between −1 to +1 to apply to the output of the first differential amplifier, where the scaled output of the first differential amplifier is applied to the differential baseband signal (the mixer output) to cancel out the IM2 signal. If the user wishes to cancel out IM2 distortion caused by RF interference, the user simulates the interference with a 2-tone test and generates a scaling factor to cancel out the calculated IM2 distortion in the baseband signal. The user may employ a digital signal processor to calculate the IM2 effects on the baseband signal under the test parameters, then generate the scaling factor needed to offset the IM2 effects. 
         [0011]    During operation of the receiver, the user applies the scaling factor, such as by applying a certain voltage to one or more pins of the receiver. The receiver then dynamically offsets the IM2 distortion as the first common node signal varies during operation and differs from the stable second common node signal. 
         [0012]    Instead of the mixer output being a baseband signal, it may be an intermediate frequency (IF) signal. Other embodiments are described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  illustrates one embodiment of the invention incorporated in a wireless RF receiver. 
           [0014]      FIG. 2  is a flowchart identifying certain steps employed in the invention. 
           [0015]      FIG. 3  is a transistor level schematic diagram that corresponds to the circuit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  illustrates one embodiment of the invention incorporated in a direct conversion wireless receiver  10  that receives a modulated RF signal. In one embodiment, the demodulator portion  12  is formed on a single IC except for the capacitor C 1 . The capacitor may be formed on-chip or off-chip. The receiver  10  may be a cell phone receiver, a keyless entry receiver in an automobile, or part of any other device. 
         [0017]    An antenna  16  receives an RF signal, which will typically be a carrier modulated by a baseband signal. The baseband signal may amplitude-modulate the carrier, frequency or phase-modulate the carrier, or modulate the carrier in another manner. 
         [0018]    A bandpass filter  18  passes the RF frequencies of interest to a low noise amplifier  20 . A transformer  22  creates a differential RF signal, RF+ and RF−. 
         [0019]    A local oscillator (LO)  24  generates a frequency that may be about the midpoint of the bandwidth of interest in the RF signal. The LO signal is converted to a differential LO signal, LO+ and LO−, by a transformer  26 . 
         [0020]    In the example used, there may be undesirable RF interference by RF signals close to the RF signal of interest, such as by RF/LO leakage. Such a combination of frequencies will create IM2 products having a frequency close to the baseband frequency or within the bandwidth of the baseband signal. IM2 products may also be generated by distortion when mixing the LO and RF signals or by other attributes in the receiver  10 . The IM2 products of interest cannot be filtered out due to their proximity to the baseband signal. 
         [0021]    The RF signal is downconverted (or demodulated) by a mixer  28 , which subtracts the LO frequency from the RF carrier frequency, leaving the baseband signal, although somewhat distorted by the IM2 products. 
         [0022]    The mixer  28  may be any conventional design. The output of the mixer  28  is a differential baseband signal with the carrier removed. The differential output terminals of the mixer  28  are connected to the power supply voltage Vcc via matched load resistors R 1   a  and R 1   b , which will typically be 50 ohms. The baseband differential output is taken across the load resistors R 1   a  and R 1   b . In  FIG. 1 , the terminals Voutp and Voutm convey positive and minus differential baseband signals, relative to a common mode voltage, and are provided on pins on the demodulator  12  IC. 
         [0023]    The baseband signal may be optionally low pass filtered to remove any undesired signals of higher frequency than the band of interest. The differential baseband signal is then processed in a conventional manner. 
         [0024]    A resistor R 3   a  and matched second resistor R 3   b  are connected in series across the mixer&#39;s differential outputs. The signal at the common node  32  of the two resistors is connected to the input of a differential amplifier  34 , having a fixed gain. In an ideal receiver, this common node  32  signal would be a fixed DC signal. However, due to inherent distortion by the mixer  34 , or due to interference, this common node  32  signal will vary due to IM2 products, distorting the baseband output of the mixer  28 . In one embodiment, Vcc is 5 volts, and the range of the baseband signals is 2-5 volts. If the baseband signal is digital (either 2 volts or 5 volts), and the performance is ideal, the common node  32  should be a relatively stable 3.5 volts. 
