Patent Publication Number: US-8112045-B2

Title: Translational loop transmitter architecture employing channel power ratio measurements for modulation accuracy calibration

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 11/078,148, filed Mar. 11, 2005; which application is a continuation of and claims priority to U.S. patent application Ser. No. 10/066,843, filed Feb. 4, 2002, now U.S. Pat. No. 6,882,831; which application claims priority to U.S. Provisional Patent Application No. 60/338,000, filed Dec. 7, 2001; and in which all above listed applications are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to wireless communications; and more particularly to the operation of a Radio Frequency (RF) transmitter within a component of a wireless communication system. 
     BACKGROUND OF THE INVENTION 
     The structure and operation of wireless communication systems is generally known. Examples of such wireless communication systems include cellular systems and wireless local area networks, among others. Equipment that is deployed in these communication systems is typically built to support standardized operations, i.e. operating standards. These operating standards prescribe particular carrier frequencies, modulation types, baud rates, physical layer frame structures, MAC layer operations, link layer operations, etc. By complying with to these operating standards, equipment interoperability is achieved. 
     In a cellular system, a governmental body licenses a frequency spectrum for a corresponding geographic area (service area) that is used by a licensed system operator to provide wireless service within the service area. Based upon the licensed spectrum and the operating standards employed for the service area, the system operator deploys a plurality of carrier frequencies (channels) within the frequency spectrum that support the subscribers&#39; subscriber units within the service area. These channels are typically equally spaced across the licensed spectrum. The separation between adjacent carriers is defined by the operating standards and is selected to maximize the capacity supported within the licensed spectrum without excessive interference. In most cases, severe limitations are placed upon the amount of adjacent channel interference that may be caused by transmissions on a particular channel. 
     In cellular systems, a plurality of base stations is distributed across the service area. Each base station services wireless communications within a respective cell. Each cell may be further subdivided into a plurality of sectors. In many cellular systems, e.g., GSM cellular systems, each base station supports forward link communications (from the base station to subscriber units) on a first set of carrier frequencies and reverse link communications (from subscriber units to the base station) on a second set carrier frequencies. The first set and second set of carrier frequencies supported by the base station are a subset of all of the carrier frequencies within the licensed frequency spectrum. In most, if not all cellular systems, carrier frequencies are reused so that interference between base stations using the same carrier frequencies is minimized but so that system capacity is increased. Typically, base stations using the same carrier frequencies are geographically separated so that minimal interference results. 
     Both base stations and subscriber units include Radio Frequency (RF) transmitters and RF receivers, together “RF transceivers.” RF transceivers service the wireless links between the base stations and subscriber units. The RF transmitter receives a baseband signal from a baseband processor, converts the baseband signal to an RF signal, and couples the RF signal to an antenna for transmission. In most RF transmitters, because of well-known limitations, the baseband signal is first converted to an Intermediate Frequency (IF) signal and then the IF signal is converted to the RF signal. The RF receiver receives an RF signal, converts the RF signal to an IF signal, and then converts the IF signal to a baseband signal, which it then provides to the baseband processor. 
     The fast growth of the mobile communications market demands multi-band RF transceivers that are small in size, low in cost, and have low power consumption. These market demands require that the architecture of the RF transceiver to be suitable for a high level of system integration on a single chip for reduced cost and miniaturized mobile device size. Low power consumption is very critical for increasing mobile device battery life and is very important for small mobile devices that include small batteries. To meet these design challenges, some RF transmitters now use translational loop architecture to convert the IF signal to an RF signal. Translational loop architectures are useful for constant envelope modulated wireless systems, such as the new generation Global Standards for Mobile Communications (GSM) and General Packet Radio System (GPRS) phones that employ Gaussian Minimum Shift Keying (GMSK) modulation. However, so far, the translational loop architecture has not been successfully applied in systems that employ a non-constant envelope modulation format, such as QPSK for CDMA (IS-95) and US-TDMA (IS-136) standardized systems, for 8-PSK for EDGE standard based mobile devices, and for mobile devices that support other non-constant envelope modulation formats, such as 16 QAM, 32 QAM, 64 QAM, 128 QAM, etc. 
     Thus, there is a need in the art for a lower power consumption RF transmitter that supports both constant envelope modulation formats and non-constant envelope formats, among other shortcomings of the prior devices. 
