Abstract:
An LO leakage canceller for a direct conversion receiver is disclosed. The canceller includes an RF input stage operating to provide at least two input signals, a local oscillator, an in-phase processor, and a quadrature processor. The local oscillator is configured to produce at least two reference signals for modulating the two input signals. Each of the in-phase and quadrature processors includes two consecutive quadrature splitters operating to substantially reduce leakage of the reference signal from the local oscillator to the RF input stage.

Description:
BACKGROUND 
     The present disclosure generally relates to direct conversion receivers used in telecommunication or data communication devices, and more specifically, to a local oscillator (“LO”) leakage signal canceller in such a receiver. 
     Wireless communication systems often require a frequency converter to translate the high frequency input signal to a lower frequency for further processing. This may be accomplished by mixing the input with a locally-generated frequency source. One approach to wireless reception may utilize direct conversion of the high frequency input signal to baseband. Direct converters translate the RF input directly to baseband with zero intermediate frequency. Since there are no intermediate frequencies, many filters may be omitted or simplified. Therefore, it may be desirable to use such direct converters in mobile cellular communications. 
     A common problem encountered in direct conversion receivers is the effective LO leakage isolation. Since frequency mixers have finite LO to RF isolation, some of the LO signal appears at the receiver input and may be observed at the antenna. If there is inadequate filtering, the magnitude of this re-radiated signal may exceed the limit set by regulatory agencies, such as Federal Communications Commission (FCC). Furthermore, the signal may have a detrimental effect on receiver performance since any LO leakage into the antenna may downconvert through the mixer to produce a spurious reception response. Such LO leakage is of a particular problem in direct conversion receivers because the leakage signal is at the same frequency as that of the input signal, and thus cannot be filtered. 
     In direct conversion receivers, the LO leakage can downconvert and contribute to a troublesome dc offset in the baseband section of the receiver. This offset appears in the middle of the downconverted signal spectrum, and may be larger than the signal itself and significantly larger than thermal and flicker noise. Thus, the LO leakage signal in the direct conversion receiver may operate to jam other nearby subscriber units. Furthermore, the LO leakage may cause unintended radiation to exceed FCC specifications. 
     SUMMARY 
     The present disclosure describes a quadrature downconverter for a direct conversion receiver. The design of the downconverter is optimized to significantly reduce LO leakage signal leaking from the local oscillator through the RF section to the antenna. 
     The quadrature downconverter includes an RF input element, at least first and second quadrature splitters, a local oscillator, and at least two sets of mixers. The RF input element is configured to provide at least first and second RF inputs. The first and second quadrature splitters are configured to receive the first and second RF inputs and provide at least first and second in-phase RF outputs and at least first and second quadrature RF outputs. 
     The local oscillator operates to provide at least first and second reference signals. Each reference signal has an in-phase reference output and a quadrature reference output. Each set of mixers has at least first and second mixers. The first mixers are configured to modulate the first or second in-phase RF output with the in-phase reference output to produce in-phase baseband signals. The second mixers are configured to modulate the first or second quadrature RF output with the quadrature reference output to produce quadrature baseband signals. The quadrature splitters and the reference outputs operate to substantially reduce any local oscillator leakage signal leaking into the RF input element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Different aspects of the disclosure will be described in reference to the accompanying drawings wherein: 
     FIG. 1 shows a conventional direct conversion receiver utilizing quadrature modulation; and 
     FIG. 2 illustrates an embodiment of the LO leakage cancellation system. 
    
    
     DETAILED DESCRIPTION 
     A conventional direct conversion receiver  100  utilizing quadrature modulation is shown in FIG.  1 . An antenna  102  receives electromagnetic energy transmitted from a transmitter (not shown). The received signal is fed to a bandpass filter, which is designated as a duplexer  104 . The designation serves to highlight the purpose of the duplexer  104  as a filter for both receive and transmit channels. A receive portion of the duplexer  104  provides for selection of the correct communication band to improve the blocking characteristics of the receiver  100 . Strong signals outside of the receiver band are attenuated and therefore do not degrade the receiver performance. 
     An output of the duplexer  104  is connected to a low noise amplifier (LNA)  106  that improves the sensitivity of the receiver. The amplifier gain of the LNA  106  may be selected in view of actual requirements. A high gain results in good sensitivity, and low gain is desirable to achieve a good dynamic range and proper intermodulation characteristics. An output of the LNA  106  is split into two channels for quadrature processing, namely an I-channel  110  and a Q-channel  112 . In the illustrated embodiment, the LNA  106  output is split into the two channels by a power splitter  108  for proper matching and power division. 
     Each of the I-channel  110  and Q-channel  112  is connected to a mixer  114 ,  116 , respectively. The mixers  114 ,  116  convert the high frequency input signal to base band where it is easier to amplify and filter the signal with amplifiers  118 ,  120  and low pass filters  122 ,  124 . 
