The present invention relates to reducing the effects of DC offset in electronic systems, and in particular, to circuits and methods that may be used to reduce DC offset in an electronic system such as a wireless receiver.
Electronic systems often include many different components that operate using voltages and currents, which are typically characterized according to whether or not they change periodically over time. Voltages and currents that do not change periodically over time are referred to as “direct current” (“DC”) signals, and voltages and currents that do change periodically over time are referred to as “alternating current” (“AC”) signals. FIG. 1 illustrates an AC signal, a DC signal and DC offset. FIG. 1 shows three waveforms. Waveform 101 is a purely AC waveform because the voltage, V1, varies periodically (in this case, sinusoidally) over time and is centered on zero volts. Waveform 102 is a purely DC waveform because it maintains the same voltage, V2, over time. Waveform 103 illustrates an AC waveform with a DC offset. Waveform 103 varies with time, but it is shifted to a voltage V3. In this case, waveform 103 is a sinusoid that is offset by a voltage V3. FIG. 2 illustrates the frequency spectrum of an AC waveform with a DC offset such as waveform 103. For instance, waveform 103 may be a sinusoidal signal oscillating at a frequency of f1. Therefore, as shown in FIG. 2, waveform 103 will have a frequency component at f1 and another component at zero frequency (i.e., DC).
In many electronic systems it is desirable to process only the AC components of signals and not the DC component. FIG. 3 illustrates an amplifier circuit that processes both the AC and DC components of a signal. Amplifier 300 may receive a varying voltage Vin as an input and generate an output Vout that is an amplified version of the input. In this example, Vin is a sinusoidal signal with a peak-to-peak amplitude of 400 mV and a DC offset of 1 volt. If amplifier 300 provides a gain of 10, the output Vout will be a sinusoidal signal having a peak-to-peak amplitude of 4 volts and a DC offset of 10 volts.
FIG. 4 illustrates one of the problems caused by DC offsets in an electronic circuit. Non-ideal amplifiers require a power supply Vdd and often include some inherent DC offset. For example, amplifier 400 is powered by a 12 volt supply and has an input referred DC offset of 150 mV, which will increase the DC offset of an input signal Vin by 1.5 volts if amplifier 400 has a gain of 10. Thus, if amplifier 400 receives a sinusoidal signal with a peak-to-peak amplitude of 400 mV and a DC offset of 1 volt, the output should be a sinusoidal signal with a peak-to-peak amplitude of 4 volts and a DC offset of 11.5 volts. However, since the power supply Vdd of amplifier 400 is only 12 volts, the output signal cannot swing to its maximum value of 13.5 volts (i.e., 11.5 v+2v) because the amplifier output is limited to a maximum value of Vdd (often less). Consequently, the output signal will reach a maximum value of 12 volts, which is referred to as “clipping.” Thus, the DC offset introduced by amplifier 400 can result in severe distortion of the AC component of the signal. This is just one of many problems caused by unwanted DC offsets.
FIGS. 5A-B illustrate problems caused by DC offset in an analog-to-digital converter. In this example, amplifier 510 receives an input signal Vin and provides an analog output signal Vout to the input of analog-to-digital converter (“ADC”) 520. ADC 520 converts the analog signal into binary (i.e., digital) values carried by N digital signal lines (i.e., where N is an integer). Both amplifier 510 and ADC 520 may, but not necessarily, be powered by the same supply Vdd. To optimize the conversion of the analog signal, it is desirable to use the full range of the ADC. When the full range is used, more bits are available to represent the analog input signal values. However, when the full range is not used, fewer bits are available to represent the analog signal values, and the digital representation of the signal is less accurate. The range of the ADC is optimized by making Vout as close to the full range of the ADC as possible (the full range of an ADC is typically, but not necessarily, a little less than Vdd).
FIG. 5B illustrates two signals 501 and 502. Signal 501 is a sinusoidal signal with a DC offset of one-half Vdd (“half-supply”). If the full range of the ADC is from zero volts to Vdd, then signal 501 may be accurately converted because signal 501 varies substantially across the full range, which in this case is an equal amount both above and below half-supply. However, as illustrated by signal 502, when an unwanted DC offset is introduced in a signal, the signal cannot use the full range of the ADC. For instance, signal 502 is a sinusoidal signal with a DC offset of three-fourths Vdd (i.e., 3Vdd/4). Therefore, signal 502 is limited to a maximum amplitude of one-fourth Vdd. Consequently, half the range of the ADC is lost because of the DC offset.
DC offsets are caused by a variety of phenomena. One source of DC offset is from second order harmonics generated by components of an electronic system. For example, if a transistor receives a sinusoidal input signal Vin (e.g., on a gate terminal), the output signal Vout (e.g., on a drain terminal) will typically include some harmonic distortion. The following equations represent the output of an electronic component as a series to illustrate DC offset generated by harmonic distortion:Vout=AVin+BVin2+CVin3+ . . .If the input, Vin, is a sinusoidal signal having a frequency ωc, then:Vout=A Sin(ωct)+B Sin2(ωct)+C Sin3(ωct)+ . . .Referring to the second term above, which is the second harmonic, the DC offset can be seen as follows:B Sin2(ωct)=B[½−Cos(2ωct)/2]It can be seen that the second harmonic introduces a DC component of B/2. Thus, second order harmonic is one source of DC offset in an electronic system.
Another source of DC offset in electronic systems is mismatch between electronic components. For example, if resistors are mismatched in a differential system, bias currents through the different resistances may produce a constant voltage difference in the system. More generally, mismatches between electronic components in amplifiers, current sources and other electronic circuits may cause the components operate at different DC operating points. These non-ideal operational conditions of the components often result in a DC offset in the system.