         [0025]    A resistor R 2   a  and matched resistor R 2   b  are also connected across the differential mixer  28  output, and their common node  36  is connected to Vcc via a capacitor C 1 , effectively creating an AC ground above a certain frequency. Thus, this common node  36  will be referenced to DC at a sufficiently high frequency (preferably the minimum frequency of the IM2 components). This common node  36  signal acts as a DC reference voltage and is applied to the other input of the differential amplifier  34 . The differential amplifier  34  thus outputs a differential signal corresponding to the difference between the two common node signals. In other words, the output of the differential amplifier  34  corresponds to the distortion in the common-mode of the differential baseband signal due to the IM2 products of interest. If the baseband signal is digital (2 volts or 5 volts), then the common node  36  should be at a relatively stable 3.5 volts if the IM2 frequency is high enough. 
         [0026]    The values of resistors R 2   a , R 2   b , R 3   a , and R 3   b  are preferably matched for simplicity. The matched value of resistors R 2   a , R 2   b , R 3   a , and R 3   b  is much greater than the value of the load resistors R 1   a  and R 1   b  so as not to significantly attenuate the signal. In one embodiment, the value of the resistors R 2   a , R 2   b , R 3   a , and R 3   b  is 1 Kohm, and the load resistors R 1   a  and R 1   b  are 50 ohms. 
         [0027]    As the amplitude of the RF signal changes, or if the RF interference changes, the common node  32  signal will vary due to a change in the IM2 products; however, the common node  36  signal should not change and acts as a reference. The output of the differential amplifier  34  reflects the distortion due to the IM2 products, but the output of the differential amplifier  34  needs to be scaled before being applied to the baseband signal. 
         [0028]    During an initial calibration of the receiver  10  during fabrication of the receiver  10 , the user performs a test on the receiver  10  to derive a scaling factor between −1 to +1 to apply to the output of the differential amplifier  34 , where the scaled output of the differential amplifier  34  is applied to the differential baseband output to cancel out the IM2 distortion under the test conditions. Thus, the proper scaling factor is that which results in minimum IM2 distortion under the calibration conditions. If the user wishes to cancel out IM2 distortion caused by RF interference, the user simulates the expected interference with a 2-tone test and generates a scaling factor to cancel out the effects of IM2 distortion on the baseband signal. A similar test is performed if the user desires to cancel IM2 distortion due to distortions generated by the receiver circuitry itself. The user may employ a digital signal processor (DSP) to calculate the IM2 effects on the baseband signal under the test parameters, then the user adjusts the scaling factor needed to offset the calculated IM2 effects by subtracting from or adding to the baseband signal. One-dimensional search algorithms such as golden-section search, backtracking, or Newton&#39;s method can be used. 
         [0029]    During operation of the receiver  10 , the user applies the scaling factor to the output of the differential amplifier  34 , such as by applying a certain voltage to one or more pins of the receiver. The receiver then dynamically offsets the IM2 distortion in the mixer output as the common node  32  signal differs from the stable common node  36  signal. 
         [0030]    In the example of  FIG. 1 , the scaling of the output of the differential amplifier  34  is performed by a multiplier  38 , which may be conventional. A differential amplifier  40  receives IM2_Ref and IM2_Set signals, derived by the DSP as previously described, and outputs the scaling factor between +1 and −1 (depending on the relative magnitudes of the IM2_Ref and IM2_Set signals). In one embodiment, the user supplies the fixed IM2_Ref and IM2_Set signals via pins on the demodulator  12  IC. Other methods may be used to generate a value between +1 and −1 to scale the output of the differential amplifier  34 , based on an empirical test of the receiver  10 . 
         [0031]    The scaled differential signals are then applied to the differential baseband signal to offset the IM2 products of interest as the common node  32  signal varies during operation by the receiver  10 . 
         [0032]    The system is calibrated (to derive the scaling factor) using IM2 components having a frequency much higher than the cutoff frequency of the resistor-capacitor network, where the impedance of the capacitor C 1  is effectively zero. Therefore, IM2 components with a frequency substantially higher than the cutoff frequency of the resistor-capacitor network will be accurately offset. At much lower frequencies, where the impedance of the capacitor C 1  is significantly greater than zero, the common node  36  signal will not have the proper amplitude and phase, so the IM2 cancellation will not be as precise. The values of the capacitor C 1  and resistors are adjusted to set the cutoff frequency. This is described in more detail below. 
         [0033]    The 3 dB cut-off frequency (or corner frequency) of the capacitor-resistor combination is given as: fk=1/((R 2   a ∥R 2   b )*C 1 *2*pi). 
