Patent Publication Number: US-6711397-B1

Title: Structures and methods for direct conversion from radio frequency modulated signals to baseband signals

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     The present invention relates to wireless radio receiver technology and, more specifically, to improved circuits and methods for the direct conversion of radio frequency modulated signals to baseband signals without requiring conversion through an intermediate frequency. 
     2. The Prior State of the Art 
     Electrical signals have proven to be an effective means of conveying data from one location to another. The further a signal is transmitted, however, the greater the decay in the signal and the greater the chance for irreversible loss in the data represented by the signal. In order to guard against this signal decay, the core electrical signal that represents the data (i.e., the baseband signal) may be modulated or superimposed on a carrier wave in the Radio Frequency (RF) frequency spectrum. 
     In order to properly interpret the signal, conventional RF receivers demodulate the baseband signal from the received signal. The data represented by the extracted baseband signal may then be interpreted by other downstream circuitry. 
     In order to perform this demodulation, typical receivers include circuitry which first converts the received radio frequency modulated signal into an intermediate frequency (“IF”) signal. This intermediate frequency signal is then converted into the baseband signal for further data processing. Receiver architectures that convert through the intermediate frequency are often called “heterodyne” receiver architectures. Naturally, circuit elements (called “IF components”) are required in order to deal with the intermediate conversion to and from the intermediate frequency. 
     It is desirable to reduce the cost, size, and power consumption of a particular receiver architecture design for strategic marketing of the receiver. This is particularly true of wireless RF receivers since those receivers are often portable and run on battery power. 
     One technology developed in order to reduce RF receiver cost, size, and power consumption is called “direct conversion.” Direct conversion refers to the direct conversion of RF modulated signals into corresponding baseband signals without requiring conversion through the intermediate frequency. Such direct conversion receiver architectures are often called “zero-IF,” “synchrodyne,” or “homodyne” receiver architectures. 
     FIG. 1 illustrates a conventional direct conversion circuit  100  in accordance with the prior art. The circuit  100  includes an antenna  101  which receives the RF modulated signal. The antenna then provides the received signal to an amplifier  102  which amplifies the signal for further processing. The amplifier  102  may be, for example, an RF low noise amplifier. 
     The amplified signal is then split into two branches, an “in-phase” branch  110 , and a “quadrature-phase” branch  120 . Each branch includes a mixer that initially receives the amplified signal. For instance, the in-phase branch  110  includes an in-phase mixer  111 , and the quadrature-phase branch  120  includes a quadrature-phase mixer  121 . A local oscillator  130  provides a sine or square wave signal as a control signal to each of the mixers. Each mixer is configured to nonlinearly process the amplified signal and control signal, resulting in output signal components at frequencies equal to the sum and difference of amplified signal and control signal frequencies, plus higher-order components at other frequencies. The circuit includes a 90-degree phase shifter  131  which causes the control signal for the quadrature-phase mixer  121  to be 90 degrees out of phase with the control signal for the in-phase mixer  111 . 
     The signal from the in-phase mixer  111  is then passed through a low pass filter  112  to a baseband amplifier  113  to complete the extraction of the baseband (difference frequency) signal from the received signal as far as the in-phase branch  110  is concerned. Likewise, the signal from the quadrature-phase mixer  121  is passed through a low pass filter  122  to a baseband amplifier  123  to complete the extraction of the baseband (difference frequency) signal as far as the quadrature-phase branch is concerned. The quadrature baseband signals are then processed by signal processing circuitry  150 . 
     The direct conversion circuit of FIG. 1 does not convert through an intermediate frequency and thus there are no IF components needed to deal with an intermediate conversion. Consequently, the direct conversion circuit of FIG. 1 is smaller, and requires less power than conventional heterodyne receiver architectures, which perform intermediate conversion. 
     However, there are some performance issues for the direct conversion circuit of FIG. 1 that limit its practical implementation. First, there is often local oscillator leakage to the antenna  101 , to the amplifier  102  input and to the mixer inputs. This results in local oscillator Direct Current (DC) self-mixing products that can overpower and degrade the baseband signals. Second, the antenna may radiate the local oscillator leakage causing an interference problem for other nearby receivers. This may also cause a time-varying self-mixing product due to radiated RF leakage reflecting off nearby objects, possible in motion, and being received back at the same antenna, then adding to the leakage component present at the antenna terminal. Third, there is a lack of RF selectivity, combined with amplifier and mixer limited dynamic range, resulting in direct AM detection of high level, in-band or adjacent channel interference. The net result of these performance degradations is that there may be some loss of data in the baseband signals and some interference with the operation of nearby receivers. 
