A quadrature subharmonic mixer comprises a polyphase filter configured to generate quadrature components of a local oscillator (LO) reference signal, a summing and scaling element configured to create additional components of the LO reference signal, and a plurality of mixer elements configured to multiply the quadrature components of the LO reference signal and the additional components of the LO reference signal with a radio frequency (RF) signal to obtain a downconverted version of the RF signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to receiver circuit architecture in a wireless portable communication device. More particularly, the invention relates to a quadrature subharmonic mixer.

2. Related Art

With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset. The different modulation and transmission schemes each have advantages and disadvantages.

As these mobile communication systems have been developed and deployed, many different standards, to which these systems must conform, have evolved. For example, in the United States, third generation portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme can be 8-quadrature phase shift keying (8QPSK), offset π/4 differential quadrature phase shift keying (π/4-DQPSK) or variations thereof and the access format is TDMA.

In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology.

Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a very low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a non-linear power amplifier, which is highly efficient, can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Further, the output in a GSM transceiver is a constant envelope (i.e., a non time-varying signal containing only a phase modulated (PM) signal) modulation signal.

One of the advances in portable communication technology is the move toward the implementation of a low intermediate frequency (IF) receiver and a direct conversion receiver (DCR). A low IF receiver converts a radio frequency (RF) signal to an intermediate frequency that is lower than the IF of a convention receiver. A direct conversion receiver downconverts a radio frequency (RF) received signal directly to baseband (DC) without first converting the RF signal to an intermediate frequency (IF). One of the benefits of a direct conversion receiver is the elimination of costly filter components used in systems that employ an intermediate frequency conversion.

When converting a received RF signal either to an intermediate frequency signal, or directly to a baseband signal, one or more mixers are used to downconvert the received RF signal. A mixer combines the received RF signal with a reference signals, referred to as a “local oscillator,” or “LO” signal. The resultant signal is the received signal at a different, and typically lower, frequency. One mixer technology used today is referred to as a “subharmonic” mixer. A subharmonic mixer uses an LO signal that is lower than, and typically on the order of one-half of the LO signal. A subharmonic mixer generally produces lower “self-mixing” components and generally reduces or eliminates feedback to the system antenna.

However, when implementing a subharmonic mixer, either the received RF signal or the LO signal must be altered to produce signals having 90° phase separation to reliably extract the in-phase (I) and the quadrature (Q) components of the received signal. Unfortunately, altering the RF signal requires the use of lossy polyphase filters and altering the LO signal requires the use of complex and inefficient phase shift circuitry to generate what is referred to as a “stair-step” function. Each of these techniques require substantial power and reduce receiver efficiency.

Therefore, it would be desirable to provide a simple and efficient subharmonic mixer.

SUMMARY

Embodiments of the invention include a quadrature subharmonic mixer, comprising a polyphase filter configured to generate quadrature components of a local oscillator (LO) reference signal, a summing and scaling element configured to create additional components of the LO reference signal, and a plurality of mixer elements configured to combine the quadrature components of the LO reference signal and the additional components of the LO reference signal with a radio frequency (RF) signal to obtain a downconverted RF signal.

DETAILED DESCRIPTION

Although described with particular reference to a portable transceiver, the quadrature subharmonic mixer can be implemented in any communication device employing a mixer.

The quadrature subharmonic mixer can be implemented in hardware, software, or a combination of hardware and software. When implemented in hardware, the quadrature subharmonic mixer can be implemented using specialized hardware elements and logic. When the quadrature subharmonic mixer is implemented partially in software, the software portion can be used to control the mixer components so that various operating aspects can be software-controlled. The software can be stored in a memory and executed by a suitable instruction execution system (microprocessor). The hardware implementation of the quadrature subharmonic mixer can include any or a combination of the following technologies, which are all well known in the art: discreet electronic components, a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit having appropriate logic gates, a programmable gate array(s) (PGA), a field programmable gate array (FPGA), etc.

