Abstract:
An image trap filter disposed between the low noise amplifier and the mixer of a radio frequency receiver that overcomes the adverse effect of process variations on image signal rejection for both a single-band radio frequency receiver and a multi-band radio frequency receiver by setting the image trap filter response at the center of the band of interest at production test. The image signal problem is presented at the input of the radio frequency receiver and the image trap filter is adjusted for the desired frequency response.

Description:
TECHNICAL FIELD 
   The present invention relates, in general, to the rejection of the image signal of a radio frequency signal converted to an intermediate frequency and, in particular, to a frequency conversion mixer especially suited to be implemented on an integrated circuit. 
   BACKGROUND OF THE INVENTION 
   Unlike wireline communications, the wireless environment accommodates essentially an unlimited number of users sharing different parts of the frequency spectrum and very strong signals coexist next to very weak signals. A radio receiver must be able to select the signal of interest, while rejecting all others. 
   Among the important problems faced by the designers of radio receivers are image rejection and monolithic integration. A radio receiver must be able to select the desired signal from its image. Otherwise, the subsequent detector circuit will be unable to distinguish between the desired signal and the image signal and, therefore, the output will be the result of the superposition of both. As wireless communications units evolve, means to reduce cost, size, and weight through monolithic integration are critical. 
   Image signal rejection relates to the ability of the radio frequency receiver to select the desired signal from the image signal of the desired signal spaced away by twice the intermediate frequency signal. This is important as the subsequent detector circuit will be unable to distinguish between the desired signals and the image signals and, therefore, the output of the detector circuit will be the result of the superposition of both. This is the essence of the image signal problem. 
   In conventional heterodyne receiver architectures, a large and expensive ceramic or Surface Acoustic Wave (SAW) filter is positioned between the low noise amplifier and the mixer to suppress the image signal. This arrangement is attractive in terms of current consumption. The arrangement defies integration, however, and results in excessive size, weight, and cost. 
   There also have been efforts to use phasing methods to achieve image signal rejection in the mixer itself. U.S. Pat. No. 5,870,670 and U.S. Pat. No. 5,678,220 provide examples of such efforts. Image reject mixers in which phasing methods are used are at best, however, only capable of achieving 30 dB of image rejection over the typical temperatures and processes used. The limitation, in terms of reliable image rejection from the phasing methods, comes from the required amplitude and phase imbalance in the local oscillator quadrature generation and intermediate frequency quadrature combining. It can be shown mathematically that achieving even 30 dB of image rejection using the phasing method requires less than 1° and 0.5 dB of phase and amplitude balance, respectively. The phasing methods of achieving image rejection, while improvements in terms of integration and cost, require additional filtering to meet overall system image rejection. 
   Other attempts at image rejection have involved image “traps” in the form of a simple series inductance-capacitance (or “L-C”) circuit across the differential line. This approach results in an excess inductance in the desired band that must be tuned out. Traditionally, a series capacitor has been used to tune out the in-band inductance. This approach suffers, however, from the fact that an additional mixer DC return is required. An on-chip choke, to provide this DC return, would be large and have considerable DC resistance. The increased space requirements add expense and the increased DC resistance in the ground return path lowers the voltage headroom on the mixer limiting its dynamic range. 
   U.S. Pat. No. 5,630,225 describes an arrangement by which a dielectric member is placed in proximity to a transmission line. The electromagnetic properties of this member alter the frequency response characteristic of the system by the formation of a notch at the image signal frequency. Such an arrangement is not amenable to monolithic integration. The dielectric member does not have the requisite electrical characteristics for such an application and the physical size of the dielectric member makes it unsuitable for monolithic integration. 
     FIG. 1  is a schematic drawing of a portion of a prior art radio frequency receiver. In  FIG. 1 , a radio frequency signal is received by a low noise amplifier  10  and, after amplification, is supplied to an image trap filter  12  that filters the image signal from the amplified radio frequency signal. The radio frequency signal then is supplied to a mixer  14  that develops the intermediate frequency signal from the radio frequency signal. 
   In modern radio frequency receivers for wireless applications, typically 50 dB of image signal filtering is required from the overall system. This image signal filtering comes from a combination of pre-select band pass filtering, image filtering and possible use of an image reject mixer. This high image signal rejection requirement means that the contribution of each portion of the receiver, where image signal rejection takes place, to the overall image signal rejection is critical. 
   In addition, in a multi-band radio frequency receiver, the image signal filter must pass all the radio frequency signal bands and must reject all the image signals simultaneously. A fundamental problem in designing an image signal filter for a multi-band radio frequency receiver is that a higher order image signal filter (i.e., a higher number of poles in the filter response) is required. This is illustrated by FIG.  2 . 
   Furthermore, problems arise when there is degradation in image signal rejection in an image trap filter due to process variations (i.e., variations in the values of components, such as resistors, capacitors and inductors, in the image trap filter). The possible variations of component values used to construct a monolithic filter requires significant margins in the image signal rejection response of an image signal filter. 
