Patent Application
20090191833
1. Field
The present embodiments relate generally to communications, and more specifically, to radio frequency receivers in communications.
2. Related Art
Radio frequency (RF) receivers used in modern communication systems may support multiple modes and networks, such as 3G wideband code division multiple access (WCDMA) and 2G Global System for Mobile communications (GSM) using time division multiple access (TDMA). A received RF signal in a certain mode may be translated into a specified frequency band and processed to retrieve the information contained in the RF signal. For example, an RF receiver may amplify, filter, and mix an analog received RF signal into in-phase (I) and quadrature (Q) signals that may be converted into digital signals for further processing.
The multiple modes may utilize the same RF receiver architecture but have different frequency ranges and linearity, noise figure, and sensitivity requirements. Noise figure is a measure of degradation of a signal to noise ratio caused by components in the RF receiver. In particular, a WCDMA system has full duplex functionality where separate receive and transmit signals may be active simultaneously. A higher power transmitted signal may leak into the receive signal in such a system. To relax linearity and noise requirements due to the transmit signal leakage, a conventional WCDMA system may include a surface acoustic wave (SAW) filter prior to a mixer and downstream processing stages. In addition, a conventional WCDMA system may include a low noise amplifier (LNA) to meet sensitivity, noise figure, and gain control requirements. Using a SAW filter, LNA, amplifiers, and other components may result in increased die size, current drain, cost, and complexity, and reduced performance of an RF receiver.
A mixer may combine the received RF signal with the I and Q pulses. The I and Q pulses may be generated by a pulse generator based on a periodic signal from a local oscillator. The pulse generator in a conventional RF receiver may generate pulses with a 150% duty cycle, including I pulses 0 and 180 degrees out of phase from the periodic signal, and Q pulses 90 and 270 degrees out of phase from the periodic signal. The I and Q pulses may control switches in the mixer to combine the received RF signal with the I and Q pulses. The switches may not be on at the same time if the rise and fall times of the I and Q pulses are ideal. However, if the 50% duty cycle I and Q pulses have non-ideal rise and fall times, the switches in the mixer may be partially on when an amplitude of the I and/or Q pulse is above a crossover threshold, resulting in degraded performance and increased current drain.
Large transmit signal leakages may also affect the gate voltage bias of a mixer, leading to degraded noise figure, 1/f noise, gain, and second order input intercept point (IIP2). Moreover, a conventional RF receiver including operational amplifiers and voltage mode mixers may have poor IIP2 and third order input intercept point (IIP3). A conventional RF receiver may also include transimpedance amplifiers that may have a pole in the modulation bandwidth. Such transimpedance amplifiers may increase current drain, integrated noise, and die area.
The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale.
By way of introduction, the preferred embodiments described below include a radio frequency (RF) receiver including a low noise amplifier, a pulse generator, a crossover correction circuit, a passive doubled balanced mixer, and a direct coupled complex filter. The low noise amplifier may convert a received voltage of an RF signal to a current and amplify the current by a specified transconductance. The current may be injected into a passive double balanced mixer, which may combine the current with in-phase (I) and quadrature (Q) pulses to generate an intermediate frequency signal, which may be a low, very low, or zero intermediate frequency signal. The I and Q pulses may have duty cycles of less than 35% and be generated by a pulse generator. A crossover correction circuit may control an amplitude crossover threshold of the I and Q pulses to reduce the amount of time switches in the mixer are simultaneously active when the I and Q pulses are rising and falling. The intermediate frequency signal generated by the mixer may include I and Q signals that are current injected into a direct coupled complex filter. The direct coupled complex filter may filter the intermediate frequency signal to a filtered analog signal for further processing. These embodiments of a RF receiver may result in a decrease in die size, current drain, cost, or complexity. In addition, there may be an improvement in gain, 1/f noise, noise figure, sensitivity, or linearity.