The software for the quadrature subharmonic mixer comprises an ordered listing of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions.

FIG. 1is a block diagram illustrating a simplified portable transceiver100including a quadrature subharmonic mixer. The portable transceiver100includes speaker102, display104, keyboard106, and microphone108, all connected to baseband subsystem110. A power source142, which may be a direct current (DC) battery or other power source, is also connected to the baseband subsystem110via connection144to provide power to the portable transceiver100. In a particular embodiment, portable transceiver100can be, for example but not limited to, a portable telecommunication device such as a mobile cellular-type telephone. Speaker102and display104receive signals from baseband subsystem110via connections112and114, respectively, as known to those skilled in the art. Similarly, keyboard106and microphone108supply signals to baseband subsystem110via connections116and118, respectively. Baseband subsystem110includes microprocessor (μP)120, memory122, analog circuitry124, and digital signal processor (DSP)126in communication via bus128. Bus128, although shown as a single bus, may be implemented using multiple busses connected as necessary among the subsystems within baseband subsystem110.

Depending on the manner in which the quadrature subharmonic mixer is implemented, the baseband subsystem110may also include an application specific integrated circuit (ASIC)135and a field programmable gate array (FPGA)133.

Microprocessor120and memory122provide the signal timing, processing and storage functions for portable transceiver100. Analog circuitry124provides the analog processing functions for the signals within baseband subsystem110. Baseband subsystem110provides control signals to transmitter150and receiver170via connection132. Although shown as a single connection132, the control signals may originate from the DSP126, the ASIC135, the FPGA133, or from microprocessor120, and are supplied to a variety of connections within the transmitter150and the receiver170. It should be noted that, for simplicity, only the basic components of portable transceiver100are illustrated herein. The control signals provided by the baseband subsystem110control the various components within the transmitter150and the receiver170.

If portions of the quadrature subharmonic mixer are implemented in software that is executed by the microprocessor120, the memory122will also include quadrature subharmonic mixer software255. The quadrature subharmonic mixer software255comprises one or more executable code segments that can be stored in the memory and executed in the microprocessor120. Alternatively, the functionality of the quadrature subharmonic mixer software255can be coded into the ASIC135or can be executed by the FPGA133. Because the memory122can be rewritable and because the FPGA133is reprogrammable, updates to the quadrature subharmonic mixer software255can be remotely sent to and saved in the portable transceiver100when implemented using either of these methodologies.

Baseband subsystem110also includes analog-to-digital converter (ADC)134and digital-to-analog converters (DACs)136and138. Although DACs136and138are illustrated as separate devices, it is understood that a single digital-to-analog converter may be used that performs the function of DACs136and138. ADC134, DAC136and DAC138also communicate with microprocessor120, memory122, analog circuitry124and DSP126via bus128. DAC136converts the digital communication information within baseband subsystem110into an analog signal for transmission to a modulator152via connection140. Connection140, while shown as two directed arrows, includes the information that is to be transmitted by the transmitter150after conversion from the digital domain to the analog domain.

The transmitter150includes modulator152, which modulates the analog information in connection140and provides a modulated signal via connection156to upconverter154. The upconverter154transforms and amplifies the modulated signal on connection156to an appropriate transmit frequency and power level for the system in which the portable transceiver100is designed to operate. Details of the modulator152and the upconverter154have been omitted for simplicity, as they will be understood by those skilled in the art. For example, the data on connection140is generally formatted by the baseband subsystem110into in-phase (I) and quadrature (Q) components. The I and Q components may take different forms and be formatted differently depending upon the communication standard being employed.

The upconverter154supplies the upconverted signal via connection158to duplexer162. The duplexer comprises a filter pair that allows simultaneous passage of both transmit signals and receive signals, as known to those having ordinary skill in the art. The transmit signal is supplied from the duplexer164to the antenna160.