   SUMMARY OF THE INVENTION 
   To overcome the shortcomings of the prior ways of achieving image signal rejection with an image trap filter considered above, a new image trap filter is provided by the present invention. One object of the present invention is to provide a new and improved image trap filter. Another object of the present invention is to provide a new and improved radio frequency receiver. A further object of the present invention is to provide a new and improved image trap filter that is particularly suited for implementation in an integrated circuit. 
   An image trap filter for filtering an image signal from a radio frequency signal, constructed in accordance with the present invention, includes a first branch having an inductor and a capacitor connected in series. At least one of the inductor and the capacitor are tunable. This image trap filter also includes a second branch connected in parallel with the first branch and has a tunable impedance. An image trap filter for filtering an image signal from a radio frequency signal, constructed in accordance with the present invention, further includes means for tuning at least one of the inductor and the capacitor in the first branch and the impedance in said second branch to resonate the first branch at the frequency of the image signal and present in the first branch at the frequency of the image signal a low impedance, present in the first branch an impedance at the frequency of the radio frequency signal, and resonate the second branch with the first branch to present a high impedance at the radio frequency. 
   It will be understood that both the foregoing general description of the invention and the following detailed description of the invention are exemplary, but are not restrictive of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. Included in the drawing are the following figures. 
       FIG. 1  is a schematic drawing of a portion of a prior art radio frequency receiver. 
       FIG. 2  is a frequency response curve that represents the conventional approach for designing an image signal filter for a multi-band radio frequency receiver. 
       FIG. 3  is a schematic drawing of a portion of a radio frequency receiver that incorporates an image trap filter constructed in accordance with the present invention. 
       FIG. 4  is a circuit diagram of an image trap filter, constructed in accordance with the present invention, with ideal switches. 
       FIG. 5  is a circuit diagram of an image trap filter, constructed in accordance with the present invention, with MOSFET switches. 
       FIG. 6A  is a simplified circuit diagram of the  FIG. 4  circuit for low-side injection of the local oscillator frequency. 
       FIG. 6B  is a circuit diagram of the functional equivalent circuit of  FIG. 4  for low-side injection of the local oscillator frequency at the frequency of the image signal. 
       FIG. 6C  is a circuit diagram of the functional equivalent circuit of  FIG. 4  for low-side injection of the local oscillator frequency at the frequency of the radio frequency signal that contains the image signal. 
       FIG. 7A  is a simplified circuit diagram of the  FIG. 4  circuit for high-side injection of the local oscillator frequency. 
       FIG. 7B  is a circuit diagram of the functional equivalent circuit of  FIG. 4  for high-side injection of the local oscillator frequency at the frequency of the image signal. 
       FIG. 7C  is a circuit diagram of the functional equivalent circuit of  FIG. 4  for high-side injection of the local oscillator frequency at the frequency of the radio frequency signal that contains the image signal. 
       FIG. 8  is RF Band and Image Band diagram of the image trap filter during use. 
       FIG. 9  is an RF Band and Image Band diagram for fine tuning of the image trap filter during receiver assembly. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  is a schematic drawing of a portion of a radio frequency receiver that incorporates an image trap filter constructed in accordance with the present invention. In  FIG. 3 , a radio frequency signal is received by a low noise amplifier  20  and, after amplification, is supplied to an image trap filter  22  constructed in accordance with the present invention. Image trap filter is tunable by means represented by block  24  to select a band when incorporated in a multi-band radio frequency receiver and to filter the image signal from the amplified radio frequency signal. The radio frequency signal then is supplied to a mixer  26  that develops the intermediate frequency signal from the radio frequency signal. 
   Referring to  FIG. 4 , an image trap filter for filtering an image signal from a radio frequency signal, constructed in accordance with a first embodiment of the present invention and arranged for low-side injection of the local oscillator frequency, includes a first branch having an inductor  30  and a capacitor made up of a plurality of capacitors  31   a ,  31   b  . . .  31   n  that are connected in series with inductor  30 . As such, the capacitor in the first branch is tunable by the selective closing of a plurality of switches  32   a ,  32   b  . . .  32   n  individually connected in series with capacitors  31   a ,  31   b  . . .  31   n.    
   An image trap filter for filtering an image signal from a radio frequency signal, constructed in accordance with the present invention, also includes a second branch connected in parallel with the first branch and having a tunable impedance, namely a capacitor  33 . Connected across capacitor  33  is a second set of capacitors  34   a ,  34   b  . . .  34   n  and a second set of switches  35   a ,  35   b  . . .  35   n  individually connected in series with capacitors  34   a ,  34   b  . . .  34   n . This arrangement makes the impedance in the second branch, namely a capacitance, tunable. 