DC offset is an important factor in many applications, but it is particularly important in the design and operation of wireless receivers. FIG. 6 illustrates an existing technique for reducing DC offset in a “direct conversion” wireless receiver. Wireless receiver 600 includes an antenna 610 for receiving RF signals. Antenna 610 is coupled through a switch 601 to a low noise amplifier 611 (“LNA”), mixer 612, filter 614, variable gain amplifier 615 (“VGA”) and analog-to-digital converter 616 (“ADC”). LNA 611 is used for amplifying high frequency signals from antenna 610 and must have sufficient bandwidth, gain and noise performance to meet system requirements. The local oscillator signal (“LO”) is generated by frequency synthesizer 630. Mixer 612 receives a local oscillator signal (“LO”) at the carrier frequency and down converts the input signal. Filter 614 is used to extract the signal of interest from the down converted signal, and VGA 615 provides appropriate gain so that the input to ADC 616 is optimizing the ADC's full range. The output of the reception channel is coupled to baseband processor 620 over N-bit digital signal lines, for example, for decoding and further processing.
DC offset in a wireless receiver may have many sources in addition to the sources described above. For example, one source of DC offset is from unwanted coupling (sometimes referred to as “leakage” or “feedthrough”) of the local oscillator (“LO”) signal into other parts of the receiver. The LO signal is typically a strong signal, and as such may couple into the communication channel and back into antenna 610. The LO signal may also couple to the input of LNA 611. In both cases the LO signal is boosted by the high gain of the LNA and, consequently, received by mixer 612 on both inputs. This is referred to as “self-mixing.” Self-mixing may also occur when the LO signal couples directly to the input of mixer 612. When the LO signal self-mixes with itself, the DC offset voltage generated at the output of mixer 612 may be very large. For instance, when the LO signal is received on both inputs of mixer 612, the LO signal is multiplied by itself. The DC offset generated by this phenomena can be seen from the following equations wherein the LO signal is modeled as a sinusoidal signal having a frequency ωc:
            Vout      ⁢                          ⁢      mixer        =          Vin      ⁢                          ⁢      1      *      Vin      ⁢                          ⁢      2                                                              Vout              ⁢                                                          ⁢              mixer                        =                        ⁢                          Sin              ⁢                                                          ⁢                              (                                                      ω                    c                                    ⁢                  t                                )                            ⁢                                                          ⁢              Sin              ⁢                                                          ⁢                              (                                                      ω                    c                                    ⁢                  t                                )                                              ;                                          ⁢                      self            ⁢                          -                        ⁢            mixing                                                        =                    ⁢                                    Sin              2                        ⁡                          (                                                ω                  c                                ⁢                t                            )                                                                    =                    ⁢                                    [                              1                -                                  Cos                  ⁢                                                                          ⁢                                      (                                          2                      ⁢                                              ω                        c                                            ⁢                      t                                        )                                                              ]                        /            2                                                        =                    ⁢                                    1              2                        -                          Cos              ⁢                                                          ⁢                                                (                                      2                    ⁢                                          ω                      c                                        ⁢                    t                                    )                                /                2                                                        Thus, the mixer output includes a constant component (i.e., ½) that has zero frequency. This term represents a DC offset at the output of the mixer resulting from self-mixing of the LO signal. Similarly, frequency components of the RF input signal may couple from the input channel to the LO input of the mixer. Such components will also self-mix and result in additional DC offset at the mixer output.
DC offset at the output of the mixer in a wireless receiver can have severe consequences on system performance. Typically, wireless receivers are designed to detect very low level signals, and therefore typically have very high gain. VGA 615, for example, may have a gain of 50 dBv or more (i.e., dBv=20 log10(Vout/Vin)), which would increase a DC offset at the mixer output by a factor of over 300. Moreover, in some applications an ADC may have a power supply as low as Vdd=1.2v or less, with a dynamic range on the order of hundreds of millivolts (e.g., 250 mV). Therefore, for accurate conversion of the analog signal, a maximum DC offset of less than a hundred millivolts may be required. This would result in a maximum allowable DC offset at the mixer output of less than a few hundred microvolts. For example, for a maximum allowable offset of 75 mV at the input of the ADC, the maximum DC offset at the output of the mixer would be about 250 μV for a VGA with a gain of 300. While these values are only an example, they clearly illustrate the importance of DC offset cancellation in electronic systems such as a wireless receiver. DC offset cancellation (i.e., DC offset reduction) is thus an important consideration in the design of electronic systems.
FIG. 6 further illustrates one existing approach to removing DC offset from a wireless receiver. According to this approach, the system is calibrated during a calibration cycle using a feedback loop. During the calibration cycle, the ADC measures the DC offset and passes the DC offset value as a digital signal to baseband processor 620. Baseband processor, in turn, provides a DC offset feedback signal to a digital-to-analog converter 621 (“DAC”). The output of the DAC is subtracted off the DC offset in the channel at 622. There are many disadvantages to the DC offset cancellation approach shown in FIG. 6. In particular, feedback loops can be slow, unstable and have limited accuracy. For instance, feedback loops always have some inherent delay around the loop, and some applications may require that the DC offset be eliminated within a period of time that is too short to accommodate such delays. Additionally, as the speed of a closed loop system is increased, such systems tend to become less stable. Moreover, accuracy of existing approaches may be compromised by the limited resolution of the ADC sampling the DC offset, as well as by the accuracy with which the digital system compensates for the DC offset (e.g., if the digital system generates a DC signal to subtract from the DC offset, the accuracy of the DC signal may be limited by the digital-to-analog conversion process). Furthermore, if DC offset varies with gain, calibration would require a different correction for each possible gain setting, which will make the system much more complex.
Thus, there is a need for improved circuits and methods for reducing DC offset, and in particular, for improved circuits and methods that may be used to reduce DC offset in wireless receivers.