DC offset correction system for a receiver with baseband gain control

A method and apparatus are provided for providing improved radio frequency (RF) receiver signal correction. For RF receiver circuitry (106) having receive path and a warmup period associated therewith and including at least one analog baseband gain control stage (218) having a gain associated therewith, the method includes the step of performing a DC correction calculation operation during the warmup period to derive a DC correction value having a first component and a second component for each of the at least one gain control stage (218). The DC correction calculation step includes the steps of performing a first closed loop correction (460) of a baseband path to derive the first component of the DC correction value and performing a second closed loop correction (462) of the receive path as a function of the (218) gain during the warmup period to derive the second component of the DC correction value. During operation after the warmup period (464 , 466), an open loop correction is performed for instantaneous DC correction as function of the PMA (218) gain and the DC correction value.

FIELD OF THE INVENTION

The present invention generally relates to radio frequency (RF) receivers, and more particularly relates to DC offset correction in a receiver with baseband gain control.

BACKGROUND OF THE INVENTION

Baseband direct current (DC) offset correction is commonly used in zero intermediate frequency (ZIF) and very low intermediate frequency (VLIF) receivers to eliminate offsets due to local oscillator leakage, filter flicker noise, and circuit mismatches. This correction is necessary to avoid clipping effects within the baseband analog signal path as well as to meet the desired receiver performance under weaker signal conditions (e.g., receiver sensitivity). Baseband gain control with large coarse steps can be employed in a receiver to achieve the necessary receiver dynamic range at a low cost and current drain while minimizing the amount of RF gain control range required. However, in continuous data reception (i.e., non-time division multiplexed) receivers such as that for wideband code-division multiple access (WCDMA) receivers, there needs to be a mechanism to quickly and accurately compensate for large DC offset steps which occur as a result of large baseband gain steps. If this is not achieved, receiver performance degradation can occur due to long DC and clipping transients. The large baseband gain changes can occur not only due to a larger step size of the baseband gain control stage but also due to RF Low Noise Amplifier (LNA) gain replacements within the baseband gain control stage. The RF LNA gain replacement is necessary to avoid applying large gain steps at the input to the baseband filters as well as to maintain a constant signal level into the received signal detector. In addition, RF standards such as High Speed Downlink Packet Access (HSDPA), a packet-based data service 3.5 G WCDMA standard, expect a large constant signal-to-noise ratio at the receiver analog-to-digital input where this is also necessary.

Accordingly it is desirable to provide a method and apparatus for DC offset correction which quickly and accurately compensates for large DC offset steps which occur as a result of large baseband gain steps while maintaining a constant signal-to-noise ratio at the receiver analog-to-digital input. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, a block diagram of an electronic communication device100, such as a cellular telephone, is shown. Although the electronic communication device100is depicted as a cellular telephone, the electronic communication device can be implemented as a pager, a laptop computer with a wireless connection, a personal digital assistant with wireless connection, or the like. The electronic communication device100includes an antenna102for receiving and transmitting radio frequency (RF) signals. A receive/transmit switch104selectively couples the antenna102to receiver circuitry106and transmitter circuitry108in a manner familiar to those skilled in the art. The receiver circuitry106demodulates and decodes the RF signals to derive information and is coupled to a controller110for providing the decoded information thereto for utilization thereby in accordance with the function(s) of the electronic communication device100.

The controller110also provides information to the transmitter circuitry108for encoding and modulating information into RF signals for transmission from the antenna102. As is well-known in the art, the controller110is typically coupled to a memory device112and a user interface114to perform the functions of the electronic communication device100. Power control circuitry116is coupled to the components of the electronic communication device100, such as the controller110, the receiver circuitry106, the transmitter circuitry108and/or the user interface114, to provide appropriate operational voltage and current to those components. The user interface114may include one or more user interface devices such as a microphone, a speaker, key inputs, such as a keypad and a display for displaying information to a user and which may also accept touch screen inputs.