     What is therefore desired are circuits and methods for direct conversion of RF modulated signals directly into baseband signals without intermediate conversion through and intermediate frequency while reducing the above-described problems related to oscillator leakage, interference with surrounding antennas, self-mixing products at the antenna, low RF selectivity, and limited dynamic range. 
     SUMMARY OF THE INVENTION 
     A direct conversion receiver converts a radio frequency modulated (RF) signal into a corresponding baseband signal without requiring conversion through an intermediate frequency. The direct conversion receiver abates local oscillator leakage, increases dynamic range and increases RF selectivity as compared to conventional direct conversion circuits. 
     After being received and amplified as necessary, the RF signal is provided to two branches of the direct conversion circuit, an “in-phase” branch and a “quadrature-phase” branch.” However, instead of the conventional one mixer per branch, each branch includes two mixers. These four mixers periodically pass on the RF signal if the corresponding control signal provided to the mixer is high. A local oscillator provides the controls signals in the form of binary waves which have the same period, nominally, as the carrier period of the RF signal. Each mixer is provided with a primary binary control signal having a duty cycle of approximately 25 percent in the logic “high” state and a corresponding binary complement signal having a duty cycle of approximately 75 percent in the logic “high” state. The quarter-period “high” states of the primary control signals are time-shifted, from one control signal to the next, to produce quadrature-phased, primary binary control signals with relative phases of approximately 0, 90, 180, and 270 degrees. 
     As for the in-phase branch, a first mixer has an output terminal coupled to the positive terminal of the operational amplifier. This first mixer is provided with a corresponding first primary binary control signal. A second mixer in the in-phase branch has an output terminal that is coupled to the negative terminal of the operational amplifier. This second mixer is provided with a second primary control signal that is 180 degrees out of phase as compared to the first primary control signal. An operational amplifier receives the signal passed by the two mixers in the in-phase branch and generates a signal that represents the difference between the two signals provided by the mixers. The resulting signal is then passed through a low pass filter to generate a first baseband signal. 
     As for the quadrature-phase branch, a first mixer has an output terminal coupled to the positive terminal of another operational amplifier. This first mixer is provided with a corresponding primary binary control signal that is 90 degrees out of phase. A second mixer in the quadrature-phase branch has an output terminal that is coupled to the negative terminal of the operational amplifier. This second mixer is provided with a primary control signal that is 270 degrees out of phase. This operational amplifier receives the signal passed by the two mixers in the quadrature-phase branch and generates a signal that represents the difference between the two signals provided by the mixers. The resulting signal is then passed through another low pass filter to generate a second baseband signal. The two quadrature baseband signals are then provided to signal processing circuitry for further processing. 
     The direct conversion circuit described herein has many advantages over conventional direct conversion circuits. Specifically, the signals provided to each mixer are balanced complementary signals. Thus, any leakage from a pair of complementary signals tends to cancel each other out. Also, the superposition of the two primary switching control signals provided to each branch is a square wave at twice the RF carrier frequency. Likewise, the superposition of the two complementary switching control signals provided to each branch is also a square wave at twice the RF carrier frequency. 
     Also, coupling of the four switching control signal waveforms in the two switching mixers of each branch via a capacitive leakage path to a distant node results in an “impulse train” at that node which is balanced. Impulse trains occur at the leading and lagging edges of the control signals thus forming a positive-going impulse train at four times the receive frequency and a coincident negative-going impulse train at four times the receive frequency. Thus, coupling along balanced leakage paths tend to cancel each other. 
     In addition, the superposition of all eight control signals is a balanced waveform that contains no spectral components at the receive frequency. These effects combine to reduce the local oscillator leakage to surrounding components including the antenna. 