In the analog front end 104, the received RF signal may be filtered by a duplex filter 108. The duplex filter 108 may include a receive filter for filtering received RF signals and a transmit filter for filtering transmit RF signals from a power amplifier in a transmitter. Noise and interference in the RF signals may be attenuated by the duplex filter 108. For example, for 3GPP Band I, the duplex filter 108 allows signals from approximately 2110 MHz to 2170 MHz to pass from the antenna 102 to the receiver, and allows signals from approximately 1920 MHz to 1980 MHz to pass from the transmitter to the antenna 102. The receive filter may be substantially isolated from the transmit filter in the duplex filter 108. However, in a full duplex WCDMA system, for example, both the received and transmitted signals may be active simultaneously. A higher power transmitted signal may leak into the received signal in the duplex filter 108, leading to a degradation of the noise figure, gain, and performance of the RF receiver 100.
The filtered RF signal from the duplex filter 108 may be input to a passive gain matching circuit 110. The passive gain matching circuit 110 steps up the impedance of the filtered RF signal such that the voltage of the filtered RF signal is multiplied by a gain while adding minimal noise. The matching circuit 110 may also include a high-pass filter to provide extra rejection and attenuation of leaked transmit RF signals that are at a lower frequency than received RF signals. The filtered and matched RF signal may be input into a low noise voltage to current amplifier 112 configured as a transconductance amplifier. The RF signal may be converted from a voltage to a current. The gain of the low noise amplifier 112 may be determined by the transconductance of the low noise amplifier 112 and the feedback resistance of the direct coupled filter 120. For example, for WCDMA SAW-less operation, the gain of the amplifier 112, passive mixer 114, and direct coupled filter 120 may be set to approximately 45 dB+/−2 dB. In this case, a gain control range of approximately 40 dB+/−1 dB with controlled input impedance would be needed in the amplifier 112. The RF signal may be input to the low noise amplifier 112 as differential signals to reduce the effects of interference and noise.
The low noise amplifier 112 may include step and binary weighted gain control to increase the dynamic range of the receiver, and to provide larger gain to weaker signals and smaller gain to stronger signals. The step gain control in the low noise amplifier 112 may adjust the gain in discrete intervals. The binary weighted gain control may further adjust the gain in binary intervals with digital control bits provided by a controller. The digital control bits may be any desired bit width to adjust the gain in binary intervals. Binary weighted gain control may be achieved by switching current away from the load. Binary weighted gain control may be implemented with any number of control bits where attenuation for each step is determined by the equation 20*log(1−2̂(n−2)/(2̂n−1)), where n is the number of control bits. However, as the number of control bits exceeds approximately five bits, the circuit complexity and die area may increase, and gain control range may diminish due to layout and device parasitics. For example, a five bit binary weighted gain control without a coarse step can achieve thirty steps for a total gain control range of approximately 29.8 dB versus a desired range. A 10.7 dB coarse step may be added to achieve a 40.3 dB total RF automatic gain control (AGC) range required for WCDMA SAW-less operation within a single stage. The coarse step gain control may add additional gain control range by reducing load resistance through adding a smaller shunt load. This may be realized with a pair of NMOS or PMOS transistors that are connected across a common mode feedback resistor. The transistors lower the output impedance while keeping the input impedance substantially constant.