Referring toFIG. 2, a high level block diagram of a low cost and low power RF receiver106in accordance with the present invention has a receive path as shown and includes an RF frontend section208. In accordance with the preferred embodiment of the present invention, the RF receiver106is optimized for RF standards such as High Speed Downlink Packet Access (HSDPA), a packet-based data service 3.5 G WCDMA standard, which require maintaining a constant signal level into the received signal detector as well as a large constant signal-to-noise ratio at the receiver analog-to-digital input.

The RF frontend section208of the receiver106includes an RF duplexor filter210, a stepped Low Noise Amplifier (LNA) amplifier212and an RF band select filter214. The RF frontend section208amplifies the RF signal from the antenna102(FIG. 1) and selects the desired RF frequency band. The receive/transmit switch104(FIG. 1) and the RF front end section208and any other blocks located prior to the quadrature mixers216(such as switches, LNAs212, filters or other attenuator stages) form a pre-mixer RF lineup which can have a low gain state or a high gain state by adjusting the LNA212to a low gain state or a high gain state, or by adjusting the gain state at the receive/transmit switch104or by adjusting other parameters within the pre-mixer RF lineup.

Following the RF frontend section208, I/Q quadrature mixers216for each of the two quadrature paths (the imaginary (I) path and the quadrature (Q) path) are employed to convert the RF signal to a low or zero Intermediate Frequency (zero IF) by mixing the received signal with the output of a lowband oscillator (LO)215. Next, a baseband analog gain control stage such as a high dynamic range Post Mixer Amplifier (PMA)218is used to provide much of the desired receiver dynamic range to minimize the cost and the power consumption of the RF receiver106. This can be accomplished by using a stepped PMA stage218using coarse steps such as three decibels (dB) over a large control range (e.g., forty-five dB). After the PMA stage218, active analog filter stages220are used to provide the necessary clipping protection against interferers and also to provide anti-aliasing protection for the analog-to-digital (A/D) converters222. Digital filters224provide the necessary channel filtering and droop/group delay compensation. The PMA218, filters220, A/D converters222and digital filters224form a baseband path.

An automatic gain control (AGC)226dynamically controls the stepped LNA and PMA stages. The AGC226is programmed to maintain a desired operating range throughout the receiver106in a manner such as to avoid over-driving or under-driving any component thereof. In accordance with the present invention, the DC offset detection and correction230is performed digitally in a DC correction feedback path after the I and Q path A/D converters222. The correction values are applied through D/A converters232to the outputs of the PMAs218. Performing the DC correction method230digitally advantageously allows for higher performance (e.g., correction accuracy, loop stability, etc.) and lower cost and lower power implementations.

Unfortunately, large baseband gain steps in the PMA218due to LNA212gain replacement applied to the PMA218(e.g., sixteen dB step) and coarse PMA218steps (e.g., three dB steps) leads to large and long DC transients in the receiver baseband receive signal path. This phenomenon occurs due to the offsets related to the LO215leakage, the mixer216and the PMA218being proportionately scaled by the indicated large baseband gain changes. The specified DC transients can cause clipping in the baseband stages as well as degrade the receiver106Bit Error Rate performance for the duration of such transients. In addition, these large DC transients can cause non-linear behavior in baseband circuits with feedback (e.g., active filter stages and sigma delta A/D's). Also, in many receivers106such as that for High Speed Downlink Packet Access (HSDPA), a packet-based data service 3.5 G WCDMA standard operating at 16-QAM/64-QAM modulation, the DC notch (i.e, high pass filter corner) due to the closed loop DC offset correction system must be kept low (<<1 KHz) to not degrade the 0.1% Bit Error Rate receiver sensitivity performance. Hence, widening of this DC notch to compensate for the large baseband gain changes is not acceptable.

In accordance with the present invention, the baseband DC offset correction system architecture and digital correction algorithm230overcomes the difficulties of prior art correction systems by employing a two step calibration procedure which is performed during the receiver warmup process. This mixed signal control algorithm is able to quickly and accurately compensate for large DC offset steps which occur as a result of large baseband gain changes.