     SUMMARY OF THE INVENTION 
     Thus, in order to overcome the above-described shortcomings as well as other shortcomings of the present devices and methodologies, an RF transmitter constructed according to the present invention includes a translational loop architecture that supports non-constant envelope modulation types, e.g., QPSK, 8-PSK, 16 QAM, 32 QAM, 64 QAM, 128 QAM, etc. The translational loop architecture of the present invention adjusts the envelope of the translational loop so that it supports non-constant envelope modulation types. The RF transmitter may be contained in a mobile device or a stationary device. 
     In particular, the RF transmitter includes an Intermediate Frequency (IF) modulator, a translational loop, an envelope time delay adjust block, an envelope adjust block, and a time delay calibration block. The IF modulator receives a modulated baseband signal and produces a modulated IF signal having a non-constant envelope. The translational loop receives the modulated IF signal and produces a modulated RF signal having a constant envelope. The envelope time delay adjust block receives an envelope signal corresponding to the modulated signal and produces a time delayed envelope signal based upon a time delay control signal. The envelope adjust block adjusts the modulated RF signal based upon the time delayed envelope signal to produce an envelope adjusted modulated RF signal. Finally, the time delay calibration block receives the envelope adjusted modulated RF signal and produces the time delay control signal. 
     In one embodiment, the time delay calibration block includes a down converter, an Analog to Digital Converter (ADC), a Low Pass Filter (LPF), a Band Pass Filter (BPF), and a level detector and control block. The down converter converts the envelope adjusted modulated RF signal to a complex baseband signal. The ADC samples the complex baseband signal. The LPF couples to the ADC and filters the complex baseband signal to produce a LPF output. The BPF also couples to the ADC and filters the complex baseband signal to produce a BPF output. With this structure, the level detector and control block receives the LPF output and the BPF output and produces the time delay control signal based upon the LPF output and the BPF output. These components of the RF transmitter may be embodied within resources resident in a coupled baseband processor. 
     The time delay calibration block of the RF transmitter determines a channel power corresponding to the RF signal. The time delay calibration block also determines an alternate channel power that is emitted by the same transmitting signal corresponding to an alternate channel or adjacent channel. The time delay calibration block then determines the time delay control signal based upon a ratio of the channel power and the alternate channel power. 
     The RF transmitter may also include an envelope detection block that produces the envelope signal. In one embodiment, the envelope detection block determines the envelope signal based upon the complex baseband signal received from the baseband processor (not the complex baseband signal generated within the time delay calibration block). In another embodiment, the envelope detection block determines the envelope signal based upon the modulated IF signal. In still another embodiment, the envelope detection block receives the envelope signal from a coupled baseband processor. 
     In at least some embodiments, the envelope signal produced by the envelope detection block is a digital signal while the time delayed envelope signal is an analog signal. With this signal format, the time delay block delays a digital envelope signal by a delay that is based upon the time delay control signal. Then, a digital to analog converter receives the output of the time delay block and produces the time delayed envelope signal. 
     Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the present invention will be more fully understood when considered with respect to the following detailed description, appended claims and accompanying drawings wherein: 
         FIG. 1A  is a system diagram illustrating a cellular system within which the present invention is deployed; 
         FIG. 1B  is a block diagram generally illustrating the structure of a wireless device constructed according to the present invention; 
         FIG. 2  is a block diagram illustrating a Radio Frequency (RF) transmitter constructed according to the present invention; 
         FIG. 3  is a block diagram illustrating in more detail the RF transmitter of  FIG. 2 ; 
         FIG. 4  is a block diagram illustrating another embodiment of the time delay calibration block of  FIG. 2  that performs the Alternate Channel Power Ratio (ACPR) measurement operations; 
         FIG. 5A  is a block diagram illustrating a low pass filter employed in an ACPR measurement block constructed according to the present invention; 
         FIG. 5B  is a block diagram illustrating a clock generation circuit constructed according to the present invention; 
         FIG. 6  is a block diagram illustrating a power level detector constructed according to the present invention; and 
         FIG. 7  is a block diagram illustrating a time delay block of the envelope time delay adjust block constructed according to the present invention; 
         FIG. 8  is a block diagram illustrating a subscriber unit constructed according to the present invention; 
         FIG. 9  is a logic diagram illustrating a method of operation according to the present invention; 
         FIG. 10A  is a graph illustrating the power spectral density of an RF signal generated by the RF transmitter of the present invention with an envelope adjust time delay mismatch; and 
         FIG. 10B  is a graph illustrating the typical power spectral density of an RF signal generated by the RF transmitter of the present invention with the envelope adjust time delay correctly set. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a system diagram illustrating a cellular system within which the present invention is deployed. The cellular system includes a plurality of base stations  102 ,  104 ,  106 ,  108 ,  110 , and  112  that service wireless communications within respective cells/sectors. The cellular system services wireless communications for a plurality of wireless subscriber units. These wireless subscriber units include wireless handsets  114 ,  118 ,  120 , and  126 , mobile computers  124  and  128 , and desktop computers  116  and  122 . When wirelessly communicating, each of these subscriber units communicates with one (or more during handoff) of the base stations  102  through  112 . Each of the subscriber units  114 - 128  and each of the base station  102 - 112  include radio frequency (RF) circuitry constructed according to the present invention. 