   Tuning of the two branches of the image trap filter of  FIG. 4  is done during assembly of the radio frequency receiver to overcome potential degradation in image signal rejection in an image trap filter due to process variations (i.e., variations in the values of components, such as resistors, capacitors and inductors, in the image trap filter). The tuning is such as to cause the first branch to resonate at the frequency of the image signal, the first branch to present a low impedance at the frequency of the image signal, the first branch to present an inductive impedance at the frequency of the radio frequency signal, and the second branch to resonate with the first branch to present a high impedance at the radio frequency. 
     FIG. 6A  is a simplified circuit diagram of the  FIG. 4  circuit for low-side injection of the local oscillator frequency.  FIG. 6B  is a circuit diagram of the functional equivalent circuit of  FIG. 4  at the frequency of the image signal.  FIG. 6C  is a circuit diagram of the functional equivalent circuit of  FIG. 4  at the frequency of the radio frequency signal that contains the image signal. The short-circuit  36  in the first branch in  FIG. 6B  represents the low impedance at the frequency of the image signal and the inductor  38  in the first branch in  FIG. 6C  represents an inductive impedance at the frequency of the radio frequency signal. Capacitor  33  in the second branch resonates with the first branch, namely the inductive impedance of the inductor  38  to present a high impedance at the frequency of the radio frequency signal. 
   For high-side injection of the local oscillator frequency, the image trap filter of  FIG. 4  includes a capacitor instead of inductor  30 , two sets of inductors instead of the two sets of capacitors  31   a ,  31   b  . . .  31   n  and  34   a ,  34   b  . . .  34   n , and an inductor instead of capacitor  33 . 
     FIG. 7A  is a simplified circuit diagram of the  FIG. 4  circuit for high-side injection of the local oscillator frequency. A tunable inductor  39  is connected in series with a capacitor  40  in a first branch that is connected in parallel with a second branch having an inductor  42 .  FIG. 7B  is a circuit diagram of the functional equivalent circuit of  FIG. 4  at the frequency of the image signal for high-side injection of the local oscillator frequency.  FIG. 7C  is a circuit diagram of the functional equivalent circuit of  FIG. 4  at the frequency of the radio frequency signal that contains the image signal for high-side injection of the local oscillator frequency. The short-circuit  44  in the first branch in  FIG. 7B  represents the low impedance at the frequency of the image signal and the capacitor  46  in the first branch in  FIG. 7C  represents a capacitive impedance at the frequency of the radio frequency signal. Inductor  48  in the second branch resonates with the first branch, namely the capacitive impedance of capacitor  46  to present a high impedance at the frequency of the radio frequency signal. 
   An image trap filter, constructed in accordance with the present invention, lends itself to balanced architectures as illustrated by  FIG. 4 , which are required for highly integrated radio frequency integrated circuit receivers due to isolation concerns. 
   As indicated above, an image trap filter, constructed in accordance with the present invention, can be incorporated in a multi-band radio frequency receiver. In such a case, as shown on  FIG. 4 , the image trap filter further includes a transformer  50  of conventional construction and operation and having a primary winding  50   a  and a secondary winding  50   b . For the embodiment of the invention being described, secondary winding  50   b  is a center-tapped, grounded winding. Primary winding  50   a  is connected to a low noise amplifier. 
   When incorporated in a multi-band radio frequency receiver, an image trap filter, constructed in accordance with the present invention, further includes a first band select filter coupled to primary winding  50   a  of transformer  50  and a second band select filter coupled to that one of the inductor and the capacitor in the first branch that is tunable, namely the capacitor made up of capacitors  31   a ,  31   b  . . .  31   n  for the embodiment of the invention illustrated by FIG.  4 . The first band select filter that is coupled across primary winding  50   a  includes a plurality of capacitors  52   a  . . .  52   n  individually connected in series with a plurality of switches  54   a  . . .  54   n . The second band select filter that is coupled to capacitor  33  also includes a plurality of capacitors  56   a  . . .  56   n  individually connected in series with a plurality of switches  58   a  . . .  58   n.    
   In use, upon reception of a radio frequency signal by the radio frequency receiver, the appropriate switches in the first band select filter and the second band select filter, corresponding to the band containing the frequency of the radio frequency signal, are closed and the receiver is conditioned to receive and process the radio frequency signal. This is shown by FIG.  8 .  FIG. 9  shows the fine tuning of the image trap filter done during assembly of the radio frequency receiver to avoid degradation of the image signal rejection due to process variations for a multi-band radio frequency receiver. 
   In  FIG. 4 , the switches by which the image trap filter selects a band and tunes to avoid degradation of the image signal rejection due to process variations are shown as ideal switches.  FIG. 5  is a circuit diagram of an image trap filter, constructed in accordance with the present invention, with MOSFET switches. 
   Although illustrated and described above with reference to certain specific embodiments, the present invention nevertheless is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention.