Abstract:
The present invention describes active double-balanced mixers. The mixers use an isolation and matching section that provides RF section input matching and superior isolation between the LO and RF port. The mixers can be implemented using an inexpensive semiconductor processing technology, obviating the need for matching inductors and facilitating a low-cost, low-power fully integrated wireless receiver.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field Of The Invention  
           [0002]    The present invention relates to mixers, and more particularly relates to self-matched fundamental and subharmonic active double-balanced mixers.  
           [0003]    2. Description of Related Art  
           [0004]    Recently there has been a great amount of activity in realizing low-cost low-power integrated wireless receivers. Wireless receivers generally comprise one or more downconversion stages, each of which contributes to power consumption. Low-power receivers can be realized if the number of downconversion stages is reduced to one. Low-cost receivers can be realized using an inexpensive semiconductor processing technology, which typically does not feature high quality factor (high-Q) inductors. Downconversion stages use a mixer for the frequency translation from the radio frequency (RF) to the intermediate frequency (IF). If the IF frequency is not equal to 0, the receiver is generally known as superheterodyne, whereas if the frequency is equal to 0, the receiver is known as homodyne or Direct Conversion Receiver (DCR).  
           [0005]    A conventional double-balanced Gilbert mixer, as shown in FIG. 1, is used in a downconversion stage for both superheterodyne receivers and DCRs. The use of a double-balanced Gilbert mixer in a DCR may lead to self-mixing, local oscillator (LO) leakage and DC offsets, which occur due to the same frequency of the RF and LO signals. A double-balanced Gilbert mixer can be modified to effectively address the problems associated with DCRs. A conventional active subharmonic double-balanced mixer, as shown in FIG. 2, is a modification of the double-balanced Gilbert mixer of FIG. 1. An active subharmonic double-balanced mixer alleviates the problem of self-mixing and DC offsets by using an LO signal that has a frequency that is approximately one-half the RF signal frequency.  
           [0006]    The input of both a double-balanced Gilbert mixer and an active subharmonic double-balanced mixer needs to be matched to the nominal impedance. In order to do this, conventional mixer input matching techniques require inductive components (e.g. high-Q integrated chip inductors or off-chip inductors) to resonate the capacitive input of the mixer. However, integrated high-Q inductors are not available in inexpensive processes. In addition, they occupy large areas of the semiconductor real estate. Finally, off-chip inductors add to the assembly and packaging cost.  
           [0007]    The applicability of a double-balanced Gilbert mixer in a DCR is limited because of the imperfect isolation between the LO and RF ports. As the LO signal is often considerably stronger than the RF signal, it leaks into the RF port through a parasitic path between the (LO and RF) ports. A LO signal coupled to the RF port mixes with the LO signal from the LO port and produces a static DC offset. A LO signal coupled to the RF port may also radiate through the antenna, due to the imperfect isolation of the receive RF path, reflect off moving objects, and penetrate the receiver. The received LO signal mixes with the LO signal from the LO port and produces a time-variable DC offset. In order to reduce DC offsets, it is advantageous to improve the isolation between the RF and LO ports.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    An object of this invention is to realize a low-cost self-matched receive mixer, eliminating the need for inductors to match the mixer input.  
           [0009]    Another object of this invention is to improve the isolation between the RF and LO ports and suppress DC offsets and LO signal leakage.  
           [0010]    In a preferred embodiment of an active self-matched double-balanced mixer both the RF and LO input can be single-ended or differential. Additionally, the IF output can be single-ended or differential. The mixer includes an RF section, a matching and isolation section and an LO section.  
           [0011]    The RF section provides voltage-to-current conversion of the input RF signal. The matching and isolation section provides: 1) a match of the RF section input to the nominal impedance; and 2) superior isolation between the RF and LO ports.  
           [0012]    The RF section (without bias) consists of two transistors. Capacitive collector loading of the RF section transistors shifts the input Miller impedance from capacitive to real and provides an RF section input match for a broad frequency range. The matching and isolation section (without bias) consists of two transistors, two resistors and four capacitors. The LO section consists of either four or eight transistors, depending whether the mixer operates as fundamental or subharmonic.  
