Abstract:
An adaptive front-end architecture for a receiver is disclosed. In one embodiment, the adaptive front-end architecture includes an input configured to receive an input signal and a linear low-noise amplifier connected to the input and configured to amplify the input signal to produce an amplified input signal. The adaptive front-end architecture further includes a first passive mixer arrangement configured to generate first a local oscillator signal and mix the first local oscillator signal with the amplified input signal to produce a first baseband output signal. The adaptive front-end architecture further includes a second passive mixer arrangement configured to generate a second local oscillator signal and mix the second local oscillator signal with the input signal to produce a second baseband output signal. The adaptive front-end architecture further includes a baseband impedance component configured to filter the first baseband signal and/or the second baseband signal using impedance translation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application 61/333,840 filed May 12, 2010, the contents of which are incorporated by reference herein in their entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to reconfigurable receiver architectures, and is more particularly, concerned with a reconfigurable receiver front-end for use with multi-standard and software-defined radios requiring high sensitivity, particularly in the presence of large out-of-band interferers. 
         [0003]    The requirements of next-generation wireless terminals are driving radio frequency integrated circuit (RFIC) design towards ubiquitous multi-standard connectivity at reduced power consumption and cost. While the use of scaled CMOS technology is required to allow economically feasible single-chip integration with a digital processor, a software-defined radio (SDR) is the preferred approach to provide a reconfigurable platform that covers a broad range of noise and/or linearity specifications while offering the best power and/or performance radio trade-off. The SDR must combine the most demanding requirements, such as, high sensitivities for cellular standards, low phase noise and high linearity in order to be competitive with dedicated single-mode radios. 
         [0004]    To relax the filtering requirements of such multi-standard radios, aiming to simplify the antenna interface of these systems, the RF front-end should be able to cope with large interferences due to the presence of other transmitters. These interferences occur at frequencies other than at the frequency of the wanted signal, and are said to be out-of-band. They are also referred to as blockers. Such out-of-band blockers can be as high as 0 dBm, especially when the interfering transmitter is less than 1 m away from the receiving radio. 
         [0005]    Receiver architectures are known which comprise a low-noise amplifier (LNA) coupled to a mixer. Input signals are applied to the LNA where it is amplified and passed to the mixer to provide an output signal. A switched LNA bypass is provided that enables the LNA to be bypassed in cases where the input signals are large. 
         [0006]    However, whilst this appears to provide a solution that enables the receiver architecture to handle both small and large input signals, this solution suffers from some disadvantages. The first disadvantage is that the bypass switch is not very linear and contributes to distortion of the signal. In addition, input matching is not achieved because the LNA, which takes care of input matching, needs to be turned off when large input signals are received. In this case, some other means of input matching needs to be provided. Moreover, whilst large signals can be present at the input, no filtering is provided at the RF input of the system, and, as such, when the signals are too large, the input power due to unwanted interference will compress the system, even when the bypass mode is activated. 
         [0007]    U.S. Patent Publication No. 2003/0159156 discloses a high linearity, low noise figure tuner front end circuit for television signals comprising first and second radio frequency paths arranged between a radio frequency input and a radio frequency output which can be selectively connected. The first path includes a mixer and the second path includes a low noise amplifier followed by a mixer. 
         [0008]    It is an object of the present disclosure to provide a reconfigurable receiver architecture that overcomes the problems associated with known receiver front-end architectures employing LNA bypass arrangements. 
       SUMMARY 
       [0009]    In accordance with a first aspect of the present disclosure, there is provided an adaptive front-end architecture for a receiver comprising: an input for receiving an input signal; a linear low-noise amplifier connectable to the input for amplifying the input signal and for providing an amplified output signal; a first passive mixer arrangement connected to the amplified output signal, the first passive mixer arrangement including a first local oscillator whose signal is mixed with the amplified output signal to provide a baseband output signal; and a bypass arrangement connectable to the input for bypassing the linear low-noise amplifier; the bypass arrangement comprising a second passive mixer arrangement that includes a second local oscillator whose signal is mixed with the input signal to provide the baseband output signal; characterised in that the adaptive front-end architecture further comprises a baseband impedance component for filtering the baseband output signal using impedance translation. 
         [0010]    By using a passive mixer arrangement in the bypass path, the problems associated with non-linearity of switches are overcome. In addition, the passive mixer arrangement is transparent in terms of impedance and assists with filtering of the signals. 
         [0011]    The first and second passive mixer arrangements each further includes selection means for connecting and disconnecting respective ones of the first and second local oscillators. 