     The RF circuitry of the present invention may be contained in any of the subscriber units  114 - 128 , any of the base stations  102 - 112  or in another wireless device, whether operating in a cellular system or not. Thus, for example, the teachings of the present invention may be applied to wireless local area networks, two-way radios, satellite communication devices, or other devices that support wireless communications. 
     The RF circuitry of the present invention supports both constant envelope and non-constant envelope modulation types. The RF transmitter section of this RF circuitry supports non-constant envelope modulation formats such as QPSK for CDMA (IS-95) and US-TDMA (IS-136) standardized systems and 8-PSK for EDGE standardized systems. The RF transmitter of the present invention also supports other non-constant envelope modulation types, e.g., QPSK, 8-PSK, 16 QAM, 32 QAM, 64 QAM, 128 QAM, etc, whether standardized or not. The structure and operation of the RF transmitter is described further with reference to  FIGS. 2-10B   
     The structure and operation of the RF transmitter of the present invention may also be implemented to service other wireless communications as well. For example, the RF transmitter may be used to service premises based Wireless Local Area Network (WLAN) communications, e.g., IEEE 802.11a and IEEE 802.11b communications, and ad-hoc peer-to-peer communications, e.g., Bluetooth. In a WLAN system, the structure would be similar to the structure shown in  FIG. 1A  but, instead of the base stations  102 - 112 , the WLAN system would include a plurality of Wireless Access Points (WAPs). Each of these WAPs would service a corresponding area within the serviced premises and would wirelessly communicate with serviced wireless devices. For peer-to-peer communications, such as those serviced in Bluetooth applications, the RF transmitter of the present invention would support communications between peer devices, e.g., lap top computer  124  and hand-held wireless device  126 . 
       FIG. 1B  is a block diagram generally illustrating the structure of a wireless device  150  constructed according to the present invention. The general structure of the wireless device  150  will be present in any of the wireless devices  114 - 128  illustrated in  FIG. 1A . The wireless device  150  includes a plurality of host device components  152  that service all requirements of the wireless device  150  except for the RF requirements of the wireless device  150 . Of course, operations relating to the RF communications of the wireless device  150  will be partially performed by the host device components  152 . 
     Coupled to the host device components  152  is the RF interface  154 . The RF interface  154  services the RF communications of the host device  150  and includes an RF transmitter  156  and an RF receiver  158 . The RF transmitter  156  and the RF receiver  158  both couple to an antenna  160 . One particular structure of a host device is described with reference to  FIG. 8 . Further, the teachings of the present invention are embodied within the RF transmitter  156  of the RF interface  154 . 
       FIG. 2  is a block diagram illustrating an RF transmitter  200  constructed according to the present invention. The RF transmitter  200  includes an I-Q Intermediate Frequency (IF) modulator  202  that receives an I-Q baseband signal from a baseband processor and converts the I-Q represented baseband signal into a modulated IF signal. The modulated IF signal produced by the I-Q IF modulator serves as the input to a translational loop  204 . The translational loop  204  produces an RF signal at the Transmit (TX) frequency that is received by an envelope adjust block  206 . Note that the modulated IF signal received by the translational loop  204  is a non-constant envelope signal (corresponding to a non-constant envelope modulation type) while the RF signal output by the translational loop  204  is a constant envelope signal. The envelope adjust block  206  adjusts the envelope of the RF signal produced by the translational loop  204  based upon a time delayed envelope signal. The envelope adjust block  206  produces an envelope adjusted RF signal that is amplified by Power Amplifier (PA)  208 . The output of the PA  208  is applied to an RF switch, a RF filter or a duplexer that couples the amplified envelope adjusted RF signal to an antenna. 