           [0013]    An advantage of the present invention over the conventional mixers includes the mixer being self-matched, without internal or external inductors. The input match is better than −10 dB in the frequency span of over 3 GHz. The present invention also has superior isolation between the LO and RF ports, suppressing DC offsets and LO leakage. Finally, the present invention can be implemented using an inexpensive digital semiconductor processing technology which does not feature inductors.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The above and further objects, features and advantages of the invention will become clear from the following more detailed description when read with reference to the accompanying drawings in which:  
         [0015]    [0015]FIG. 1 is a schematic diagram of a conventional fundamental active double-balanced mixer.  
         [0016]    [0016]FIG. 2 is a schematic diagram of a conventional subharmonic active double-balanced mixer.  
         [0017]    [0017]FIG. 3 is a schematic diagram of a self-matched fundamental active double-balanced mixer according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 4 is a schematic diagram of a self-matched subharmonic active double-balanced mixer according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Two preferred active double-balanced mixer embodiments are depicted in FIGS. 3 and 4. FIG. 3 is a self-matched fundamental active double-balanced mixer and FIG. 4 is a self-matched subharmonic active double-balanced mixer. An RF section provides voltage-to-current conversion of the input RF signal. A matching and isolation section is used for RF input matching in both the fundamental and subharmonic active double-balanced mixers. A matching and isolation section also provides superior isolation between the LO and RF mixer ports. Therefore, the LO signal is less likely to couple to the RF port, thereby reducing the DC offset level and LO leakage.  
         [0020]    An LO section can be implemented with two differential transistor pairs, as shown in FIG. 3, or four differential transistor pairs, as shown in FIG. 4. In the subharmonic embodiment, the LO signal leakage to the RF port is further reduced by using unequal LO and RF frequencies. The mixer load in FIG. 3 and FIG. 4 is passive. Alternatively, the load may be active.  
         [0021]    In a self-matched fundamental active double-balanced mixer, as shown in FIG. 3, the RF signal is input to the bases of transistors Q 1  and Q 2 , which operate in common-emitter configurations. The RF input can be either single-ended or differential. The transistors Q 1  and Q 2 , as shown in FIG. 3, perform voltage-to-current conversion. C 1  and C 2  are decoupling capacitors. The collectors of Q 1  and Q 2  are capacitively coupled to the bases of transistors Q 3  and Q 4 , which also operate in common-emitter configurations. Capacitors C 5  and C 6  ensure that AC ground is presented to transistors Q 3  and Q 4  and define the load of the transistors Q 1  and Q 2 . The collectors of transistors Q 3  and Q 4  are coupled to the LO section, or resistors coupled to the LO section, if the goal is to further suppress third-order intermodulation distortion. The RF section input impedance is matched by proper sizing of resistors R 5  and R 6  and capacitors C 3 , C 4 , C 5  and C 6 . The LO-RF port isolation is improved by the collector-emitter isolation of transistors Q 3  and Q 4  and attenuation of resistors R 5  and R 6 .  
         [0022]    The LO section consists of two differential pairs (Q 5 -Q 6  and Q 7 -Q 8 ). The LO signal is divided into two components phase-shifted by 180° , each of which is applied to the base of two transistors of distinct differential pairs in the LO section. The LO section translates the RF signal to the intermediate frequency, which is equal to the absolute value of the difference between the RF and LO frequencies. In a fundamental active double-balanced mixer for a DCR, the RF and LO frequencies are approximately equal, therefore, the IF frequency is approximately equal to 0. The collectors of Q 5 , Q 6 , Q 7  and Q 8  are coupled to the mixer load, which could be passive or active. In FIG. 3, the mixer load is represented by resistors R 9  and R 10 . Voltage source Vbb1 provides bias current for the RF section. Voltage source Vbb2 provides bias for the matching and isolation section. Voltage source Vcc is the mixer supply voltage. Resistors R 1  and R 2  are RF section bias resistors, whereas resistors R 7  and R 8  are matching and isolation section bias resistors. Resistors R 3  , R 4  and R 11  linearize the RF section.  