A signal received by antenna160will be directed from the duplexer162to the receiver170. The receiver170includes a downconverter172, a filter chain180, and a demodulator178. The downconverter172includes, among other elements, a quadrature subharmonic mixer250. If implemented using a direct conversion receiver (DCR), the downconverter172converts the received signal from an RF level to a baseband level (DC). Alternatively, the received RF signal may be downconverted to an intermediate frequency (IF) signal, depending on the application. The downconverted signal is sent to the filter chain180via connection174. The filter chain comprises a least one filter stage to filter the received downconverted signal as known in the art.

The filtered signal is sent from the filter chain180via connection176to the demodulator. The demodulator178recovers the transmitted analog information and supplies a signal representing this information via connection186to ADC134. ADC134converts these analog signals to a digital signal at baseband frequency and transfers the signal via bus128to DSP126for further processing.

FIG. 2is a block diagram illustrating an embodiment of the receiver170ofFIG. 1. The receiver170receives a signal via antenna160, which supplies the received signal at an RF frequency level via the duplexer (not shown) to low noise amplifier (LNA)202. The LNA202amplifies the received signal and provides the amplified signal on connection204to a balun206. The balun206receives the single ended signal on connection204and generates a differential RF signal on connections208aand208b. Alternatively, other devices capable of generating a differential RF signal may be used in place of the balun206. The output of the balun206on connections208aand208brepresent the received RF signal having portions that are separated in phase by 180°. For example, the signal on connection208acan be at 0° and the signal on connection208bcan be at 180°. The output of the balun206is supplied via connections208aand208bto the quadrature subharmonic mixer250.

The quadrature subharmonic mixer250receives quadrature LO signals on connections284,286,288and290that are 45° offset in phase from each other. The 0° and 90° signals on connections284and286form one of the quadrature LO signals and the 45° and 135° signals on connections288and290form the other quadrature LO signals. A frequency reference signal, also called a “local oscillator” signal, or “LO,” is generated by a synthesizer210on connection212. The LO signal on connection212, which comprises a differential signal having 0° and 180° phase components, is supplied to a phase altering element220. The phase altering element220generates the quadrature LO signals on connections284,286,288and290that are supplied to the mixer250. The frequency of the LO signal determines the frequency to which the quadrature subharmonic mixer250downconverts the signal received from the balun206on connections208aand208b. In accordance with an embodiment of the invention, the LO signal supplied to the quadrature subharmonic mixer250via connection212comprises a plurality of quadrature phase offset signals that allow the quadrature subharmonic mixer250to operate on an RF signal that has not been phase shifted. In this manner, lossy polyphase filtering in the RF path is avoided. The output of the quadrature subharmonic mixer250is provided on connections214aand214bto the filter chain180. The signal on connection214ais the downconverted in-phase baseband signal (IBB) and the signal on connection214bis the downconverted quadrature baseband signal (QBB).

The signal on connection214is supplied to the filter chain180and then to the demodulator (not shown) via connection176.

FIG. 3is a block diagram illustrating the quadrature subharmonic mixer250. The quadrature subharmonic mixer250receives the LO signal via connections212aand212b, which corresponds to the connection212inFIG. 2. In this embodiment, and as an example only, the LO signal is at a frequency of 900 megahertz (MHz), which is one-half of the 1.8 gigahertz (GHz) radio frequency (RF) receive signal. The signal on connections212aand212bis the LO reference signal supplied by the synthesizer210ofFIG. 2. In one embodiment, the LO signal on connections212aand212bare supplied to a first polyphase filter252. The polyphase filter is one manner of generating the quadrature components of the LO signal. Alternatively, dividers or a quadrature voltage controlled oscillator (VCO) can be used to generate the quadrature components of the LO signal. Further, a single, two-stage polyphase filter can be used instead of the two single-stage polyphase filters252and258.