     Furthermore, the circuit results in reduced AM detection in the mixers due to the use of switches for mixers, as opposed to “active” mixer implementations such as Gilbert cells. Switching mixers can be implemented with active semiconductor switching devices where the degree of non-linearity incurred across the switch itself in the “on” state is less than with Gilbert cells or semiconductor diode ring devices. Therefore, larger second order and third order intercept values can be realized. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to describe the manner in which the above-recited and other advantages and features of the invention can be obtained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 illustrates a direct conversion circuit in accordance with the prior art in which one mixer is provided for each of the in-phase and quadrature-phase branches; 
     FIG. 2 illustrates a direct conversion circuit in accordance with the present invention in which two mixers are provided for each branch, and in which the output from the mixers is provided to operational amplifiers; 
     FIG. 3 is a signal diagram of a RF signal received at the direct conversion circuit of FIG. 2; 
     FIG. 4A i  is a signal diagram of a primary control signal provided to a mixer; 
     FIG. 4A ii  is a signal diagram of a signal that represents the complement of the signal in FIG. 4A i;    
     FIG. 4B i  is a signal diagram of a primary control signal that is 180 degrees out of phase compared to the primary control signal of FIG. 4A i;    
     FIG. 4B ii  is a signal diagram of a signal that represents the complement of the signal in FIG. 4B i;    
     FIG. 4C i  is a signal diagram of a primary control signal that is 90 degrees out of phase compared to the primary control signal of FIG. 4A i;    
     FIG. 4C ii  is a signal diagram of a signal that represents the complement of the signal in FIG. 4C i;    
     FIG. 4D i  is a signal diagram of a primary control signal that is 270 degrees out of phase compared to the primary control signal of FIG. 4A i;    
     FIG. 4D ii  is a signal diagram of a signal that represents the complement of the signal in FIG. 4D i;    
     FIG. 5A is a signal diagram of the signal output from the upper-most mixer of the in-phase branch as shown in FIG. 2; 
     FIG. 5B is a signal diagram of the signal output from the lower-most mixer of the in-phase branch; 
     FIG. 5C is a signal diagram of the signal output from the operational amplifier of the in-phase branch; 
     FIG. 5D is a signal diagram of the signal (i.e., the baseband signal) output from the low pass filter of the in-phase branch; 
     FIG. 6A is a signal diagram of the signal output from the upper-most mixer of the quadrature-phase branch as shown in FIG. 2; 
     FIG. 6B is a signal diagram of the signal output from the lower-most mixer of the quadrature-phase branch; 
     FIG. 6C is a signal diagram of the signal output from the operational amplifier of the quadrature-phase branch; 
     FIG. 7 is a circuit diagram of a mixer that may be used as the mixer shown in FIG. 2; 
     FIG. 8 is a circuit diagram of another mixer that may be used as the mixer shown in FIG. 2; and 
     FIG. 9 illustrates a flowchart of a method implemented by the circuit of FIG. 2 for extracting the baseband signal from the received RF signal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention extends to both circuits and methods for the direct conversion of radio frequency (RF) modulated signal into a corresponding baseband signal without requiring conversion through an intermediate frequency. This description will first define certain terms that are to be applied throughout this description and in the claims. The description will then detail a specific embodiment of the present invention. 
     In this description and in the claims, “radio frequency” or “RF” is defined as any frequency that is suitable for wireless communications. Typical RF frequencies range from several kilohertz to tens of gigahertz. In the future, it is possible that wireless communication using greater frequencies may become possible. It is intended that such frequencies also be included within the definition of “radio frequency” so long as such frequencies are suitable for wireless communications. 
     In this description and in the claims, a “baseband signal” is an original signal before it is modulated on a radio frequency wave in preparation for wireless transmission at the transmitter end. The baseband signal is typically extracted or demodulated from the radio frequency wave at the receiver end with, of course, some level of distortion from the original signal. The baseband signal most directly represents the data involved in the transmission. 
     In this description and in the claims, “radio frequency modulated signal”, “RF modulated signal” and “RF signal” refer to a signal in which a baseband signal is modulated on an RF carrier wave. The details of embodiments of the present invention will now be described. 
     FIG. 2 illustrates a direct conversion circuit  200  in accordance with the present invention. The direct conversion circuit may be implemented in, for example, a wireless receiver and operates to directly convert an RF signal into a corresponding baseband signal without requiring an intermediate conversion through an intermediate frequency. 