         [0023]    In a self-matched subharmonic active double-balanced mixer as shown in FIG. 4, the RF signal is input to the bases of transistors Q 11  and Q 12 , which operate in common-emitter configurations. The RF input can be either single-ended or differential. The transistors Q 11  and Q 12 , as shown in FIG. 4, perform voltage-to-current conversion. C 1  and C 2  are decoupling capacitors. The collectors of Q 11  and Q 12  are capacitively coupled to the bases of transistors Q 9  and Q 10 , which also operate in common-emitter configurations. Capacitors C 5  and C 6  ensure that AC ground is presented to transistors Q 9  and Q 10  and define the load of the transistors Q 11  and Q 12 . The collectors of transistors Q 9  and Q 10  are coupled to the LO section, or resistors coupled to the LO section, if the goal is to further suppress third-order intermodulation distortion. The RF section input impedance is matched by proper sizing of resistors R 5  and R 6  and capacitors C 3 , C 4 , C 5  and C 6 . The LO-RF port isolation is improved by the collector-emitter isolation of transistors Q 9  and Q 10  and attenuation of resistors R 5  and R 6 . The LO section consists of four differential pairs (Q 1 -Q 2 , Q 3 -Q 4 , Q 5 -Q 6  and Q 7 -Q 8 ). The LO signal is divided into four components phase-shifted by 90° , each of which is applied to the bases of two transistors in the LO section (e.g. LO shifted by 90° is applied to the bases of Q 3  and Q 5 ). The LO section translates the RF signal to the intermediate frequency, which is equal to the absolute value of the difference between the RF and a double of the LO frequencies. In a subharmonic active double-balanced mixer for a DCR, the RF frequency is approximately equal to twice the LO frequency, therefore, the IF frequency is approximately equal to 0. The collectors of Q 1 -Q 8  are coupled to the mixer load, which could be passive or active. In FIG. 4, the mixer load is represented by resistors R 9  and R 10 . Voltage source Vbb1 provides bias current for the RF section. Voltage source Vbb2 provides bias for the matching and isolation section. Voltage source Vcc is the mixer supply voltage. Resistors R 1  and R 2  are RF section bias resistors, whereas resistors R 7  and R 8  are matching and isolation section bias resistors. Resistors R 3 , R 4  and R 11  linearize the RF section.  
         [0024]    The following table compares a typical conventional to a typical self-matched fundamental (subharmonic) active double-balanced mixer in terms of input matching and LO-RF isolation. All numbers refer to the RF frequency 2GHz, supply voltage 3 V, and power dissipation 6.5 mW. The input is matched to 2*50 Ω; the output load is 2*1000 Ω.  
                                                                                 Conventional fundamental   Conventional subharmonic   Self-matched fundamental   Self-matched subharmonic           active double-balanced   active double-balanced   active double-balanced   active double-balanced           mixer (FIG. 1)   mixer (FIG. 2)   mixer (FIG. 3)   mixer (FIG. 4)                                    Input   −2   −2   −11   −11       Matching S11 [dB]       LO-RF   −24   −29   −42   −48       Isolation [dB]                  
 
         [0025]    While these figures are representative for input matching and LO-RF isolation of the conventional and claimed mixers, they may vary due to different processing parameters, mixer specifications and design goals, and the implemented tradeoff between noise, power dissipation, input matching, linearity, conversion gain and frequency.  
         [0026]    While this invention describes an active double-balanced mixer, it is possible to design a single-balanced mixer with improved LO-RF isolation and matching, which has an RF section comprising one transistor, a matching and isolation section comprising one transistor, and an LO section comprising one or two differential transistor pairs, depending on whether the mixer operates as fundamental or subharmonic.  
         [0027]    While this invention has been described in its preferred form, it will be appreciated that changes may be made in the form, construction, procedure and arrangement of its various elements and steps without departing from its spirit or scope.