Abstract:
A method for generating an amplified radio frequency (RF) signal is provided. In-phase (I) and quadrature (Q) signals are received and interleaved so as to generate a time-interleaved signal. Delayed time-interleaved signals are then generated from the time interleaved signal, and each of the delayed time-interleaved signals is amplified so as to generate a plurality of amplified signals. The amplified signals are then combined with a transformer, where the delayed time-interleaved signals are arranged to generate a filter response with the transformer.

Description:
TECHNICAL FIELD 
     The invention relates generally to transformer power combiners and, more particularly, to transformer power combiners with a filter response. 
     BACKGROUND 
     Turning to  FIG. 1 , an example of a conventional radio frequency (RF) transmitter  200  can be seen. As shown in this example, RF circuitry is included on an integrated circuit (IC)  102  that uses a transformer combiner to generate the desired power. The scheme employed by the IC  102  is a polar modulation scheme, which uses a polar modulator  108  to decompose in-phase (I) and quadrature (Q) signals into amplitude and phase. The phase information is provided to the power amplifiers  112 - 1  to  112 - 4  (PAs), which are digital PAs, and the amplitude information is provided to the power controller  110 , which controls the power output of each of PAs  112 - 1  to  112 - 4 . Since each of the PAs  112 - 1  to  112 - 4  receive the same signal and the respective outputs are applied to primary windings  116 - 1  to  116 - 4  of transformer  114 , the power from each “branch” can be added together by secondary winding  108  to generate an output signal for IC  102 . Depending on the amount of deliverable power desired, the number of PAs  112 - 1  to  112 - 4  can be increased or decreased. 
     One issue with this transmitter  100  is that there is no filter response introduced that allows for any analog filtering. The usual reason is that phase modulation is very sensitive, and introduction of predistortion to induce a filter response would create a mismatch between the power controller  110  and the PAs  112 - 1  to  112 - 4 . As a result, an off-chip band-pass filter  104  is provided between the IC  102  and load  106  (which may be an antenna and matching circuitry). Other conventional RF transmitter also have similar drawbacks. For example, quadrature modulators usually includes an off-chip PA, and Linear Amplification with Nonlinear Components (LINC) transmitters usually include off-chip Wilkinson combiners and band-pass filters. Thus, there is a need for an improved RF transmitter. 
     Some example of other conventional systems are: Haldi et al., “A 5.8 GHz 1 V Linear Power Amplifier Using a Novel On-Chip Transformer Power Combiner in Standard 90 nm CMOS,”  IEEE J. of Solid - State Circuits , Vol. 43, No. 5, May 2008, pp. 1054-1063; Lai et al., “A 1V 17.9 dBm 60 GHz Power Amplifier in Standard 65 nm CMOS,” 2010  IEEE Intl. Solid - State Circuits Conf . (ISSCC), Feb. 10, 2010, pp. 424-425; Chang et al., “A 77 GHz Power Amplifier Using Transformer-Based Power Combiner in 90 nm CMOS,” 2010  IEEE Custom Integrated Circuits Conf . (CICC), Sep. 19-22, 2010, pp. 1-4; Kim et al., “A Linear Multi-Mode CMOS Power Amplifier With Discrete Resizing and Concurrent Power Combining Structure,”  IEEE J. of Solid - State Circuits , Vol. 46, No. 5, May 2011; and U.S. Pat. No. 7,777,570. 
     SUMMARY 
     An embodiment of the present invention, accordingly, provides an apparatus. The apparatus comprises an interleaver that is configured to receive an in-phase (I) signal and a quadrature (Q) signal; a distribution circuit that is coupled to the interleaver; a plurality of power amplifiers (PAs), wherein each PA is coupled to the distribution circuit; and a transformer having a plurality of primary windings and a secondary winding, wherein each primary winding is coupled to at least one of the PAs, and wherein the distribution circuit introduces at least one of a plurality of delays between the output of the interleaver and each PA so as to generate a filter response with the transformer. 
     In accordance with an embodiment of the present invention, the distribution circuit further comprises: a plurality of channels, wherein each channel is coupled to at least one of the PAs; and a plurality of delay circuits, wherein each delay circuit is coupled between at least two of the channels. 
     In accordance with an embodiment of the present invention, the plurality of channels further comprises a first set of channels and a second set of channels that are interleaved with one another, and wherein each channel from the second set of channels further comprises an inverter. 
     In accordance with an embodiment of the present invention, the interleaver further comprises: a first mixer that is configured to receive the I signal and a first interleaving signal; a second mixer that is configured to receive the Q signal and a second interleaving signal; and a combiner that is coupled to the first mixer, the second mixer, and the distribution circuit. 
     In accordance with an embodiment of the present invention, the apparatus further comprises a band-pass filter that is coupled to the secondary winding of the transformer. 
     In accordance with an embodiment of the present invention, the first and second interleaving signals are 90° out-of-phase. 
     In accordance with an embodiment of the present invention, a method is provided. The method comprises receiving I and Q signals; interleaving the I and Q signals so as to generate a time-interleaved signal; generating a plurality of delayed time-interleaved signals from the time interleaved signal; amplifying each of the delayed time-interleaved signals so as to generate a plurality of amplified signals; and combining the amplified signals with a transformer, wherein the delayed time-interleaved signals are arranged to generate a filter response with the transformer. 
     In accordance with an embodiment of the present invention, the step of generating further comprises: delaying the time-interleaved signal by a first amount to generate a first delayed time-interleaved signal from the plurality of delayed time-interleaved signals; delaying the time-interleaved signal by a second amount to generate a second delayed time-interleaved signal from the plurality of delayed time-interleaved signals, wherein the second amount is the sum of the first amount and a predetermined delay; delaying the time-interleaved signal by a third amount to generate a third delayed time-interleaved signal from the plurality of delayed time-interleaved signals, wherein the third amount is the sum of the second amount and the predetermined delay; and delaying the time-interleaved signal by a fourth amount to generate a fourth delayed time-interleaved signal from the plurality of delayed time-interleaved signals, wherein the fourth amount is the sum of the third amount and the predetermined delay. 
     In accordance with an embodiment of the present invention, the step of interleaving further comprises: mixing the I signal with a first interleaving signal; mixing the Q signal with a second interleaving signal; and combining the mixed I and Q signals to generate the time-interleaved signal. 
     In accordance with an embodiment of the present invention, the steps of mixing and combining the mixed I and Q signals further comprise: outputting, during a first period of the first and second interleaving signals, the I signal; outputting, during a second period of the first and second interleaving signals, the Q signal; outputting, during a third period of the first and second interleaving signals, an inverse of the I signal; and outputting, during a fourth period of the first and second interleaving signals, an inverse of the Q signal. 
     In accordance with an embodiment of the present invention, the steps of delaying the time-interleaved signal by the second amount and delaying the time-interleaved signal by the fourth amount further comprise: delaying and inverting the time-interleaved signal by the second amount to generate the second delayed time-interleaved signal; and delaying and inverting the time-interleaved signal by the fourth amount to generate the third delayed time-interleaved signal. 
     In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises an interleaver that is configured to receive an I signal and a Q signal; a distribution circuit that is coupled to the interleaver so as to generate first, second, third, and fourth delayed time-interleaved signals; a first PA that is coupled to the distribution circuit so as to receive the first delayed time-interleaved signal; a second PA that is coupled to the distribution circuit so as to receive the second delayed time-interleaved signal; a third PA that is coupled to the distribution circuit so as to receive the third delayed time-interleaved signal; a fourth PA that is coupled to the distribution circuit so as to receive the fourth delayed time-interleaved signal; a transformer having a first primary winding, a second primary winding, a third primary winding, a fourth primary winding, and a secondary winding, wherein the first primary winding is coupled to the first PA, and wherein the second primary winding is coupled to the second PA, and wherein the third primary winding is coupled to the third PA, and wherein the fourth primary winding is coupled to the fourth PA, and wherein the first, second, third, and fourth delayed time-interleaved signals are introduced so as to generate a filter response with the transformer. 
     In accordance with an embodiment of the present invention, the distribution circuit further comprises: a first channel that is coupled between the interleaver and the first PA; a first delay circuit that is coupled to the interleaver; a second channel that is coupled between the first delay circuit and the second PA; a second delay circuit that is coupled to the first delay circuit; a third channel that is coupled between the second delay circuit and the third PA; a third delay circuit that is coupled to the second delay circuit; and a fourth channel that is coupled between the third delay circuit and the fourth PA. 