     The circuit includes an antenna  201 , which receives an RF signal in which a baseband signal is modulated on an RF carrier wave. The RF signal then passes through an RF low noise amplifier  202 , which amplifies the RF signal in preparation for further processing. FIG. 3 illustrates a waveform of an example amplified RF signal in which the magnitude of the RF signal is plotted vertically, and in which the passage of time is plotted horizontally. 
     The amplified RF signal from the output terminal of the RF amplifier  202  is then provided to an “in-phase” branch  210  and a “quadrature-phase” branch  220 . Instead of each branch conventionally having one mixer, each branch contains two mixers. For example, the in-phase branch  210  includes mixers  211  and  212 , and the quadrature-phase branch  220  includes mixers  221  and  222 . Each mixer  211 ,  212 ,  221  and  222  receives the amplified RF signal and is configured to pass the RF signal only if a corresponding control signal provided to the mixer is high. Mixers  211 ,  212 ,  221 , and  222  represent examples of a means for passing the RF signal at the carrier frequency of the RF signal. FIG.  7  and FIG. 8 illustrate circuit diagrams of mixers that are suitable for the present invention and also represent examples of a means for passing the RF signal. 
     A local oscillator  230  represents an example of a means for controlling the means for passing so that all of the means for passing (i.e., the mixers) pass out of phase with each other as described herein. The local oscillator  230  provides the control signals that control whether or not each mixer passes the amplified RF signal. The oscillator  230  provides a total of eight control signals to the mixers in aggregate. A primary control signal and its binary complement are provided to each mixer. 
     For example, the oscillator  230  provides a primary control signal 0° (see FIG. 4A i ) and an inverted control signal 10° (see FIG. 4A ii ) to the mixer  211 , a primary control signal 180° (see FIG. 4B i ) and an inverted control signal 1180° (see FIG. 4B ii ) to the mixer  212 , a primary control signal 90° (see FIG. 4C i ) and an inverted control signal 190° (see FIG. 4C ii ) to the mixer  221 , and a primary control signal 270° (see FIG. 4D i ) and an inverted control signal 1270° (see FIG. 4D ii ) to the mixer  222 . 
     Each inverted control signal is the binary complement of the primary control signal. Each primary/inverted signal pair represents a balanced control signal, the control signal being “balanced” since the superposition of the primary and inverted control signals results in a substantially constant signal. 
     Each primary control signal 0°, 90°, 180° and 270° is a binary periodic wave having a duty cycle (high state condition) of approximately 25%. The primary control signal 90° is approximately 90 degrees out of phase with the primary control signal 0°; the primary control signal 180° is approximately 180 degrees out of phase with the primary control signal 0°; and the primary control signal 270° is approximately 270 degrees out of phase with the primary control signal 0°. 
     The signal processing of the in-phase branch  210  with respect to the example amplified RF signal of FIG. 3 will now be described with reference to FIGS. 5A through 5D. FIG. 5A illustrates the output signal from the mixer  211 , which passes the amplified signal only when the primary control signal 0° (see FIG. 4A i ) is high. FIG. 5B illustrates the output signal from the mixer  212 , which passes the amplified signal only when the primary control signal 180° (see FIG. 4B i ) is high. 
     The output signals shown in FIGS. 5A and 5B are passed to an operational amplifier  250  (see FIG.  2 ). The signals optionally pass first through the low pass filters  231  and  232  respectively in order to eliminate any high frequency anomalous components generated at the mixers. The operational amplifier  250  generates a signal that has the magnitude at any given time equal to the magnitude of the signal from the mixer  211  (FIG. 5A) minus the magnitude of the signal from the mixer  212  (FIG.  5 B). This resulting signal is shown in FIG.  5 C. 
     The signal of FIG. 5C then passes through a low pass filter  251  which filters out high frequency components. The resulting baseband signal is illustrated in FIG.  5 D. This baseband signal component is then amplified by the baseband amplifier  252  for further processing by the signal processing circuitry  270 . 
     The signal processing of the quadrature-phase branch  220  with respect to the example amplified RF signal of FIG. 3 will now be described with reference to FIGS. 6A through 6C. 
     FIG. 6A illustrates the output signal from the mixer  221 , which passes the amplified signal only when the primary control signal 90° (see FIG. 4C i ) is high. FIG. 6B illustrates the output signal from the mixer  222 , which passes the amplified signal only when the primary control signal 270° (see FIG. 4D i ) is high. 