         [0034]    IM2 components with a frequency below fk will not be amplified by the differential amplifier  34  with the right phase and amplitude if IM2 calibration is done using a signal with IM2 component frequencies &gt;&gt;fk. Therefore, the IM2 cancellation will not be as effective. 
         [0035]    The frequency response of the filter using R 2   a , R 2   b  and C 1  is given by: H(f)=1/(1+j*(f/fk)), assuming R 1   a , R 1   b &lt;&lt;R 2   a , R 2   b.    
         [0036]    The IM2 cancellation capability will follow the same shape since the differential amplifier  34  and multiplier  38  are assumed to have constant gain vs frequency. 
         [0037]    Therefore, at the corner frequency fk, the IM2 components will only be cancelled by 3 dB. At 10*fk, the IM2 components will be cancelled by 20 dB. At 100*fk the IM2 components will be cancelled by 40 dB, if all other blocks are assumed to be ideal. 
         [0038]    In practical situations, the IM2 intercept point (IP2) improvement will be about 20 dB. Therefore, it is recommended to select the resistor R 2   a  and R 2   b  and capacitor C 1  values such that the corner frequency fk is less than one-tenth the minimum frequency of the IM2 components of interest. 
         [0039]    A very low corner frequency fk will increase settling time, and a good trade-off needs to be made for fk. 
         [0040]    At very high IM2 frequencies, delay and gain roll-off of the differential amplifier  34  and multiplier  38  may adversely affect the cancellation for those IM2 components. 
         [0041]    The compensation circuit automatically adjusts for process variations, temperature variations, and supply voltage variations, due to the voltages at the common nodes  32  and  36  similarly tracking with such variations. 
         [0042]    Although the invention was described with respect to a direct conversion receiver where the output of the mixer is a baseband signal, it may also be applied to certain receivers that generate an intermediate frequency (IF). For example, the invention is useful when the receiver uses a low-IF and the baseband signal bandwidth is the same order of magnitude as its center frequency, or when the receiver uses an IF signal and the baseband signal bandwidth is much smaller than its center frequency. 
         [0043]    The demodulator  12  can be a part of many types of receivers, such as direct conversion quadrature receivers, where in-phase and quadrature signal paths are employed, and the invention is used in each path. The baseband signal may be digital or analog and may use FSK, ASK, or other standards. In one embodiment, the demodulator is a separate IC that is a component in a receiver. 
         [0044]      FIG. 2  summarizes one embodiment of method employing the present invention. It is assumed that the user wishes to calibrate the receiver to cancel IM2 components that occur due to an interfering RF signal at a frequency close to the desired RF signal. 
         [0045]    In step  101 , the user calibrates the receiver using a 2-tone RF signal to simulate the actual environment having the interfering signal. 
         [0046]    In step  102 , the user uses a DSP to identify the IM2 Ref and IM2 Set scaling voltages needed for cancelling the IM2 components derived during calibration. 
         [0047]    In step  103 , the user applies the IM2 Ref and IM2 Set scaling voltages to pins of the demodulator IC to create a scaling factor between +1 and −1. 
         [0048]    In step  104 , the receiver operates and mixes the received RF signal (containing interference) with a LO signal. 
         [0049]    In step  105 , the mixer generates a differential output signal (e.g., a baseband signal), and the resistors R 3   a  and R 3   b  provide a first common node signal that varies during operation, where the variation is related to the IM2 components. The first common node signal may vary with the RF signal amplitude due to the IM2 components. 
         [0050]    In step  106 , the capacitor C 1  and resistors R 2   a  and R 2   b  provide a second common node signal that is relatively stable due to C 1  acting as an AC short to the power supply. This signal acts as a reference voltage representing a lack of IM2 distortion. 
         [0051]    In step  107 , the two common node signals are applied to the differential amplifier  34  to generate a difference signal. 
         [0052]    In step  108 , the difference signal is scaled (or multiplied) by a scaling factor based on the calibration signals IM2_Ref and IM2_Set. 
         [0053]    In step  109 , the scaled difference signal is applied to the outputs of the mixer (assumed to be a baseband signal) to compensate for the IM2 distortion of the baseband signal. 
         [0054]    Other methods may also be employed depending on the nature of the IM2 distortion and other factors. 
         [0055]      FIG. 3  is a transistor level schematic diagram that corresponds to the circuit of  FIG. 1 . The circuit is self-explanatory to those skilled in the art after reading the present disclosure. 
         [0056]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications that are within the true spirit and scope of this invention.