     The mixers  114 ,  116  can be passive or active, and the choice between the two depends mostly on the intended application. Passive mixers have good large signal performance and a high third-order intercept point but suffer from high conversion losses and the need for a strong local oscillator signal. Therefore, such mixers are avoided in battery-powered equipment. An active mixer has a high conversion gain, can be driven by a low power local oscillator but instead has a lower third order intercept point and a slightly poorer noise figure than the passive mixer. 
     In a direct conversion receiver, a local oscillator signal may be at the same frequency as the received input signal. The signal from a local oscillator  126  may be split into two quadrature reference signals that are 90° apart by a quadrature splitter  128 . The outputs of the quadrature splitter  128  are then amplified by amplifiers  130 ,  132 , and the amplified signals are provided to the mixers  114 ,  116 . Those outputs constitute second inputs to the mixers  114 ,  116 , and operate to provide in-phase and quadrature input signals at the baseband. 
     When the circuit of FIG. 1 is in operation, there may be a leakage signal with the same frequency and some arbitrary phase shift relative to the local oscillator (“LO”)  126 . This signal may leak across the mixers  114 ,  116 , the amplifier  106 , and the duplexer  104 , into the antenna  102  as shown in  134 . 
     The signal may leak from the LO  126  through the power splitter  128  into the mixers  114 ,  116 . The split signals impinging on the mixers  114 ,  116  pass through to the RF inputs at levels described by the mixers&#39; LO to RF isolation specification. These signals then sum at the power splitter  108 , and pass through the LNA  106  at a level described by its reverse isolation specification. The summed signal then easily passes through the duplexer  104  and gets applied to the antenna  102  as shown  134 . The mixers  114 ,  116  and the duplexer  104  may reduce the level of the leakage signal somewhat, however, substantial portion of the LO leakage signal may pass through to the antenna  102 . 
     FIG. 2 illustrates an embodiment of the present disclosure, which is configured to significantly reduce the LO leakage signal. In the illustrated embodiment, the quadrature stage of the direct conversion receiver is configured to reduce the LO leakage signal by effectively canceling the LO leakage signal. The cancellation is premised on a fact that two signals that differ in phase by 180° cancel each other and produce a zero result if they are added together. 
     In the configuration of FIG. 2, an RF signal is received at an antenna  250  and is directed to a bandpass filter, which is designated as a duplexer  252 . An output of the duplexer  252  is connected to a low noise amplifier (LNA)  254  that improves the sensitivity of the receiver. The amplified RF signal is then split into two channels, I-and Q-channels  202 ,  204 , by a power splitter  256 . I-and Q-channels  202 ,  204  are independently processed with quadrature LO signals by multiplying the input with quadrature LO signals in mixers  214 ,  216 ,  218 ,  220 . 
     The LO signals are split in quadrature through a second set of quadrature splitters  210 ,  212 . Each of the quadrature splitters  210 ,  212  produces two signals, one with no phase shift and another with 90° phase shift. The LO signals leak across the LO/mixer isolation as shown in  222 ,  224 . The LO leakage signals are then summed in the first set of quadrature splitters  206 ,  208 . The signal routed through the 0° phase shift is summed with the signal routed through the two successive 90° phase shifts. The two 90° phase shifts generated in the successive quadrature splitters  210 ,  206  or  212 ,  208  serve to produce signals that are 180° out of phase. Two signals that differ in phase by 180° cancel each other and produce a zero result if they are added together. Thus the summing of the two signals, one with no phase shift and another with 180° phase shift, at the power splitter  256  will substantially reduce the LO leakage signals leaking back through the LNA and the duplexer  252  into the antenna  250 . Therefore, the LO leakage signals across the LO to RF isolation are canceled. 
     Op amp  226  recombines the quadrature components making up the I channel. Op amp  228  recombines the quadrature components making up the Q channel. 
     The local oscillator  230  provides a reference signal for the system. Since two reference signals are required in the illustrated configuration, the LO signal may be split in a frequency divider  232  to produce two signals that are ½pulse shifted. In the illustrated embodiment, the cosine signal is directed to the I-channel  202  and the sine signal is directed to the Q-channel  204 . 
     One of the advantages of using the above-described LO leakage rejection system in direct conversion receivers includes the increase of up to 30 dB or more rejection of the LO leakage. This increase may reduce the size, cost, and complication of the rest of the components in the system. 
     While specific embodiments of the invention have been illustrated and described, other embodiments and variations are possible. For example, although the present disclosure has been described in terms of a wireless cellular system, the LO leakage cancellation design may be used in other systems such as portable cellular phones, cordless phones, pagers, carrier frequency systems, TV cable systems, and other related systems that include direct conversion receivers. 
     All these are intended to be encompassed by the following claims.