     The output signals shown in FIGS. 6A and 6B are passed to an operational amplifier  260 . The signals optionally pass first through the low pass filters  241  and  242  respectively in order to eliminate any high frequency anomalous components generated at the mixers. The operational amplifier  260  generates a signal that has the magnitude at any given time equal to the magnitude of the signal from the mixer  221  (FIG. 6A) minus the magnitude of the signal from the mixer  222  (FIG.  6 B). This resulting signal is shown in FIG.  6 C. 
     The signal of FIG. 6C then passes through a low pass filter  261  which filters out high frequency components. Because the signal of FIG. 6C remains at zero except for some high frequency variation that is reasonably balanced above and below zero, the high-frequency signal is essentially filtered out entirely so as to be at the zero state. If there was a signal from the operational amplifier  260 , this signal would then pass through a baseband amplifier  262  and on for further processing at the signal processing circuitry  270 . 
     The resulting baseband signal is zero at the quadrature branch because the in-phase branch control signals 0° and 180° are timed to be in phase with the peaks of the amplifier RF signal shown in FIG.  3 . Thus, the quadrature-phase control signals are timed to be high where the RF signal transitions between positive and negative magnitudes. This timing is an optional feature provided by signal processing circuitry  270 . If there were no such timing control, both the in-phase and quadrature-phase branches would provide a baseband signal component to the signal processing circuitry  270 . Thus, the signal processing circuitry  270  must either be able to control the timing of the local oscillator  230 , or be able to evaluate two baseband signal components to reconstruct the original baseband signal. 
     One skilled in the art will recognize that as long as the primary control signals 0° and 180° are timed to be in phase with the peaks of the amplifier RF signal shown in FIG. 3, that the baseband signal may be extracted from the in-phase branch alone. Thus, the quadrature phase branch may be eliminated in this context. However, the output from the quadrature-phase branch may be useful for the signal processing circuitry to time the oscillator to so that the primary control signals 0° and 180° are in phase with the peaks of the amplified RF signal. For example, the signal processing circuitry may adjust the timing until the signal provided by the quadrature-phase branch is reduced almost to zero magnitude. 
     As explained above, the circuit of FIG. 2 allows the baseband signal to be extracted from an RF signal for further processing by downstream signal processing circuitry. The advantages of the circuit of FIG. 2 as compared to the conventional direct conversion circuits will be explained after the following description of the mixers. 
     FIG. 7 illustrates in further detail the mixer  211  of FIG.  2 . The other mixers  212 ,  221 , and  222  of FIG. 2 are structured similar to the mixer shown in FIG. 7 except for the control signals they receive are, of course, different as described above. 
     The mixer  211  is often called a “T” switch and is composed of two on-off Single Pole, Single Throw (SPST) switches  710  and  720  connected in series between the input signal (i.e., the amplified RF signal) and the output signal. The mixer also includes a shunt SPST switch  730  connecting the common node of the series switches to ground. The control signal 0° is provided to the gate of the series switches which may each be an nMOS transistor. The complementary control signal 10° is provided to the gate of the shunt switch  730  which may also be an nMOS transistor. 
     Each of the switches  710 ,  720  and  730  is closed when the control signal voltage applied at its gate is high, and is otherwise open. When the control signal 0° is high (and the control signal 10° is low), the switches  710  and  720  are closed and the switch  730  is open. Thus, the RF signal passes through the switches  710  and  720  and is output to the operational amplifier. 
     When the control signal 0° is low, switches  710  and  720  are open. Thus, the mixer is off and the RF signal is not passed to the operational amplifier. Also in this off condition, since the control signal 0° is low, the complementary control signal 10° is high thereby grounding the common node of the series switches to ground through the shunt switch  730 . This improves the off condition by reducing leakage current in the series switches. 
     In another embodiment of the mixer, three or more switches may be coupled in series between the amplified RF signal input and the output. In this embodiment, a shunt switch would be provided for each common node thus grounding each common node when the mixer is off. This alternative embodiment is especially advantageous for high frequency RF signals thus extending the operating frequency range of the direct conversion circuit of FIG.  2 . 
     FIG. 8 illustrates a mixer  800  in accordance with an alternative embodiment of the invention. The mixer includes three series switches  810 ,  820  and  830  coupled between the input terminal that receives the RF signal and the output terminal. The mixer also includes a shunt switch  840  coupled between ground and the common node of switches  810  and  820 , and a shunt switch  850  coupled between ground and the common node of switches  820  and  830 . 