Referring toFIG. 3, a block diagram of the details of the invention depict a single quadrature channel (I or Q) in accordance with the present invention and includes a DC offset correction system which allows for dynamic immediate compensation for large DC offset steps introduced due to large baseband gain changes. Long DC transients that can otherwise occur due to large baseband PMA gain changes from either LNA gain replacements in the PMA stage or coarse PMA steps taken can be avoided. This dynamic open loop correction is achieved instantaneously in a manner such as to avoid any DC transients while the closed loop system is maintained in a held state. A typical mixed signal DC offset correction system architecture is described in U.S. Pat. No. 6,560,447. The present invention includes additional elements as described herein. Following the 1-or multibit A/D converter222, the feedback loop includes a digital loop filter410which is typically implemented using a leaky integrator first order loop filter structure. The closed loop bandwidth is set by appropriately setting the gain (K) of the loop filter410. Following the loop integrator410, the DC correction D/A converter232applies the correction value at the PMA218output in a closed loop fashion.

During the first step of the DC offset correction warmup sequence, the “DC Calibration Step1Enable” signal is set high to short to ground the input of the PMA. This allows the DC correction algorithm to calibrate out only those offsets due to the elements located at and after the PMA stage220(e.g., PMA, IF amplifier414, IF filter416, biquads418, output buffer420, comparator, and DC correction DAC232stages) signaling the multiplexer412to couple the input of the PMA218to ground. At the end of this calibration step, the high to low transition of the indicated “DC Calibration Step1Enable” signal causes a load pulse422to be generated to load the acquired DC offset located at the loop filter410output into the cal_offset register424. This acquired offset (cal_offset)424is then later used by the DC offset correction algorithm to dynamically only compensate for those offsets located at the PMA input whenever a large PMA gain change occurs to avoid any transients in the feedforward signal path.

Following the specified initial calibration step, the shorting to ground at the PMA412input is removed by signaling the multiplexer412to directly connect the mixer output to the PMA input. Subsequently, a second closed loop DC offset correction warmup step is performed to quickly calibrate out those additional offsets located at the PMA input due to LO215leakage as well as mixer216related static offsets. The result of this second closed loop correction step is loaded from the loop filter410into the integ_out register425at the completion of this second closed loop correction step. The load_integ_out signal generated by an external controller unit is used to load the integ_out register425when a pulse is present on this signal.

At the completion of the two-step warmup process of the present invention, the integrator410output will settle to a value which reflects:
integ_out=cal_offset+pma_in_offset
where pma_in_offset reflects only those offsets located at the PMA218input due to the LO215leakage and mixer216stage offsets. As mentioned earlier, cal_offset reflects those other offsets located in the baseband signal except for those at the PMA218input. During receiver106data reception, when the PMA218gain is altered due to either LNA212gain replacements or coarse PMA218gain steps applied, a load pulse is generated from the PMA gain change detect logic426. This is done to dynamically compensate for only those DC offsets located at the PMA218input to avoid DC transients by updating the current loop integrator register428content with:
integ_update=pma_gain_chg*(integ_out−cal_offset)+cal_offset
where integ_update reflects the new value that will be stored into the integration register428to compensate for the PMA218gain change. Also, pma_gain_chg is the detected PMA218gain change on a linear scale after dB to linear conversion at circuit430. Substituting for integ_out based upon the previous equation, results in:
integ_update=pma_gain_chg*(cal_offset+pma_in_offset−cal_offset)+cal_offset=(pma_gain_chg*pma_in_offset)+cal_offset

Thus, based upon the above result, the circuit of the present invention advantageously only compensates for those offsets at the PMA218input using the detected PMA218gain change value and not for those offsets located after the PMA stage. Alternately, if integ_out (which also includes offsets due to stages after the PMA218at the completion of the second closed loop DC correction) had been scaled using the PMA218gain change amount, it would lead to additional DC transients in the feedforward signal path depending upon the amount of offsets located after the PMA218stage.