Although the step and binary weighted gain control may increase the total gain control range, the input impedance of the low noise amplifier 112 remains substantially constant because the current drain in the transconductance stage is kept substantially constant. The gain of the amplifier 112 may be controlled by an AGC state machine, for example. The active PMOS load devices 218 and 220, the common-mode feedback circuit 234, and resistors R3 and R4 in
The low noise amplifier 112 may also minimize desensitization of the receiver 100 that are due to large blocking signals. A large continuous wave or modulated blocking signal at the input of the receiver 100 may cause nonlinear effects and receiver desensitization due to a rise in the noise figure. Non-linear mixing of bias circuit noise with a large blocking signal, for example, may contribute to a rise in the noise figure. An active bias generator 201 (shown in
The current generated by the low noise amplifier 112 may drive a passive double balanced mixer 114. The current may be split into a current going to an I mixer, IRFI, and a current going to a Q mixer, QRFI. The currents may be input to the mixer 114 as differential signals. The mixer 114 may combine the currents with I and Q pulses to produce a set of currents IP and QP comprising an intermediate frequency signal, which may be a zero intermediate frequency baseband signal. A zero intermediate frequency baseband signal may be produced when the receiver 100 is in a direct conversion mode, e.g., when the frequency of the received RF signal and the frequency of a periodic signal from a local oscillator 118 are approximately equal. The currents IP and OP may be generated as differential signals to reduce the effects of interference and noise. A pulse generator 116 may generate the I and Q pulses based on the periodic signal from the local oscillator 118. The I and Q pulses may have a duty cycle of less than 35%. Other duty cycles above 35% are possible, however, the performance of the amplifier 112 and direct coupled filter 120 may be degraded when the duty cycle is more than 35%. The duty cycle of the I and Q pulses may range from 20-35%. The I pulses may include a first pulse zero degrees out of phase from the periodic signal, and a second pulse 180 degrees out of phase from the periodic-signal. The Q pulses may include a third pulse 90 degrees out of phase from the periodic signal, and a fourth pulse 270 degrees out of phase from the periodic signal. The pulse generator 116 may generate the I and Q pulses using latches, combinatorial logic, sequential logic, or other logic.
The pulse generator 116 may also include a crossover correction circuit to control an amplitude crossover threshold of the I and Q pulses. The crossover correction circuit may adjust the amplitude crossover threshold to reduce the amount of time the I and Q pulses are simultaneously active. The amplitude crossover threshold may include a crossing point of the I and Q pulses when one pulse is rising and the other is falling, for example. In other words, because the I and Q pulses may control switches in the mixer 114, it is desirable to adjust the amplitude crossover threshold of the I and Q pulses so that the switches are not turned on at the same time. Performance of the RF receiver 100 may be improved and current drain may be reduced when the amplitude crossover threshold is adjusted in this fashion. Using the mixer 114 to combine I and Q currents with less than 35% duty cycle crossover corrected I and Q pulses may result in an improvement in gain, noise figure, linearity, and IIP2, as well as a reduction in 1/f noise and noise figure degradation when a large blocker signal is present.
Crossover correction in the pulse generator 116 may be beneficial in a WCDMA full duplex SAW-less receiver where the noise figure of the receiver 100 should be very low in the presence of a large transmitter signal. For example, gain, noise figure, IIP3, IIP2 and 1/f noise may be improved by approximately 3 dB, 2.5 dB, 3 dB, 15 dB and 15 dB, respectively. Adjusting the gate threshold and sharpening the fall time of the pulse may result in additional pulse shaping. The crossover between the I and Q pulses can be further reduced by decreasing the channel width to length (W/L) ratio of the PMOS device relative to the NMOS inverter device of a complementary inverter. For example, a complementary rail to rail inverter may be optimized with a PMOS device W/L ratio of 2.5 and an NMOS device W/L ratio of 1. Alternatively, by making the W/L ratio of the PMOS device equal to 1 and the W/L ratio of the NMOS device equal to 1, the falling edge may be made sharper than the rising edge, and the crossover is reduced. If a rail to rail output AND or NOR gate is used instead of an inverter, the complementary output devices can be sized to have similar effect.
The currents IP and QP comprising the intermediate frequency signal produced by the mixer 114 may be filtered by a direct coupled complex filter 120. The filter 120 may perform second ordering filtering of the intermediate frequency signal. The filter 120 may include voltage mode operational amplifiers with current inputs. Current mode operational amplifiers may also be used in the filter 120 to filter the intermediate frequency signal. The filter 120 may produce filtered analog signals IPF and QPF that may be converted to digital signals and digital signal processed downstream in the RF receiver 100 to retrieve the information contained in the received RF signal. The filtered analog signals IPF and QPF may be generated as differential signals.
Configuring the filter 120 in this fashion may allow elimination of the pole within the modulation bandwidth used in conventional RF receivers. Using a direct coupled complex filter 120 may result in a relaxed linearity requirement in downstream processing stages, and reduction in noise, die size, and current drain. For example, for 3GPP Bands II, III, and V, where a 3G system co-exists with a 2G GSM system, a 3G receiver is required to meet narrow band blocker at 2.7 MHz and 3.5 MHz. A conventional receiver may achieve this requirement by putting a pole in a modulation bandwidth. This is not desirable for a SAW-less receiver as it may significantly degrade the noise figure of the receiver. Also, in a conventional receiver, the pole in the modulation bandwidth is typically realized as RC feedback in a transimpedance amplifier. This transimpedance stage, with a non-complex pole, adds current drain and die area. The lower frequency of the mixer pole also contributes to additional die area. The transfer function of the complex filter following this transimpedance stage in a conventional receiver typically has to correct for the in-band pole, resulting in higher in-band ripple and higher group delay ripple. In-band pole and noise figure degradation may be eliminated by using the direct coupled filter 120 with second order complex filtering. For example, the noise figure of a WCDMA receiver may be improved by approximately 0.3 dB by using the direct coupled filter 120 and eliminating the in-band pole and biquad noise peaking. Group delay in-band ripple for a WCDMA/GSM compressed-mode receiver may be reduced from approximately 139 ns to 65 ns.
If the received RF signal is single-ended, a source degeneration circuit 236 may convert the single-ended received RF signal to a differential RF signal for use within the low noise amplifier 112. An inductor 202 center tapped to ground and capacitors 204 and 206 may be included in the source degeneration circuit 236 to optimize noise, input impedance, and linearity. The received RF signal is then amplified by a gain and converted from a voltage to a current in the low noise amplifier 112. The active bias generator 201, which minimizes the up-conversion of low frequency noise with the presence of strong blockers, may include NMOS transistors 208 and 210, a reference current source 212, and resistors R1 and R2. The NMOS transistor 210 may operate in the deep sub-threshold region.
The matched RF signal may be provided to the low noise amplifier 112, which may be configured as a transconductance amplifier. The matched RF signal voltage may be converted into a current and amplified by a gain in the low noise amplifier 112. The low noise amplifier 112 may generate differential currents RFI and RFIX. The currents RFI and RFIX may be split to go to an I mixer and Q mixer in the passive double balanced mixer 114. The low noise amplifier 112 may include a step gain control section 214 controlled by a signal AGC_STEP, and a binary weighted gain control section 216 controlled by signals AGCx and AGCx_B, where x is the bit width of the signals AGCx and AGCx_B. The signals AGC_STEP, AGCx, and AGCx_B may be provided by a controller (not shown) to vary the gain as desired. In particular, the step gain control may adjust the gain in discrete intervals with the AGC_STEP signal. The NMOS transistors in the step gain control 214 may be positioned in parallel to common mode feedback resistors R3 and R4 and PMOS transistors 218 and 220. This may result in a reduction of the output impedance of the low noise amplifier 112, which provides a step gain, while the input impedance is unaffected. In addition, a differential amplifier 234 may compare a reference voltage Vref with a divided voltage to adjust the common mode feedback voltage Vcmfb. The common mode feedback voltage Vcmfb may drive the gates of the PMOS transistors 218 and 220. When the transistors in the step gain control section 214 are on, the load is shunted, reducing the load impedance and decreasing the gain. The input impedance remains substantially constant because the transconductance of the stage remains substantially constant.
The binary weighted gain control 216 may further adjust the gain in binary intervals with the AGCx and AGCx_B signals. In particular, the possible binary intervals may depend on the bit width of the AGCx and AGCx_B signals. Each bit of the AGCx and AGCx_B signals may correspond to a stage in the binary weighted gain control 216. In
The pulse generator 116 may generate the I and Q pulses using latches, combinatorial logic, sequential logic, or other logic. For the I current, the pulse generator 116 may produce less than 35% duty cycle I pulses including a first pulse zero degrees out of phase from the periodic signal, and a second pulse 180 degrees out of phase from the periodic signal. For the Q current, the pulse generator 116 may produce less than 35% duty cycle Q pulses including a third pulse 90 degrees out of phase from the periodic signal, and a fourth pulse 270 degrees out of phase from the periodic signal. The pulse generator 116 may also include an I/Q crossover correction circuit that adjusts an amplitude crossover threshold of the I and Q pulses. The amplitude crossover threshold includes the crossing point of the I and Q pulses when one pulse is rising and the other is falling. In particular, because the I and Q pulses may control switches in the mixers 114, it is desirable to adjust the amplitude crossover threshold of the I and Q pulses so that the switches are not turned on at the same time. The crossover correction circuit may operate in parallel with the pulse generator 116, and adjust the I and Q pulses provided to the mixers 114.
The passive double balanced mixers 114 may include transistors, active elements, passive elements, or other components to combine the differential currents RFI and RFIX from the low noise amplifier 112 with the I and Q pulses from the pulse generator 116. Currents comprising the intermediate frequency signal generated by the mixers 114 may be of a desired frequency lower than the received RF signals for downstream processing purposes. The direct coupled complex filters 120 may receive and filter the current-injected intermediate frequency signals. The filters 120 may perform second ordering filtering of the intermediate frequency signal. The filter 120 may include voltage mode operational amplifiers with current inputs. Current mode amplifiers, including current conveyors and other types of operational amplifiers, may also be used in the filter 120. The order of the filter and structure of the filter is not limited to second order and may be tailored to a particular application. The filtered signal generated by the filters 120 may be amplified by low noise amplifiers 302 prior to conversion to a digital signal in the analog to digital converter 304. Alternately, the filtered signal may be provided to the analog to digital converter 304 without amplification. The resulting digital signal may be processed downstream using digital signal processing to retrieve the information contained within the received RF signal.
The function of the crossover correction circuit in the pulse generator 116 is clarified with reference to
The gated clock circuits 904, 906, 908, and 910 may generate the OUT_180, OUT_270, OUT_0, and OUT_90 pulses, respectively, based on the enable signal EN, the inverted enable signal ENX, the periodic signal CLK, and the inverted periodic signal CLKX. In particular, the outputs of the D flip flops 912, 914, 916, and 918 are ANDed with the periodic signal CLK or inverted periodic signal CLKX to generate the I and Q pulses. The gated clock circuit 904 generates the OUT_180 pulse based on the enable signal EN and the periodic signal CLK. The gated clock circuit 906 generates the OUT_270 pulse based on the enable signal EN and the inverted periodic signal CLKX. The gated clock circuit 908 generates the OUT_0 pulse based on the inverted enable signal ENX and the periodic signal CLK. The gated clock circuit 910 generates the OUT_90 pulse based on the inverted enable signal ENX and the inverted periodic signal CLKX.
A digital control signal SPI may be provided to an adjustable voltage generator 1002. The control signal SPI may be provided by a controller, such as through a serial to parallel interface, to variably adjust the crossover threshold of the I and Q pulses. The voltage Va generated by the generator 1002 may be coupled to a differential amplifier 1004. The differential amplifier 1004 may be configured to compare the desired crossover threshold set by the voltage Va with the voltage Vf output from the differential amplifier 1004. The differential amplifier 1004 may adjust the voltage Vf so that it tracks the voltage Va. An optional temperature compensation circuit 1006 may receive and adjust the voltage Vf and output a gate voltage bias Vg. The temperature compensation circuit 1006 may compensate for variations in the threshold voltage. The gate voltage bias Vg biases the crossover threshold of the I and Q pulses, and may compensate for temperature, process, or other variations. The gate voltage bias Vg may be adjusted so that it is less than the threshold voltage Vth added to the voltage Vcm, where Vcm is the common mode voltage at the input of the direct coupled filter 120. If the crossover correction circuit 1000 does not include the temperature compensation circuit 1006, then the crossover threshold may be biased by the voltage Vf. The crossover threshold of the I and Q pulses may be adjusted with other techniques, including level shifting the I and Q pulses with a voltage divider, sizing of the transistors comprising the mixer 114, or other techniques.