     An envelope detection block  210 , an envelope time delay adjust block  212 , and a time delay calibration block  214  work in cooperation to produce the time delayed envelope signal. Envelope detection block  210  detects the envelope of the IF signal using one of three (or more) techniques/embodiments. In a first embodiment, the envelope detection block  210  receives an envelope indication from a coupled baseband processor. In a second embodiment, the envelope detection block  210  measures I and Q components of the baseband signal at the input of the I-Q IF modulator to calculate the envelope. In a third embodiment, the envelope detection block  210  measures the envelope of the IF signal at the output of the I-Q IF modulator  202 . 
     The envelope signal, which represents a magnitude of the modulated signal, is output by the envelope detection block  210  and received by the envelope time delay adjust block  212 . The envelope time delay adjust block  212  outputs a time delayed envelope signal. The time delayed envelope signal, with respect to the envelope signal, has been delayed so that it correctly corresponds to the RF signal output of the translational loop  204 . Thus, the delay introduced by the envelope adjust time delay  212  corresponds to the delay introduced by the translational loop  204 , and in addition, the delay introduced by the I-Q IF modulator  202  when the envelope is detected from the baseband signal at the input to the I-Q IF modulator  202 . Thus, the RF signal output of the envelope adjust block  206  is a phase and magnitude matched RF signal having a non-constant envelope. 
     A time delay calibration block  214  adjusts the delay introduced into the detected envelope by the envelope time delay adjust block  212 . The operation of the time delay calibration block  214  will be described in detail with reference to  FIGS. 3-6  and  9 - 10 B. Generally speaking, the time delay calibration block  214  adjusts the time delay introduced by the envelope time delay adjust block  212  until the ratio of the signal level within a subject channel to the signal level of an adjacent channel (or an alternate channel) satisfies the operating requirements of the RF transmitter  200 . 
       FIG. 3  is a block diagram illustrating in more detail the RF transmitter  200  of  FIG. 2 . As shown, the I-Q IF modulator  202  receives an IF reference signal. In the described embodiment, the IF reference signal is equal to ⅙ of the transmit frequency (TX) reference signal. A phase shifter  304  receives the IF reference signal (I component) and produces a corresponding Q component by shifting in phase the I component by 90 degrees. An I-Q mixer  302  separately mixes the I and Q components of the baseband signal with the I and Q components of the IF reference signal. The output of the I-Q mixer  302  is summed and band pass filtered by block  306 , which produces the modulated IF signal. 
     The translational loop  204  includes a Phase and Frequency Detector (PFD)  308  that receives the modulated IF signal and produces an output that is received by a charge pump  310 . Low Pass Filter (LPF)  312  filters the output of the charge pump  310  and the output of the LPF  312  drives a TX Voltage Controlled Oscillator (VCO)  314 . The output of the TX VCO  314  serves as one input to the envelope adjust block  206 , which is a mixer in the illustrated embodiment. The translational loop  204  also includes a mixer  316  that receives the output of the TX VCO  314  and a 7/6 TX reference signal from a local oscillator. The output of mixer  316  with ⅙ of the TX frequency serves as a second input to the PFD  308 . 
     The envelope detection block  210  implements the second embodiment described above by calculating the I and Q components in a digital format. The envelope detection block  210  also includes processing operations to determine the magnitude of the envelope by calculating the square root of (I 2 +Q 2 ). When the envelope detection block  210  implements the third embodiment described above, it digitizes the modulated IF signal produced at the output of the I-Q IF modulator and determines the magnitude thereof. 