The delay element (z−X)432shown on the PMA218control path is employed to time align the dynamic DC offset compensation applied at the DC correction D/A232output with the PMA218gain change application to avoid any DC transient that can otherwise occur due to timing misalignment of the indicated DC offset compensation at the output of the subtractor430. In addition, the “dB to linear” conversion block430shown converts the PMA gain change value (which is typically on a dB scale) to a linear scale since the DC correction signal path operates on a linear scale. The digital DC correction signal is applied to the D/A converter232and thence to the DC correction subtractor430for application of the correction value to the signal.

Referring toFIG. 4, the flow chart450summarizes how the two-step DC offset correction warmup process can be used as part of a practical receiver warmup process. Note that the LNA's212and the receive/transmit switch104, when possible, are placed in a low gain state during the two-step DC offset correction warmup process460,462to minimize the related warmup times since a strong on- or off-channel signal can degrade the acquisition performance of the closed loop system. During the first step of the warmup sequence460, the input of the PMA is shorted to ground to perform a first closed loop calibration of the baseband path. During the second step of the warmup sequence462, the PMA218input is directly connected to the mixer output and the PMA is placed in a high gain mode. A second closed loop calibration of the receive path is performed with these settings. Note that the high PMA gain setting maximizes the offsets located at the PMA218input into the A/D222input and this ensures the desired correction performance under receiver sensitivity conditions (where DC correction performance is most crucial).

After DC correction warmup455, the closed loop DC offset correction system is placed in the open loop correction mode464and is held in that mode by placing the DC correction feedback path in a hold mode, holding the DC correction value in the register428. During normal operation of the AGC system466, an open loop correction method is employed to dynamically compensate for DC offset steps introduced by large baseband gain changes instantaneously by performing an open loop correction in response to altering of the PMA218gain.

FIG. 5shows the PMA218circuit topology in accordance with the preferred embodiment of the present invention. The PMA218circuit topology achieves a constant DC offset due to the PMA218stage alone across the PMA218gain control range. If the DC offset due to this PMA218stage varied significantly over the PMA218control range, additional warmup calibration steps310,312would have been required for each PMA218gain setting. This is undesirable since there is a desire to minimize receiver warmup times to maximize battery life of the wireless device.

Gain control is achieved in this PMA218by steering current away from virtual ground of the operational amplifier510to differential virtual ground in 3 dB steps. The biggest contributor to DC offset is mismatch in input differential stage of the operational amplifier510. Since the gain control is achieved through attenuation in front of the amplifier510, the topology512,514provides nearly constant DC offset over the whole AGC226range. When signal “DCOC” is logic 1, PMA218inputs are AC shorted and a common mode voltage is fed into the input of the PMA218to calculate static offset of the lineup over the AGC226range.

The plots ofFIGS. 6,7and8depict system simulation results for the DC offset correction system of the present invention.FIG. 6shows an example600of a long transient lasting greater than 200 chips period (>pb 52 us) that can occur at the receiver A/D converter222if there is no dynamic compensation for PMA218gain changes. The output of the PMA218is shown in plot602, the output from a 1-bit feedback comparator is shown in plot604, the DC correction summer440output in plot606, the receiver A/D converter222input in plot608, and the digital offset correction D/A converter232output in plot610. In600, a PMA218gain change of sixteen dB is applied at time612to compensate for an LNA212gain change. Also, a high DC offset correction loop bandwidth attempts to minimize the DC transient as much as possible without going into a loop oscillatory condition. In plot608(signal at A/D input), it can be seen that at time612there is a significant DC transient (approximately 52 us duration) even with the loop running in a high bandwidth mode. This long acquisition transient can also be seen at time612in plot608(signal at receiver A/D converter222) while reacting to the sixteen dB PMA gain change. Such a long DC transient degrades the receiver performance over the specified DC transient period.