The amplitude crossover threshold may be adjusted to the adjusted crossover threshold 1204 by a crossover correction circuit, such as the circuit 1000. The adjusted crossover threshold 1204 is biased down from the non-adjusted amplitude crossover threshold 1202 such that there is a different crossover point for the OUT_0 and OUT_90 pulses. At the adjusted crossover threshold 1204, the overlap between the OUT_0 and OUT_90 pulses is substantially reduced. In this case, the transistors 402 and 408 controlled by the OUT_0 pulse and the transistors 410 and 416 controlled by the OUT_90 pulse are not substantially turned on at the same time. The crossover threshold may also be adjusted for the pulses OUT_90 and OUT_180, OUT_180 and OUT_270, and OUT_0 and OUT_270 that are 90 degrees out of phase from one another.
The topology of the direct coupled complex filter 120 may be similar in appearance to a Tow Thomas Biquad filter, however, the filter 120 includes no resistors at the input that add noise and significantly degrade linearity (e.g., voltage mode operation). Since the filter 120 does not have a resistor at the input and instead operates in current mode, the gain is determined by the transconductance of the circuit shown in
At Step 1402, a voltage of a received radio frequency signal may be converted to a current and amplified. The RF signal may be converted by a low noise amplifier that converts a voltage to a current. The amplifier may include step control or binary weighting control to adjust the amplification. At Step 1404, pulses with a duty cycle of less than 35% may be generated based on a periodic signal. The pulses may include in-phase (I) and quadrature (Q) pulses, such as pulses that are zero, ninety, one hundred eighty, and two hundred seventy degrees out of phase from a periodic signal. The pulses may be generated with latches, and/or other combinatorial or sequential components. A crossover threshold of the pulses may be adjusted at Step 1406, such that an overlap between a rising and a falling pulse is substantially reduced. The crossover threshold may be adjusted so that switches in a mixer controlled by the pulses are not turned on simultaneously.
At Step 1408, the current generated at Step 1402 may be mixed with the pulses generated at Steps 1404 and 1406 to produce an intermediate frequency signal, which may be a zero intermediate frequency baseband signal. The intermediate frequency signal may include an in-phase signal and a quadrature signal, and may be currents. The mixing at Step 1408 may be performed by a mixer comprising NMOS transistors, for example, to combine the RF signal with I and Q pulses. At Step 1410, the intermediate frequency signal may be filtered to a filtered analog signal. A complex direct coupled filter may perform the filtering at Step 1410. The resulting filtered analog signal may be converted to a digital signal and digitally signal processed to retrieve information in the RF signal at Step 1412. Additional filtering, matching, and other steps may be included in the method 1400 to process the received RF signal to attain desired performance characteristics.
A radio frequency receiver has been disclosed comprising a low noise transconductance amplifier operable to amplify and convert a received voltage of a radio frequency signal to a first current; a pulse generator operable to generate pulses from a periodic signal, the pulses having a duty cycle of 35% or less; a double balanced mixer operable to combine the first current with the pulses to produce a second current, the second current comprising an intermediate frequency signal; and a direct coupled complex filter operable to filter the second current to a filtered analog signal. The radio frequency receiver may further comprise a crossover correction circuit operable to control a crossover threshold of the pulses, where the pulses comprise in-phase pulses and quadrature pulses; the crossover threshold comprises a crossing point of an amplitude of the in-phase pulses and the quadrature pulses, the crossing point selected to reduce the amount of time the in-phase pulses and quadrature pulses are simultaneously active; and the intermediate frequency signal comprises an in-phase signal and a quadrature signal. The double balanced mixer may comprise CMOS transistors where the crossover correction circuit is operable to bias a gate voltage of the CMOS transistors to select the crossing point. The duty cycle of the pulses may be between 20% and 35%.