     The envelope time delay adjust block  212  includes a time delay block  322  and a Digital to Analog converter (DAC)  324 . The time delay calibration block  214  includes a mixer  326  that mixes the RF signal produced by mixer  206  (envelope adjust block) with a local oscillator signal at a frequency equal to the TX carrier frequency to down convert the modulated TX signal to a complex baseband signal at the output of the mixer  326 . An Analog to Digital Converter (ADC)  328  receives the output of the mixer  326  and converts the signal to a digital equivalent of the complex baseband signal. Both a Low Pass Filter (LPF)  330  and a Band Pass Filter (BPF)  332  filter the complex baseband signal. As an example of an operation that would be performed in an EDGE standard based cellular system, the LPF  330  would have a cutoff frequency (F c ) of 200 kHz, and the BPF  332  would have an F o  of 500 kHz and a Bandwidth of 200 kHz. 
     The outputs of the LPF  330  and the BPF  332  are received by a level determination and control block  334  that produces the time delay control signal that is an input to the time delay block  322 . In the described embodiment, the LPF  330  produces a signal that corresponds to a subject channel and the BPF  332  produces a signal that corresponds to an alternate channel (or adjacent channel). The level determination and control block  334  uses these signals to calculate an Alternate Channel Power Ratio (ACPR) that is the ratio of the Channel Power (CP) to the Alternate Channel Power (ACP). The ACPR is then compared to an allowable limit. If the ACPR meets this limit, the input to the time delay block  322  is properly set. However, if the ACPR does not meet this limit, the level determination and control block  334  adjusts the time delay control signal to the time delay block  322  until the limit is met. 
       FIG. 4  is a block diagram illustrating another embodiment of the time delay calibration block  214  of  FIG. 2  that performs the ACPR measurement operations. As shown in  FIG. 4 , for hardware efficient filtering of the alternate channel signal, the BPF  332  includes a DDFS Look Up Table (LUT)  402 , a complex mixer  404 , and a LPF  406 . Further, the level detection and control block  334  includes a power level detector  410  and a time delay control signal block  412 . The power level detector  410  outputs detected levels of the CP and the ACP. Based upon a ratio of these measurements, the ACPR, the time delay control signal block  412  produces the time delay control signal. 
       FIG. 5A  is a block diagram illustrating a LPF employed in an ACPR measurement block constructed according to the present invention. The LPF includes filter operation blocks  502 ,  506 ,  510 , and  512  and gain operator  504 . The low pass filter of  FIG. 5A  may be employed for both/either of LPFs  330  and  406  of  FIG. 4 . 
       FIG. 5B  is a block diagram illustrating a clock generation circuit constructed according to the present invention. As was described with reference to  FIG. 3 , various reference signals must be produced for the RF transmitter  200 . These frequencies include ⅙ TX, TX, and 7/6 TX. In order to produce these frequencies, the clock generation circuit of  FIG. 5B  includes a Local Oscillator  552  that produces a clock reference signal. A Phase Locked Loop (PLL)  554  receives the clock reference signal and produces a frequency-multiplied output based thereupon. A first divider  556  receives the frequency-multiplied output and divides the signal to produce the 7/6 TX reference signal. A second divider  558  receives the frequency-multiplied output and divides the signal to produce the ⅙ TX (IF) reference signal. Further, in order to produce the TX reference signal (for use by the time delay calibration block  214 ), the 7/6 TX reference signal and the ⅙ TX reference signal are mixed via mixer  560  to produce the TX reference signal. 
       FIG. 6  is a block diagram illustrating a power level detector constructed according to the present invention. As is shown, the power level detector includes a first mixer  602  that is employed to produce the Power Spectral Density (PSD) of the signal output by the LPF  330 . Further, a second mixer  604  is employed to produce the PSD of the signal output by the BPF  332 . Filtering blocks  606  and  610  filter the outputs of the mixers  602  and  604 , respectively. Subsequently, the outputs of filtering blocks  606  and  610  are down sampled by down sampling blocks  608  and  612 , respectively, to produce the CP and ACP signals, respectively. Note that the input to the power level detector  410  is at a sample rate of 26 MHz and that the output of the power level detector  410  is at a sample rate of 10 kHz. 