         [0012]    The present disclosure comprises an adaptive radio front-end arranged for operating in a first and a second operating mode. The front-end comprises an input terminal arranged for receiving an input signal, a linear low-noise amplifier (LNA) connected to the input terminal, a baseband impedance component characterised by a baseband filtering profile of the impedance arranged for filtering the appropriate signal (the appropriate signal may be either the input signal or either the amplified input signal) and shared by the output of a first and a second mixing means, a first mixing means connected (in series) to the input terminal of the front-end and the baseband impedance component, a second mixing means connected (in series) to the linear LNA output and the baseband impedance component. The first and the second mixing means comprises switches. 
         [0013]    The linear LNA is arranged for providing input matching. The linear LNA is further arranged for providing linear amplification of the input signal in the second operating mode. The LNA is designed with high output impedance. As such, the LNA can be seen as an amplifier that amplifies the input voltage, V IN , into an output current, I OUT , where I OUT =A*V IN . The output current flows into the output impedance, Z OUT , seen by the LNA and therefore provides the conversion into voltage. The gain, G, is then G=V OUT /V IN =I OUT *Z OUT /V IN =A. Here, Z OUT  is determined by the element loading the LNA. In this case, the loading element consists of a mixer loaded with a filtering component. Through the frequency-transparency of the mixer, the baseband filtering of the mixer load is up-converted to the RF input of the mixer, and becomes a bandpass filter. The LNA is thus loaded by a bandpass impedance, Z OUT , and hence the LNA gain (G=I OUT *Z OUT /V IN ) is also bandpass, or frequency selective. 
         [0014]    In the first operating mode, the first mixing means is arranged for sampling the input signal to the baseband impedance component, by means of a first oscillating input frequency. Further, the first mixing means provides an input impedance consisting of the baseband impedance, up-converted to the first oscillating input frequency by means of impedance translation. As such, the baseband steep filtering profile is up-converted to the mixer input for providing out-of-band signal filtering at the input terminal of the front-end. The filter centre frequency is thus determined by the oscillating input frequency of the mixer means (first oscillating input frequency). Such steep out-of-band filtering at the RF input cannot be achieved by other on-chip techniques known in the art. For example, passive filtering needs high-Q elements, unavailable on-chip and not tuneable, and active filtering is noisy, non-linear and frequency-limited. The first operating mode can be used in the presence of large input signals when no LNA gain is desired. This provides linear behaviour and power savings. Strong out-of-band interferers will consequently be filtered at the input of the front-end, which prevents compression and distortion of subsequent blocks. This operation provides an improvement over the prior art in the form of an LNA bypass switch as described with reference to  FIG. 1  below, since the latter forms a nonlinear switch, added to the chain. In the current embodiment, the switch itself is the mixer, and its linearity is of no concern. 
         [0015]    In the second operating mode, where the input signal is very weak, the second mixing means is arranged for down-converting the amplified LNA output signal to the (same) baseband impedance component, by means of a second oscillating input. Note that, here, the LNA can be seen as an amplifier that amplifies the input voltage into an output current. The output current flows into the output impedance seen by the LNA, and therefore provides the conversion from current to voltage. The second mixing means provides an input impedance consisting of the baseband impedance, up-converted to the oscillating input, by means of impedance translation, for providing out-of-band signal filtering at the output of the LNA. Out-of-band blockers are hence filtered at the LNA output before they are amplified in the voltage domain. As a result, the presence of out-of-band blockers does not cause compression of the signals. The second mode is used at very weak desired input power, and for providing low noise amplification, without resulting in compression of the signal before it reaches the second mixing means, at the LNA output. The oscillating input to either one of the mixing means determines the frequency band of operation, where each of the mixing means can be disabled by disabling its oscillating input, or opening the switches of the other mixing means. 
         [0016]    In either mode of operation, the LNA can be left on, and used for input matching purposes, since the LNA output can be isolated from the front-end output by opening the second mixer switches. In the mixer-first mode, that is, where the LNA is bypassed, the LNA can be reconfigured to provide low gain through good input matching. Therefore, power savings can be achieved. As another example, part of the input matching can be achieved by the mixer connected to the input. In this case, the LNA can then provide the remainder necessary for input matching at lower input powers. 
         [0017]    In an embodiment, the baseband impedance component (ZFILT) comprises a capacitor, thereby achieving a low-pass filter at baseband, and accordingly achieving a bandpass filter at the high-frequency input of each mixing means, filtering around the oscillating input frequency. 
         [0018]    In another embodiment, the baseband impedance component comprises a low impedance, resistive component, thereby achieving a low impedance at baseband, and a low impedance at the high-frequency input of each mixing means, for low voltage swing, and little compression in the case of strong interferers. 
         [0019]    In an embodiment, the baseband impedance component comprises any other active of passive impedance or combination thereof. 