The polyphase filter252receives the LO signal on connections212aand212b, and generates 90° offset signals on differential pair connections254and256. Each of the differential pair connections254and256represent a differential pair signal representing the LO signal supplied to the polyphase filter252, offset in phase by 90°. The differential pair252and the differential pair256supply the 90° offset phase signals to a second polyphase filter258. The second polyphase filter258operates similar to the first polyphase filter252and generates 90° offset phase signals on differential pair connections262and264. The polyphase filters252and258comprise resistive (R) and capacitive (C) circuitry having a low pass path and a high pass path. The low pass path delays the phase of the input signal by 45° and the high pass path advances the phase of the input signal by 45°.

The output of the second polyphase filter258is supplied to a vector sum and scale element300. As will be described below inFIG. 4, the vector sum and scale element300receives the differential pair signals representing 0°, 90°, 180° and 270° from the polyphase filter258and generates 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315° phase signals on differential pair connections272,274,276and278.

The 0° and 180° local oscillator signal on differential pair272and the 90° and 270° local oscillator signal on differential pair274are supplied to the buffer280. Similarly, the 45° and 225° local oscillator signal on differential pair276and the 135° and 315° local oscillator signal on differential pair278are supplied to the buffer282. The buffers280and282buffer the signals and provide amplified and wave-shaped versions of the 0°, 45°, 90°, 135°, 180°, 225°, 270° and 315° phase signals on differential pairs284,286,288and290. The 0° and 180° local oscillator signal on differential pair284and the 90° and 270° local oscillator signal on differential pair on286are supplied to the mixer core292. The 45° and 225° local oscillator signal on differential pair288and the 135° and 315° local oscillator signal on differential pair290are supplied to the mixer core294. The mixer core292and the mixer core294also receive the received radio frequency (RF) signal via connection208aand208b. These signals are received from the balun206(FIG. 2) as described above. In accordance with this embodiment of the quadrature subharmonic mixer, the RF signal on connections208aand208bis at an exemplary frequency of 1.8 GHz, and the local oscillator signal on connections212aand212is at one-half the RF frequency, or 900 MHz. The quadrature LO signals on connections284,286,288and290supplied to the mixer cores292and294are 45° offset in phase from each other. The 0°/180° and 90°/270° signals on connections284and286, respectively, form one of the quadrature LO signals and the 45°/225° and 135°/315° signals on connections288and290, respectively, form the other quadrature LO signals. In this manner, the quadrature subharmonic mixer250downconverts the received RF signal to recover the baseband data without altering the phase of the received RF signal. Instead, the phase of LO reference signal is altered as described above, avoiding loss in the RF path and improving receiver sensitivity.

The output of the mixer core292is supplied via differential pair214a, and includes the in-phase portion of the baseband signal (IBB), and the output of the mixer core294on differential pair214bcontains the quadrature portion of the baseband signal (QBB). In this manner, by implementing a quadrature subharmonic mixer250as shown inFIG. 3, polyphase filtering is eliminated from the RF path and only occurs in the LO path. In this manner, the RF loss of the circuitry used to implement the quadrature subharmonic mixer250can be reduced.

FIG. 4is a block diagram illustrating an embodiment of the vector sum and scale element300ofFIG. 3. The 0°, 90°, 180° and 270° local oscillator input signals derived by the first polyphase filter252and the second polyphase filter258are supplied via connections302,304,306and308, respectively. The capacitors312,314,316and322, are the passive components that provide the 0° output on connection318and the 45° output on connection324. If the 0° input on connection302and the 90° input on connection304are vector summed to provide the 45° signal on connection324, the magnitude of the 45° signal on connection324would have a magnitude equal to √{square root over (2)}, if the magnitude of the 0° signal on connection302and the 90° signal on connection304each equal magnitude1. In accordance with this embodiment of the invention, to equalize the magnitude of the 45° signal on connection324to the magnitude of the signal on connections318and334, the capacitors312,314,322and316function to scale the output of the 45° vector on connection324to have a magnitude equal to the magnitude of the 0° signal on connection318and the 90° signal on connection334.