       FIG. 7  is a block diagram illustrating a time delay block  322  of the envelope time delay adjust block  212  constructed according to the present invention. The time delay block  322  receives as its input the envelope signal produced by the envelope detection block  210  and the time delay control signal produced by the time delay calibration block  214 . The time delay block  322  includes a plurality of serially coupled delay elements  702 - 712 . Multiplexer  714  receives as its input the envelope signal and the output of each of the serially coupled delay elements  702 - 712 . The multiplexer  714  receives as its control input the time delay control signal from the time delay calibration block  214 . As its output, the multiplexer  714  produces the time delayed envelope signal. The delay introduced by the delay elements  702 - 712  is determined based upon expected minimum and maximum delays that are required to compensate for the delay introduced by the translational loop  204  (and the I-Q IF modulator  202 ). 
       FIG. 8  is a block diagram illustrating a subscriber unit  802  constructed according to the present invention. The subscriber unit  802  operates within a cellular system, such as the cellular system described with reference to  FIG. 1A  and according to the operations previously described with reference to  FIGS. 2-7  and as will subsequently be described with reference to  FIGS. 9-10B . The subscriber unit  802  includes an RF unit  804 , a processor  806  that performs baseband processing and other processing operations, and a memory  808 . The RF unit  804  couples to an antenna  805  that may be located internal or external to the case of the subscriber unit  802 . The processor  806  may be an Application Specific Integrated Circuit (ASIC) or another type of processor that is capable of operating the subscriber unit  802  according to the present invention. The memory  808  includes both static and dynamic components, e.g., DRAM, SRAM, ROM, EEPROM, etc. In some embodiments, the memory  808  may be partially or fully contained upon an ASIC that also includes the processor  806 . A user interface  810  includes a display, a keyboard, a speaker, a microphone, and a data interface, and may include other user interface components. The RF unit  804 , the processor  806 , the memory  808 , and the user interface  810  couple via one or more communication buses/links. A battery  812  also couples to and powers the RF unit  804 , the processor  806 , the memory  808 , and the user interface  810 . 
     The RF unit  804  includes the RF transmitter components described with reference to  FIG. 2  and operates according to the present invention to adjust the envelope of an RF signal produced by a translational loop contained therein. The structure of the subscriber unit  802  illustrated is only one particular example of a subscriber unit structure. Many other varied subscriber unit structures could be operated according to the teachings of the present invention. Further, the principles of the present invention may be applied to base stations, as are generally described with reference to  FIG. 1A . 
       FIG. 9  is a logic diagram illustrating a method of operation according to the present invention. Operation according to the present invention is initiated upon power up or reset. In such case, the operations of  FIG. 9  will be performed along with a number of other operations required to set/reset the operation of a corresponding RF transmitter  200 . The operations of  FIG. 9  will be described with additional reference to  FIGS. 10A and 10B .  FIG. 10A  is a graph illustrating the power spectral density of an RF signal generated by the RF transmitter  200  of the present invention with the time delay control signal improperly set.  FIG. 10B  is a graph illustrating the power spectral density of an RF signal generated by the RF transmitter  200  of the present invention with the time delay control signal properly set. 
     Referring again to  FIG. 9 , upon power up or reset, the calibration operations of the present invention are initiated (step  902 ). In such case, the components of the RF transmitter  200 , e.g., RF transmitter  200  of  FIG. 2 , operate to produce the RF signal. The RF signal may have a power spectral density such as that shown in  FIG. 10A . With the operations of the present invention, the CP (central lobe of the PSD) and the ACP (side lobe of the PSD) are measured (step  904 ). Then, the ratio of the CP to the ACP (or the ACP to the CP) is determined, i.e., ACPR. The ACPR is then compared to an ACPR threshold (step  906 ). This ACPR threshold relates directly to the permissible interference that operation on the channel may produce in the alternate channel (or adjacent channel). 
     With the PSD shown in  FIG. 10A , the ACPR does not meet the threshold and the time delay control signal provided by the time delay calibration block  214  to the envelope time delay adjust block  212  is adjusted (step  908 ). Operation returns from step  908  to step  904  where the CP and ACP are again measured and the ACPR is determined. After one or more iterations of step  908 , the PSD of  FIG. 10B  is produced such that the ACPR threshold is met at step  906 . From step  906 , with the ACPR met, operation proceeds to step  910  wherein the currently set time delay control signal produced by the time delay calibration block  214  is used. Operation remains at step  912  until a calibration event occurs. A calibration event may occur periodically or when a threshold is met. 
     The invention disclosed herein is susceptible to various modifications and alternative forms. Specific embodiments therefore have been shown by way of example in the drawings and detailed description. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the claims.