         [0020]    In an additional embodiment, the present disclosure also relates to a method for receiving an (RF) input signal by an adaptive receiver front-end (method for adaptively down-converting an RF signal by a receiver front-end). The method comprises at least two operating modes; a first operating mode comprising the steps of receiving an input signal, amplifying the received signal by means of a LNA, down-converting the amplified signal by means of a sampling mixer means (having a first input oscillating frequency) to a baseband frequency; and a second operating mode comprising the steps of down-converting the received signal by means of sampling mixer means (having a first input oscillating frequency) to a baseband frequency. The method further comprises the step of selecting the operating mode by enabling or disabling the oscillating input of each of the mixing means. The oscillating input to either one of the mixing means determines the frequency band of operation, where each of the mixing means can be disabled by disabling its oscillating input, or thus opening the switches of the other mixing means. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0021]    For a better understanding of the present disclosure, reference will now be made, by way of example only, to the accompanying drawings in which: 
           [0022]      FIG. 1  illustrates a prior art receiver front end; 
           [0023]      FIG. 2  illustrates an example receiver front-end, in accordance with an embodiment; 
           [0024]      FIG. 3  illustrates example impedance translational properties of sampling mixers, in accordance with an embodiment; 
           [0025]      FIG. 4  illustrates the receiver front-end of  FIG. 2  operating as a low noise amplifier, in accordance with an embodiment; and 
           [0026]      FIG. 5  illustrates the receiver front-end of  FIG. 2  operating as a mixer, in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto but. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. 
         [0028]      FIG. 1  illustrates a prior art receiver front end. In  FIG. 1 , a state-of-the-art receiver architecture  100  is shown. The architecture  100  comprises a low-noise amplifier (LNA)  110  coupled to a mixer  120 . An input signal  130  is applied to the LNA  110 , where it is amplified and passed to the mixer  120  where it is mixed with a local oscillator (LO) signal  140  to provide an output signal  150 . An LNA bypass  160  is provided which is coupled to the input  130  and to the input of the mixer  120 . As shown, the LNA bypass  160  comprises switch elements  162 ,  164  which when closed effectively enables the input signal  130  to bypass the LNA  110 . 
         [0029]    The architecture  100  is able to handle either large or small input signals. In case of small input signals, the switches  162 ,  164  in the LNA bypass  160  are open so that the input signal  130  does not bypass the LNA  110 . The LNA  110  amplifies the input signal  130  and the amplified signal is passed to the mixer  120 . In the mixer  120 , the amplified signal is down-converted, using the LO signal  140 , to provide a low frequency output signal  150 . At low frequencies, a baseband section (not shown) filters the output signal  150 . 
         [0030]    In case of large input signals, for example, greater than 0 dBm, switches  162 ,  164  of the LNA bypass  160  are closed and the LNA  110  is bypassed. This means that the input signal  130  is applied to the mixer  120  without gain, and the system can better handle large signals. 
         [0031]    As discussed above, however, the architecture  100 , when receiving large input signals, suffers from the following disadvantages: the bypass switch is not very linear and contributes to distortion of the signal; input matching is not achieved as the LNA, which takes care of input matching, needs to be turned off when large input signals are received; and as no filtering is provided at the RF input of the system, the input power due to unwanted interference tends to compress the system, even when the bypass mode is activated. 
         [0032]    In accordance with the present disclosure, an improved adaptive receiver front-end architecture  200  is provided.  FIG. 2  illustrates an example receiver front-end, in accordance with an embodiment. In  FIG. 2 , the architecture  200  comprises a linear LNA  210  having input terminals  215  to which input signals  220  are applied, a baseband impedance component  230 , a first mixer arrangement  240  connected in series with the input terminals  215  and a second mixer arrangement  250  connected in series with the LNA  210 . The first mixer arrangement  240  utilises a first LO signal LO 1  and the second mixer arrangement  250  utilises a second LO signal LO 2  as shown. Each of the first and second mixer arrangements  240 ,  250  is also connected in series with the baseband impedance component  230 . An output  260  is also provided as shown. 
         [0033]    Each of the first and second mixer arrangements  240 ,  250  has LO inputs and switches for switching between two modes of operation, namely, an LNA-first mode and a mixer-first mode. It is possible to switch between two modes of operation by turning the individual mixer arrangements  240 ,  250  on or off by disabling the respective LO inputs, LO 1  and LO 2 , and opening the respective switches. These modes of operation will be described in more detail with respect to  FIGS. 4 and 5  below. 
         [0034]    It will be appreciated that while the first and second mixing arrangements  240 ,  250  are shown as being in the I path, the first and second mixing arrangements  240 ,  250  could similarly be connected to the Q path for baseband output signals. Only the output signal  260  from the I path is shown in  FIG. 2  for clarity. 