FIGS. 7 and 8depict700,800using the specified gain compensation algorithm for offsets located only at the PMA218input to eliminate DC transients at the receiver A/D input222, thereby highlighting the correction behavior of the two-step DC correction process of the present invention. Referring toFIG. 7, the output of the PMA218is shown in plot702, the feedback comparator output is shown in plot704, the DC correction subtractor440output in plot706, the receiver A/D converter222input in plot708, and the digital offset correction D/A converter232output in plot710. Shorting of the PMA218input is depicted during time period712to calibrate out and store away offsets in the baseband path located after the PMA218in the cal13offset register424and, next in the warmup process462, correcting out the additional offsets located in the receive path, such as LO215leakage and mixer216offsets (with the PMA218input connected to the mixer216) can be seen inFIG. 7, particularly the second longer upward sloping line in plot710. At time714, the PMA218gain is reduced by sixteen dB to compensate for an LNA212state change. Also, the DC correction DAC output plot710shows how the DAC232output is instantaneously reduced by a particular amount at time714to compensate for only those offsets located at the PMA218input due to the specified PMA218gain change. As a result of this instantaneous open loop correction at the DC correction DAC232output, notice there is not a transient in the A/D222input plot708unlike what occurs in600(FIG. 6). This exemplifies the instantaneous correction behavior of the open loop correction algorithm of the present invention to eliminate such undesirable transients which can otherwise occur as a result of large PMA218gain changes.

The plot800ofFIG. 8shows how DC transients are avoided at the receiver A/D222input at time814even when the offsets located after the PMA218stage (e.g., IFA, IFF, biquads, output buffer, and DC correction DAC) are larger than those offsets located at the PMA218input. Similar toFIGS. 6 and 7,FIG. 8includes the output of the PMA218shown in plot802, the feedback comparator output shown in plot804, the DC correction summer440output in plot806, the receiver A/D converter222input in plot808, and the digital offset correction D/A converter232output in plot810.

Thus, the present invention includes a method for improved DC correction in radio frequency (RF) receiver circuitry having a warmup period and a receive path associated therewith, wherein the RF receiver circuitry includes a DC correction feedback path and a baseband path including at least one baseband analog gain control stage having a gain associated therewith. The at least one baseband analog stage may include at least one post mixer amplifier (PMA).

The method includes the step of performing a DC correction calculation operation during the warmup period to derive a DC correction value having a first component and a second component for each of the at least one gain control stage, wherein the step of performing a DC correction calculation operation during the warmup period includes the steps of: performing a first closed loop correction of the baseband path to derive a first component of the DC correction value and performing a second closed loop correction of the receive path as a function of the gain during the warmup period to derive a second component of the DC correction value. The method also includes the step of during operation of the RF receiver circuitry after the warmup period performing an open loop correction as a function of the gain and the DC correction value to perform instantaneous DC correction. In addition, the step of performing an open loop correction includes the steps of placing the DC correction feedback path in a hold mode and, in response to altering of the gain, performing an open loop correction as a function of the DC correction value.

In accordance with another embodiment of the present invention, the at least one baseband analog control stage includes an input and an output and the step of performing a first closed loop correction to derive a first component of the DC correction value includes the steps of connecting the input of the at least one baseband analog control stage to ground and storing a baseband path closed loop DC offset value as the first component of the DC correction value. The RF receiver circuitry additionally includes a pre-mixer RF lineup coupleable to the at least one baseband analog gain stage and having a high gain state and a low gain state and the at least one baseband analog gain stage having an input and an output. The step of performing a second closed loop correction as a function of the gain during the warmup period to derive a second component of the DC correction value includes the steps of coupling the pre-mixer RF lineup to the input of the at least one baseband analog gain stage, setting the gain of the at least one baseband analog gain stage to a high gain, setting the gain of the pre-mixer lineup to a low gain state, and storing the second closed loop DC offset value as the second component of the DC correction value.