Similarly, the capacitors326,328,332and336are the passive components used to generate the 135° signal on connection342using the 90° input on connection304and the 180° input on connection306. Similarly, the capacitors338,344,348and352are the passive components used to generate the 225° signal on connection354using the 180° signal and the 270° signal on connections306and308, respectively. Finally, the capacitors356,358,366and364are the passive components used to generate the 315° signal on connection368using as input the 0° signal on connection302and the 270° signal on connection308. Similar to the 45° output on connection324, the 135° output, the 225° output and the 315° output are each scaled to have a magnitude equal to the magnitude of the 0°, 90°, 180° and 270° signals. The architecture in the vector sum and scale element300reduces current and power consumption by using only passive components to perform the vector summation and scaling.

FIG. 5is a block diagram illustrating a subharmonic500. The subharmonic mixer500comprises a switching core506, a load (implemented as a low pass filter (LPF))508and a gmstage502. The gmstage502receives a radio frequency voltage signal, VRF, via connections208aand208band provides a current signal, I, via connections504aand504b. Essentially, the gmstage502functions as a voltage to current converter. The output of the gmstage502on connections504aand504bis supplied to the switching core506. The switching core506also receives the 0°/180° and the 90°/270° quadrature local oscillator (LO) signals via connections284and286, respectively, and provides the differential in-phase baseband (IBB) output via connection214a. The switching core506corresponds to the mixer core292ofFIG. 3and switches the current on connections504aand504bto create harmonics that are filtered out by the load508resulting in the differential baseband signal on connection214a. Although not shown inFIG. 5, when the 45°/225° and the 135°/315° quadrature LO signals are applied to the mixer core, the resultant output is the differential quadrature baseband (QBB) signals corresponding to connection214binFIGS. 2 and 3. In a subharmonic mixer500, the LO signal is effectively multiplied by two in the switching core. The RF signal is multiplied by 2*LO and downconverted to a baseband level signal.

FIG. 6is a schematic diagram illustrating the switching core506ofFIG. 5. The switching core506comprises transistors602,604,606,608adapted to receive the output of the gmstage502via connection504aand transistors612,614,616and618adapted to receive the output of the gmstage502via connection504b. The transistors can be implemented using, for example, heterojunction transistor (HBT) technology, or can be implemented using complimentary metal oxide semiconductor (CMOS), bi-CMOS, or other technology. The transistors602,604,606and608receive the 0°, 180°, 90° and 270° LO signals on connections622,624,626and628, respectively. The transistors612,614,616and618, receive the 270°, 90°, 180° and 0° LO signals on connections632,634,636and638, respectively. The output of the switching core506is the in-phase portion (IBB) of the baseband signal on connection214a, and the quadrature portion (QBB) of the baseband signal on connection214b.

FIG. 7is a flow chart700describing the operation of an embodiment of the quadrature subharmonic mixer250ofFIG. 3. The steps in the blocks of the flowchart may be performed in the order shown, out of the order shown or may be performed concurrently. In block702a local oscillator reference signal is provided. In block704, the quadrature components of the local oscillator reference signal are generated.

In block706, the 0°, 90°, 180° and 270° outputs are summed and scaled by the vector sum and scale element300(FIG. 4) to provide the 0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° local oscillator reference signals that will be provided to the mixer cores. In block708, the additional quadrature components (i.e., the 45°, the 135°, the 225°, and the 315°) signals are scaled as described above inFIG. 4.

In block712, the quadrature components of the local oscillator signal are multiplied with the received RF signal in the mixer core292and294to produce the downconverted in-phase (I) and quadrature (Q) baseband signals on connections214aand214b.

FIG. 8is an alternative embodiment of the quadrature subharmonic mixer ofFIG. 3. In this embodiment, an LO signal at twice (2×) the LO frequency of 900 MHz (1.8 GHz in this embodiment) is supplied to a “divide-by-two” divider810in the quadrature subharmonic mixer800. Alternatively, a signal at four times the LO frequency can be supplied to a “divide-by-four” divider. The divider810then creates the quadrature LO signals on connections262and264that are supplied to the vector and sum element300.

While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.