         [0035]      FIG. 3  illustrates example impedance translational properties of sampling mixers, in accordance with an embodiment. The sampling mixers may be sampling mixers used to provide filtering in both operational modes of the receiver front-end architecture in accordance with the present disclosure. For a baseband filtering profile, Z IN , as shown by  310 , no filtering properties are obtained as shown by  320 . When Z IN  is up-converted to a radio frequency (RF) input frequency using an RF LO input to a sampling mixer as shown in  330  using impedance translation, filtering is provided as shown in  340 . This is possible because the first mixer arrangement  240  in  FIG. 2  is passive, and therefore transparent in terms of impedance. 
         [0036]    In accordance with the present disclosure, impedance translational properties of sampling mixers can be used in both modes of operation, namely, as an LNA and a mixer, of the front end architecture  200  shown in  FIG. 2 . This pre-attenuates the input out-of-band interference signal. Overall, a much better linearity can be achieved. Finally, the linearity of the mixer is very high, as it is now not influenced by the linearity of a bypass switch or LNA. Therefore, the front-end architecture  200  has the advantages that: it provides a much better immunity to blockers or interference signals; it provides RF filtering; it can bypass the LNA; and it is much more linear. 
         [0037]    This means that a front-end receiver architecture can be provided that features a highly linear LNA for low noise amplification and a down-converter that can reconfigure to a mixer-first architecture. This is done in an elegant manner that allows for very highly linear operation, mainly to cope with strong, out-of-band unwanted interferences within the two modes of operation, namely, a LNA-first mode and mixer-first mode, where switching between the two modes is achieved by turning the mixers on or off. This is achieved by disabling the local oscillator input to the mixers and opening the mixer switches as described above with reference to  FIG. 2 . 
         [0038]      FIG. 4  illustrates the receiver front-end of  FIG. 2  operating as a low noise amplifier, in accordance with an embodiment. Components that have previously been described in relation to  FIG. 2  bear the same reference numerals. The first passive mixer arrangement  240  connects directly to the input  215 , that is, is directly coupled to the antenna (not shown) to receive the input signals  220 . The output of the first mixer arrangement  240  connects its output to the baseband impedance component  230 ′. In this case, the baseband impedance component  230 ′ is shown as a capacitor. In case the LNA-first operation is not desired, because of a very large interference at the input  215  as shown at  410 , the second mixer arrangement  250  is disabled with its switches open and there is no mixing with the second LO signal LO 2 . The first mixer arrangement  240  is now enabled, and directly down-converts the input signal  220  to baseband as shown by the graph  420 . Now, the LNA  210  is not involved in this signal path and there is no output from the LNA  210  at  430 . The signal path is indicated by arrow  270 . Here, bypass switches that can affect the linearity are not needed. In fact, the bypass switches form part of the first mixer arrangement  240 , and these bypass switches also bypass the second mixer arrangement  250 . This has the advantage that the LNA  210  can still be left in place to guarantee input matching. 
         [0039]    The filtering profiles are now described with reference to  FIG. 4 . In  FIG. 4 , the second mixer arrangement  250  at the LNA output  430  is disabled (LO 2  is arranged such that the switches are open), and the first mixer arrangement  240  is enabled (LO 1  is provided with appropriate oscillating signal). The baseband filter profile, in this embodiment achieved by means of a capacitor  230 ′, is up-converted to the input, providing the filter profiles as indicated as  410 ,  420 . Profile  410  illustrates the profile at the input  215  and profile  420  illustrates the profile at the baseband impedance component  230 ′. Out-of-band filtering is provided at the front-end input  215 . An unwanted blocker or interference signal is directly filtered at the input, preventing it from compressing the system. This mode of operation is to be used at large input power where no LNA gain is desired, achieves power consumption savings and can handle out-of-band interference signals greater than 0 dBm. 
         [0040]      FIG. 5  illustrates the receiver front-end of  FIG. 2  operating as a mixer, in accordance with an embodiment. In  FIG. 5 , the first mixer arrangement  240  is disabled (LO 1  is arranged such that the switches are open), while the second mixer arrangement  250  is enabled (LO 2  is provided with an appropriate oscillating signal). No filtering is present at the input  215 , other than provided by the LNA  210 . The profile of the signal received at the input  220  is shown at  510 . The LNA  210  provides a low noise amplification signal  540  to the second mixer arrangement  250 . The low noise amplification signal  520  provides an up-converted filter profile as shown by  520  at the LNA output  540 , filtering the unwanted blocker or interference signal as shown at  510 . As such, the output of the LNA  210  will only compress at very much higher blocker power. 
         [0041]    This second mode of operation is to be used in the presence of weak input power and can still filter out-of-band interference signals, and to provide low noise amplification and down-conversion. This mode of operation can handle out-of-band interference signals greater than 0 dBm (since the LNA output is prevented from compressing) while providing a low noise figure (NF). 
         [0042]    The present disclosure will be described with respect to particular embodiments and with reference to certain drawings but the disclosure is not limited thereto. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.