The RF receiver circuitry may also include a Low Noise Amplifier (LNA) coupled to the at least one baseband analog control stage and having a high gain state and a low gain state, and the step of performing a first closed loop correction to derive a first component of the DC correction value would further include the step of placing the LNA in the low gain state before the step of storing a baseband path closed loop DC offset value. The RF receiver circuitry may additionally include a Low Noise Amplifier (LNA) coupled to the at least one baseband analog control stage and having a high gain state and a low gain state and at least one mixer coupled to the LNA, each of said at least one mixer having an input and an output, wherein the step of performing a second closed loop correction as a function of the gain during the warmup period to derive a second component of the DC correction value would include the steps of coupling the output of the at least one mixer to the input of the at least one baseband analog gain stage, setting the gain of the at least one baseband analog gain stage to a high gain, placing the LNA in the low gain state, and storing a receive path closed loop DC offset value as the second component of the DC correction value.

The step of storing the receive path closed loop DC offset value also includes the step of storing the receive path closed loop DC offset value to derive a stored DC correction value, wherein the stored DC correction value is a combination of the stored first component of the DC correction value and the receive path closed loop DC offset value which is the second component of the DC correction value. The method further includes the step of during operation of the RF receiver circuitry after the warmup period performing an open loop correction as a function of the gain and the stored DC correction value, wherein the step of performing an open loop correction may include the step of performing an open loop correction as a function of the gain, the stored DC correction value and a predetermined delay, wherein the predetermined delay corresponds to a time duration necessary to align altering of the gain to application of the DC correction value to the receive path.

The RF receiver circuitry further includes a DC correction feedback path and the step of performing an open loop correction includes the steps of placing the DC correction feedback path in a hold mode, enabling operation of the automatic gain control, and performing an open loop correction as a function of the gain and the stored DC correction value. The step of performing an open loop correction also includes the step of performing an open loop correction as a function of the stored DC correction value in response to altering of the gain.

The present invention is a radio frequency (RF) receiver circuitry having a warmup period associated therewith. The RF receiver circuitry includes at least one baseband analog gain control stage having a gain associated therewith, the at least one gain control stage having a first input for receiving a control signal to control the gain thereof, a second input and an output. Automatic gain control circuitry is coupled to the at least one gain control stage for generating and providing the control signal thereto and DC offset correction circuitry is coupled to the at least one gain control stage for receiving information therefrom. During the warmup period the second input of the at least one gain control stage is coupled to ground to perform a first closed loop correction to derive a first component of a DC correction value, the first component of the DC correction value being stored in the DC offset correction circuitry.

The RF receiver circuitry of Claim also includes a lowband oscillator (LO) and at least one mixer, each of the at least one mixer having an input coupled to the LO and an output coupleable to one of the at least one baseband analog gain control stage. During the warmup period the second input of each of the at least one gain control stage is coupled to one of the at least one mixer to perform a second closed loop correction as a function of the gain to derive a second component of the DC correction value. Also, during operation after the warmup period, the second input of each of the at least one gain control stage is coupled to one of the at least one mixer to perform an instantaneous open loop correction as a function of the gain and the DC correction value, wherein the DC correction value consists of a combination of the first component and the second component thereof. The RF receiver circuitry also includes two quadrature paths, an imaginary (I) path and a quadrature (Q) path, wherein each of the two quadrature paths comprise one of the at least one baseband analog gain control stage and one of the at least one mixer. The RF receiver circuitry further includes a Low Noise Amplifier (LNA) coupled to the at least one PMA and the automatic gain control circuitry, the LNA having a high gain state and a low gain state.

The DC offset correction circuitry includes storage means for storing the DC correction value and, when performing the second closed loop correction during the warmup period to derive the second component of the DC correction value, the automatic gain control circuitry signals the at least one gain control stage to set the at least one gain control stage to a high gain and signals the LNA to place it in the low gain state and a closed loop DC offset value is stored in the storage means as the second component of the DC correction value, combining with the first component to derive the DC correction value. The at least one gain control stage also includes at least one post mixer amplifier (PMA) having a PMA gain, wherein during the warmup period the second input of the at least one PMA is coupled to the at least one mixer to perform a second closed loop correction as a function of the gain to derive a second component of the DC correction value.