     As mentioned above, the direct conversion circuit  200  operates to receive wireless RF signals. Accordingly, the direct conversion circuit should preferably be manufactured using a technology that is suited for RF radio or wireless technologies. These technologies might include, for example, deep submicron CMOS, CMOS-on-SOI, BiCMOS, SiGe and GaAs semiconductor technologies. Deep submicron CMOS is preferred since it is low cost and has high integration capability. 
     The direct conversion circuit described herein has many advantages over conventional direct conversion circuits. Specifically, the signals provided to each mixer are balanced complementary signals. Thus, any leakage from a pair of complementary signals tends to cancel each other out. Also, the superposition of the two primary switching control signals provided to each branch is a square wave at twice the RF carrier frequency. Likewise, the superposition of the two complementary switching control signals provided to each branch is also a square wave at twice the RF carrier frequency. Therefore, this superposition contains only even-order harmonics of the receive frequency thus reducing interference with the received signal. 
     Also, coupling of the four switching control signal waveforms in the two switching mixers of each branch via a capacitive leakage path to a distant node results in an “impulse train” at that node which is balanced. Impulse trains occur at the leading and lagging edges of the control signals thus forming a positive-going impulse train at four times the receive frequency and a coincident negative-going impulse train at four times the receive frequency. Thus, coupling along balanced leakage paths tend to cancel each other. 
     In addition, the superposition of all eight control signals is a balanced waveform that contains no spectral components at the receive frequency. These effects combine to reduce the local oscillator leakage to surrounding components including the antenna. 
     Furthermore, the circuit results in reduced AM detection in the mixers due to the use of switches for mixers, as opposed to “active” mixer implementations such as Gilbert cells. Switching mixers can be implemented with active semiconductor switching devices where the degree of non-linearity incurred across the switch itself in the “on” state is less than with Gilbert cells or semiconductor diode ring devices. Therefore, larger second order and third order intercept values can be realized. 
     FIG. 9 flowcharts a method  900  of extracting a baseband signal from the RF signal and may be performed for each branch both in-phase and quadrature-phase. First, the method performs a step for periodically passing the RF modulated signal to an operational amplifier at alternating input terminals at double the frequency of the carrier frequency of the radio frequency modulated signal. For the in-phase branch, the passed RF signal is represented by the combination of the signal of FIG.  5 A and the signal of FIG.  5 B. 
     Now specifically describing the corresponding acts associated with this step, the signal of FIG. 5A (in which the RF signal is periodically represented) is passed to the positive terminal of the operational amplifier (act  920 ). Also, the signal of FIG. 5B (in which the RF signal is periodically represented at 180 degrees out of phase) is passed to the negative terminal of the operational amplifier (act  930 ). This passing may occur as described above. 
     Next, the difference signal is created (act  940 ), the difference signal representing the difference in magnitude at any given time between the passed signal at the positive terminal (the signal of FIG. 5A) and the passed signal at the negative terminal (the signal of FIG. 5B) of the operational amplifier. The difference signal from the in-phase branch is represented by the signal of FIG.  5 C. The difference signal is then filtered (act  950 ) to remove high frequency components. The resulting signal for the in-phase branch is illustrated in FIG.  5 D. 
     In summary, the circuit described herein overcomes many of the disadvantages inherent in conventional direct conversion circuits thus allowing direct conversion to be more readily adopted and thus improving on prior state of the art. 
     Optimal performance is achieved when the primary control signals 90°, 180° and 270° are very close to exactly 90 degrees, 180 degrees, and 270 degrees, respectively, out of phase from the primary control signal 0°. However, as the phases vary from 90 degrees, 180 degrees, and 270 degrees, the performance of the direct conversion amplifier will gradually degrade. Realizing that exact precision in phasing is difficult, if not impossible, to achieve, there will likely be some variance from 90 degrees, 180 degrees, and 270 degrees in practical implementation. Thus, in this description and in the claims, these degree values are emphasized to be “approximate.” “Approximate” in this context means that the phase should be close enough to the value that the direct conversion amplifier illustrated in FIG. 2 is suitable for receiving a wireless RF signal and extracting a baseband signal from the RF signal. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.