The pulses may comprise in-phase pulses and quadrature pulses where the in-phase pulses comprise a first pulse zero degrees out of phase from the periodic signal and a second pulse one hundred eighty degrees out of phase from the periodic signal, and the quadrature pulses comprise a third pulse ninety degrees out of phase from the periodic signal and a fourth pulse two hundred seventy degrees out of phase from the periodic signal. The pulse generator may comprise a first latch operable to generate the quadrature pulses and a first latch output based on the periodic signal and a second latch output, and a second latch operable to generate the in-phase pulses and the second latch output based on the periodic signal and the first latch output. The first latch and the second latch may comprise a gated clock circuit.
The radio frequency receiver may further comprise an active bias generation circuit configured to bias the amplifier such that low frequency bias noise in the radio frequency signal is inhibited in the presence of large blocking signals. The active bias generation circuit comprises a reference current source coupled to a supply voltage and a node; a first NMOS transistor with a source coupled to ground, a drain coupled to the node, and a gate coupled to the received voltage; and a second NMOS transistor with a source coupled to the received voltage, a drain coupled to the supply voltage, and a gate coupled to the node. The low noise transconductance amplifier is further operable to amplify and convert the received voltage of the radio frequency signal to the first current using step weighting and binary weighting. The radio frequency receiver may further comprise a gain matching circuit operable to step up an impedance of the received voltage to match an input impedance of the low noise transconductance amplifier.
A method of receiving a radio frequency signal has been disclosed comprising converting and amplifying a voltage of the radio frequency signal to a first current; generating pulses from a periodic signal, the pulses having a duty cycle of 35% or less; mixing the first current with the pulses to produce a second current, the second current comprising an intermediate frequency signal; and filtering the second current to a filtered analog signal. The method may further comprise correcting a crossover threshold of the pulses, where the pulses comprise in-phase pulses and quadrature pulses; correcting the crossover threshold comprises selecting a crossing point of the in-phase pulses and the quadrature pulses to reduce the amount of time the in-phase pulses and quadrature pulses are simultaneously active; and the intermediate frequency signal comprises an in-phase signal and a quadrature signal. Correcting the crossover threshold may comprise biasing a gate voltage of a CMOS transistor to select the crossing point. The duty cycle of the pulses may be between 20% and 35%.
The pulses may comprise in-phase pulses and quadrature pulses, where the in-phase pulses comprise a first pulse zero degrees out of phase from the periodic signal and a second pulse one hundred eighty degrees out of phase from the periodic signal; and the quadrature pulses comprise a third pulse ninety degrees out of phase from the periodic signal and a fourth pulse two hundred seventy degrees out of phase from the periodic signal. Generating pulses in the method may comprise generating the quadrature pulses and a first latch output based on the periodic signal and a second latch output, and generating the in-phase pulses and the second latch output based on the periodic signal and the first latch output. The method may further comprise step weighting and binary weighting the amplification of the voltage of the radio frequency signal to the first current. The method may also comprise inhibiting low frequency bias noise in the radio frequency signal in the presence of large blocking signals.
An analog front end of a radio frequency receiver has been disclosed, comprising a low noise amplifier operable to amplify and convert a received voltage of a radio frequency signal to a first current, the low noise amplifier configured to step weight and binary weight the amplification; a pulse generator operable to generate in-phase and quadrature pulses based on a periodic signal, the in-phase and quadrature pulses having an approximately 25% duty cycle and comprising a first pulse zero degrees out of phase from the periodic signal, a second pulse one hundred eighty degrees out of phase from the periodic signal, a third pulse ninety degrees out of phase from the periodic signal, and a fourth pulse two hundred seventy degrees out of phase from the periodic signal; and a mixer operable to combine the first current with the in-phase and quadrature pulses to produce a second current, the second current comprising an intermediate frequency signal. The analog front end may further comprise a crossover correction circuit operable to control an amplitude crossover threshold of the in-phase and quadrature pulses, such that the in-phase and quadrature pulses are not active simultaneously; and a direct coupled complex filter operable to filter the intermediate frequency signal to a filtered analog signal.
Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims.
As used herein, the phrases “coupled with,” “coupled between,” or like phrases, are defined to mean directly connected to or indirectly connected through one or more intermediate components. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.