     In accordance with an embodiment of the present invention, the second and fourth channels further comprise first and second inverters, respectively. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of an example of a conventional RF transmitter; 
         FIG. 2  is a diagram of an example of an RF transmitter in accordance with an embodiment of the present invention; 
         FIG. 3  is an example of a timing diagram depicting the operation of the interleaver and distribution circuit of  FIG. 2 ; and 
         FIG. 4  is a diagram of the filter response of the transmitter of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Turning to  FIG. 2 , an example of an RF transmitter  200  in accordance with an embodiment of the present invention can be seen. As shown, the transmitter includes transformer  114  and PAs  112 - 1  to  112 - 4 , similar to system  100 . In this example, four PAs  112 - 1  to  112 - 4  are shown for the sake of illustration, but depending on the desired output power the number can be increased or decreased. Transmitter  200 , however, does not operate based on polar modulation but, instead, operates as a time-interleaved pulse width modulation (PWM) transmitter. To do this, the I and Q signals are upsampled at a rate of four times the center frequency F C . The interleaver  204  (which generally comprises mixers  206 - 1  and  206 - 2  and combiner or adder  208 ) then interleaves the I and Q signals into a single stream using interleaving signals PH 1  and PH 2 . The distribution circuit  209  (which generally comprises delay circuits  210 - 1  to  210 - 3  and inverters  212 - 1  and  212 - 2 ) then distributes the interleaved stream to PAs  112 - 1  to  112 - 4 , where, in this example, there is a channel for each of the PAs  112 - 1  to  112 - 4 . The distribution circuit  209  also introduces a delay for each channel generating signals IN 1  to IN 4  that would create a filter response with the transformer  114  (namely, an analog finite impulse response). Since a filter response is created or introduced with transformer  114 , an on-chip band-pass filter  214  can be used instead of an off-chip band-pass filter (i.e.,  104 ), reducing cost. 
     To illustrate the operation of transmitter, a timing diagram is shown in  FIG. 3 . As shown in this example, the interleaving signals PH 1  and PH 2  have a cycle of 0→1→0→−1→0 and are 90° out-of-phase with one another. In period T 1 , signals PH 1  and PH 2  are 1 and 0, respectively, which allows the I signal to be provided as signal IN 1  (with the channel for PA  112 - 1  having a nominal delay). In period T 2 , signals PH 1  and PH 2  are 0 and 1, respectively, allowing the Q signal to be provided as signal IN 1 . In period T 3 , signals PH 1  and PH 2  are −1 and 0, respectively, allowing an inverse of the I signal to be provided as signal IN 1 . Additionally, because of the use of delay circuit  210 - 1  (which is generally a ½ cycle delay but the length of the delay is usually related to the upsample rate and/or the number of PAs), the channel for PA  112 - 2  receives the I signal from period T 1 , which is then inverted by inverter  212 - 1  and provided as signal IN 2 . Similarly, for period T 4 , an inverse of the Q signal is provided as signals IN 1  and IN 2 , and a similar result is shown for periods T 5  to T 10  for signals IN 1  to IN 4 . As should be apparent, the structure of the distribution circuit  209  (which is similar to the construction for a finite impulse response (FIR) filter) and the timings for the interleaving signals PH 1  and PH 2 , the signals IN 1  to IN 4  are the same or are “aligned.” This creates notches at certain frequencies (as shown, for example, in  FIG. 4  with notches at F C /2, 3F C /2, and 2F C ), which is the filter response created with transformer  114 . 
     As a result of employing the interleaver  204  and distribution circuit  209  in a manner so as to create a filter response with transformer  114 , several advantages can be realized. A complete on-chip combining of the I and Q signals along with parallel PAs  112 - 1  to  112 - 4  through a transformer  114  to increase the total output power is also achieved. As an example, if individual PAs  112 - 1  to  112 - 4  can deliver 250 mW of power, the combined power output can be 1 W with fully integrated solution. The filter response with transformer  114  also provides a sufficient amount of suppression that relaxes requirements for analog band-pass filter  214  following the transformer  114 , allowing for an on-chip implementation with a low Q-factor. Additionally, adding more parallel PAs (greater than the four shown) to reach higher output power creates more room to improve the filter response of the transformer with more notch locations and better stop-band attenuation. Moreover, the fully digital transmitter architecture allows flexibility to adjust out-of-band noise and image cancellation using different PWM schemes to meet the spectral mask requirements without using external off-chip components. 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.