Differential mixer injection with optional step gain control

[0001] The present invention relates to wireless communications, and in particular to a fully differential front end receiver architecture providing broadband performance.

[0002] Receiver front ends for wireless communication systems typically require a low noise amplifier (LNA) followed by a double-balanced mixer to pre-amplify incoming signals and subsequently down-convert those signals to an appropriate intermediate frequency (IF). Due to ever-present competitive concerns, there is a need to provide high performance and competitive pricing. Product cost is directly proportional to the number of components necessary to implement a design, and given the need to provide circuitry capable of performing over various frequency bands and in different modes, the ability to provide architectures capable of supporting multiple frequency bands without requiring redundant circuit elements for each band significantly reduces overall cost.

[0003] Unfortunately, providing architectures capable of performing over multiple frequency bands often requires more expensive components to maintain performance levels over a broad frequency range. Further, operation over multiple modes often subjects receiver circuitry to varying input signal levels, which often results in poor performance or distortion. Previous mixing circuits suffer from poor second-order intermodulation performance, and are unable to handle significant fluctuations in signal levels associated with the incoming signal.

[0004] Accordingly, there is a need for a broadband receiver front end capable of being implemented in a cost-effective manner while achieving high levels of performance. There is a further need for such a receiver capable of handling significant fluctuations in signal levels associated with the received signals.

[0005] The present invention provides a fully differential receiver front end including a differential low noise amplifier and a transformer-based mixer for down-converting a radio frequency (RF) signal to an appropriate intermediate frequency (IF) signal or baseband signal in a direct conversion system. The receiver front end has exceptional second-order intermodulation performance, and may include circuitry providing a reduced gain in order to handle incoming signals having high signal levels. The receiver front end is very broadband and is able to cover multiple communication modes or bands using a common switching core in the mixer. The invention is also applicable to quadrature-based receiver front ends.

[0006] In one embodiment, a differential RF input signal drives the plus and minus terminals of a center tapped primary winding, which couples the differential RF input signal to the plus and minus terminals of a center tapped secondary winding. From the secondary winding, the coupled, differential RF signal drives a double-balanced mixer core operating in response to a differential local oscillator signal. The output of the mixer core is a differential IF signal representative of the differential RF input signal down-converted at the local oscillator frequency.

[0007] A step reduction in gain associated with the RF input signal may be implemented using bypass circuitry coupled across the primary of the transformer, wherein a relatively small resistance or impedance is coupled across the primary to effectively reduce the load of the differential amplifiers driving the primary. Preferably, the bypass circuitry is implemented such that both windings of the primary are treated equivalently. In one embodiment, opposingly configured PMOS or NMOS transistors are used to implement the bypass circuitry, such that when the opposing transistors are turned on, the effective channel resistance is applied across a primary of the transformer. In an alternative embodiment, a symmetrically structured multi-fingered MOS device may be used as or in association with the bypass circuitry to provide a reduced gain when the RF input signal is at a high level. The MOS device can be realized in either NMOS or PMOS.

[0008] In quadrature-based systems, an additional quadrature mixer may be implemented by placing the primary of a second transformer in parallel with the primary of the first transformer to derive the quadrature phase IF signals. Preferably, any step gain control is provided simultaneously to the primaries of both transformers, such that the in-phase and quadrature phase IF signals are treated consistently.

[0009] Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

[0010] The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.

[0011] FIG. 1 is a block representation of a mobile terminal configured according to one embodiment of the present invention.

[0012] FIG. 2 is a schematic representation of a receiver front end architecture according to one embodiment of the present invention.

[0013] FIG. 3 is an alternative bypass circuit according to one embodiment of the present invention.

[0014] FIG. 4 is another alternative bypass circuit embodiment according to the present invention.

[0015] FIG. 5 is a schematic representation of a quadrature-based receiver front end architecture according to one embodiment of the present invention.

[0016] The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

[0017] The present invention is preferably incorporated in a mobile terminal 20, such a mobile telephone, personal digital assistant,,or the like. The basic architecture of a mobile terminal 20 is represented in FIG. 1 and may include a receiver front end 22, a radio frequency transmitter section 24, an antenna 26, a duplexer or switch 28, a baseband processor 30, a control system 32, a frequency synthesizer 34, and a user interface 36. The receiver front end 22 receives information bearing radio frequency (RF) signals from one or more remote transmitters provided by a base station (not shown). A filter 38 minimizes broadband interference in the received RF signal and a low noise amplifier (LNA) 40 pre-amplifies the filtered RF signal. The pre-amplified RF signal is then sent to a down-conversion and digitization circuitry 42, which down-converts the signal to one or more intermediate frequency (IF) signals and digitizes the IF signals into one or more digital streams for baseband processing. The receiver front end 22 typically uses one or more mixing frequencies generated by the frequency synthesizer 34.

[0018] The baseband processor 30 processes the digitized signals to extract the information or data bits conveyed in the received signal. This processing typically comprises demodulation, decoding, and error correction operations. As such, the baseband processor 30 is generally implemented in one or more digital signal processors (DSPs).

[0019] On the transmit side, the baseband processor 30 receives digitized data, which may represent voice, data, or control information, from the control system 32, which it encodes or modulates for transmission. The encoded data is output to the transmitter 24, where it is used by a modulator 44 to modulate a carrier signal that is at a desired transmit frequency. Power amplifier circuitry 46 amplifies the modulated carrier signal to a level appropriate for transmission from the antenna 26. The amplified signal is sent to the duplexer or switch 28 and antenna 26 through a matching network 48.

[0020] A user may interact with the mobile terminal 20 via the interface 36, which may include interface circuitry associated with a microphone, speaker, keypad, and display. The interface circuitry typically includes analog-to-digital converters, digital-to-analog converters, amplifiers, and the like. Additionally, it may include a voice encoder/decoder, in which case it may communicate directly with the baseband processor 30.

[0021] As illustrated in FIG. 2, the present invention provides a fully differential receiver front end section combining the functions of a low noise amplifier and a down-conversion mixer. The improved circuitry provides exceptional second-order intermodulation performance, and is capable of performing over a very broad band of frequencies, covering multiple frequency bands associated with different wireless communication technologies. As illustrated, a differential RF input signal (RF_IN RF_IN) drives the bases of two transistors 50, 52, the emitters of which are controlled by differential degeneration circuitry 54. As those skilled in the art will appreciate, the differential degeneration circuitry 54 provides good common mode rejection, which enhances second order harmonic performance.

[0022] The differential version of RF_IN appearing at the collectors of transistors 50, 52 is represented by signals RF and RF. The RF and RF signals are coupled to opposing ends of two series-connected primary windings, P and P of a transformer 58. A DC supply voltage 60 is applied at center tapped primary transformer coil. The transformer 58 is preferably a completely balanced transformer having center-tapped primary and secondary windings, which form primary windings P and P and secondary windings S and S. Preferably, the transformer 58 is a Balun transformer capable of being implemented directly in an integrated circuit containing all or a portion of the receiver front end circuitry.

[0023] At the junction of the two secondary windings S and S, a center-tapped current sink is provided. The current sink is illustrated as a bi-polar transistor 62 capable of sinking a controlled amount of current. Preferably, a capacitor 64 couples the collector of transistor 62 to ground to provide a good AC ground, and a current sink control voltage, V_CS, is used to control current flow through transistor 62.

[0024] During operation, the differential signals RF and RF are coupled from the primary winding P to the secondary winding S and the primary winding P to the secondary winding S. Although not illustrated, additional passive components may be consistently provided across any of the windings of transformer 58 in a manner controlling signal gain in traditional fashion.

[0025] The coupled RF and RF signals appearing across the secondary windings S and S are used to drive mixing circuitry comprised primarily of transistors 66-72. The coupled RF signal appearing across the secondary winding S drives the emitters of transistors 66 and 68. Similarly, the coupled RF signal appearing across secondary winding S drives the emitters of transistors 70 and 72. To mix the RF and RF signals with an appropriate local oscillator, differential local oscillator signals LO and LO are provided by local oscillator circuitry 74. The LO signal drives the bases of transistors 66 and 72, while the LO signal drives the bases of transistors 68 and 70 in a fully differential mixing topology. Mixing the coupled RFand RF signals with the LO and LO signals of the local oscillator provides a differential intermediate frequency output, consisting of IF and IF signals.

[0026] The collectors of transistors 66 and 70 are coupled to form the IF output, and the collectors of transistors 68 and 72 are coupled to form the IF output. Current is supplied to the mixing circuitry from a supply voltage 76 through a filter network. The filter network may include a resistor 78 and capacitor 80 in parallel with one another and coupling the collectors of transistors 66 and 70 to the supply voltage 76. Resistor 82 and capacitor 84 may be coupled in parallel with one another and between the collectors of transistors 68 and 72 and the supply voltage 76. The respective filter networks are configured to control the gain and frequency response of the mixing circuitry in traditional fashion. The load between the collectors of transistors 66, 68, 70, and 72 and the supply voltage 76 will vary depending on the desired intermediate frequency. For example, inductive loading may be required for higher intermediate frequencies.

[0027] The present invention provides an exceptionally well-balanced RF input to the mixer to enhance second-order intermodulation performance. Further, by the removal of a differential amplifier from the Gilbert cell-type mixer switching core, any performance limitations based on mismatched active components are significantly reduced through the use of passive fully differential transformer. Further, since the mixer core is driven in a differential manner directly from the secondary windings S and S, there is additional headroom to allow greater signal swings, because the traditional LNA transistor amplifier circuitry is replaced with the transformer 58.

[0028] Given the ability of the receiver front end of the present invention to operate over very wide frequency bands supporting various communication technologies, the levels for received signals may vary greatly. Significant changes in the signal level of RF_IN may result in gain compression as a result of transistors 50 and 52 approaching or entering saturation. Since the gains of transistors 50 and 52 are directly controlled by the impedance of the two primary coils P and P of transformer 58, the present invention provides bypass circuitry 86, which is capable of coupling a small resistance across the entire primary (P and P) to reduce the load at the collectors of transistors 50 and 52. In the embodiment illustrated in FIG. 2, the small resistance coupled across the primary of transformer 58 is provided by two oppositely connected PMOS transistor switches 88 and 90. For example, if the primary of transformer 58 provides a load of 300 Ohms, a PMOS configuration for transistors 88 and 90 may provide a resultant load of 10 Ohms when the bypass circuitry is biased on, which would result in a stepped, gain reduction of 10 to 15 dB.

[0029] The gates of the transistor switched 88 and 90 are coupled together and controlled by step gain control circuitry 92, which may be provided by any type of control logic, including the control system 32. The source of transistor 88 and the drain of transistor 90 are coupled to one end of the primary winding, and the drain of transistor 88 and the source of transistor 90 are coupled to the other end of the primary winding. When the transistors 88 and 90 are biased off, a very high impedance is presented across the primary winding of the transformer 58 and the transistors 88 and 90 are effectively out of the circuit, wherein the gains of transistors 50 and 52 are controlled by the impedance of the primary winding of the transformer 58.

[0030] When a large signal is received, and the gain associated with transistors 50 and 52 must be reduced to maintain linear operation and avoid compression of RFand RF, transistors 88 and 90 are simultaneously turned on to effectively short the primary of the transformer 58 with the channel resistance of the respective devices. In effect, the channel resistance of transistors 88 and 90 are placed in parallel with the entire primary of transformer 58, which effectively reduces the load applied to and the resultant gain associated with transistors 50 and 52. Thus, the gain for large signals is reduced so that the transistors 50 and 52 can operate in a linear fashion.

[0031] The amount by which the gain can be reduced is a function of the channel resistance. If the channel resistance of the PMOS transistors 88 and 90 illustrated in FIG. 2 is not low enough to adequately reduce gain, NMOS transistors, which typically have a lower channel resistance than PMOS transistors, may be used to provide greater step reduction in gain, as illustrated in FIG. 3 with transistors 88 and 90. Those skilled in the art will recognize various ways in which to selectively bypass or switch in resistance across the primary winding of transformer 58 to achieve a desired reduction in the gain associated with transistors 50 and 52. Notably, the winding ratios for the primary and secondary windings of transformer 58 may vary depending on the desired gain. In the preferred embodiment, the primary and secondary windings have a winding ratio of 1:1; however, the ratios of P to S and P to S may vary wherein the effective number of turns for the secondary windings may be greater than those of the corresponding primary windings to add an effective amount of voltage gain.

[0032] The transistors 88 and 90 are used in opposing fashion in an effort to maintain balance between the differential RF input signals, RF and RF. Accordingly, using two transistors 88 and 90 and coupling them across the primary in an opposing fashion compensates for any variances in the effective impedance provided across the primary windings P and P, and thus, the load provided to transistors 50 and 52. This differential structure helps balance any parasitic capacitors and resistance. Notably, the balanced implementation of transistors 88 and 90 may be implemented using a multi-finger or inter-digitated MOSFET device as illustrated in FIG. 4. Notably, in an effort to maintain balance across the primary channel resistances, the amount of drain area (D) and source area (S) are balanced. Those skilled in the art will recognize other techniques for providing a balanced load in parallel with the entire primary of the transformer 58.

[0033] As illustrated in FIG. 5, the present invention may be incorporated into quadrature-based mixing systems. In the embodiment illustrated, the quadrature-based front end is capable of operating in three communication modes: EGSM, DCS, and PCS. Accordingly, the illustrated embodiment includes an in-phase mixing section and a quadrature phase mixing section, as well as three LNA sections, 94, 96, 98, corresponding to the EGSM, DCS, and PCS modes, respectively. Depending on the mode of operation, each of the LNAs 94, 96, 98 will receive the RF input signal RF_EGSM, RF_DCS, or RF_PCS, and generate a differential version of the signal using the corresponding transistors 50 and 52 and the differential degeneration circuitry 54, as described above. This concept can be extended to more than 3 bands and may be used to support any combination of bands.

[0034] In this quadrature mixing embodiment, there are two transformers 58I and 58Q, which both have center-tapped primary windings forming individual primary windings P and P, respectively. These primary windings P and P, including the center tap, for both transformers 58I and 58Q are coupled together. The supply voltage 60 provides bias to both primaries for each transformer 58I and 58Q. Transformers 58I and 58Q include secondary windings, which are center-tapped to form windings S and S. The center taps for the secondaries of transformers 58I and 58Q are coupled to current sinks 100 and 102, respectively, which are driven by a common current sink reference 104.

[0035] As described above, the RF and RF signals are coupled from the P and P windings to the S and S windings, respectively, for each transformer 58I and 58Q. As such, the RF and RF signals are available to drive the in-phase and quadrature phase mixing cores.

[0036] Depending on the frequency needed for down-conversion, each mixing core will operate at a given local oscillator frequency; however, as in traditional quadrature fashion, the quadrature phase local oscillator frequency will operate 90 degrees out of phase from the in-phase local oscillator frequency. Accordingly, the quadrature phase local oscillator 74Q provides differential quadrature local oscillator signals LO_Q and LO_Q, which are 90 degrees out of phase from the differential in-phase local oscillator signals LO_Iand LO_I provided by the in-phase local oscillator 74I.

[0037] For in-phase mixing, the RF signal is coupled from the primary winding P to the secondary winding S of transformer 58I to drive the emitters of transistors 70I and 72I. The RF signal is coupled from primary winding P to the secondary winding S of transformer 58I to drive the emitters of transistors 66I and 68I. The local oscillator signal LO_I drives the bases of transistors 66I and 72I, while the LO_I signal drives the bases of transistors 68I and 70I. The collectors of transistors 66I and 70I are coupled together to provide the negative component of the differential intermediate frequency signal, IF_I while the collectors of transistors 68I and 72I are coupled together to form the positive component of the differential in-phase intermediate frequency signal, IF_I. The collectors of transistors 66I and 70I are coupled to a supply voltage 76 via a network, illustrated as including resistor 78I and capacitor 80I in parallel. Similarly, the collectors of transistors 68I and 72I are coupled to the supply voltage 76 via resistor 82I and capacitor 84I. Thus, the differential RF signals RF and RF are mixed with differential local oscillator signals LO_I and LO_I to provide differential intermediate frequency signals IF_I and IF_I.

[0038] For quadrature phase mixing, the RF signal is coupled from the primary winding P to the secondary winding S of transformer 58Q to drive the emitters of transistors 70Q and 72Q. The RF signal is coupled from primary winding P to the secondary winding S of transformer 58Q to drive the emitters of transistors 66Q and 68Q. The LO_Q signal drives the bases of transistors 66Q and 72Q, while the LO_Q signal drives the bases of transistors 68Q and 70Q. The collectors of transistors 66Q and 70Q are coupled together to provide the negative component of the differential intermediate frequency signal, IF_Q while the collectors of transistors 68Q and 72Q are coupled together to form the positive component of the differential quadrature phase intermediate frequency signal, IF_Q. The collectors of transistors 66Q and 70Q are coupled to a supply voltage 76 via a network, illustrated as including resistor 78Q and capacitor 80Q in parallel. Similarly, the collectors of transistors 68Q and 72Q are coupled to the supply voltage 76 via resistor 82Q and capacitor 84Q. Thus, the differential RF signals RF and RF are mixed with a differential local oscillator signals LO_Q LO_Q to provide differential intermediate frequency signals IF_Q and IF_Q.

[0039] Gain control as described above can be provided to any of the LNAs 94, 96, 98, and is preferably applied equally to the primary and secondary windings of transformers 58I and 58Q, as illustrated. Thus, when the step gain control circuitry 92 controls the bypass circuitry 86 to turn on transistors 88 and 90, the channel resistances of the respective transistors are coupled across the primary windings of both transformers 58I and 58Q uniformly and simultaneously.

[0040] Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. For example, the PNP-type, bipolar transistors may be used to form the mixing core of the mixing circuitry instead of NPN-type bipolar transistors. Further, the mixing core may be formed with various types of transistor technologies, including MOS. Accordingly, the transistors may be generically represented as having an input, bias control input, and output, which correspond to an emitter, base, and collector of an NPN-type bipolar transistor. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.