Abstract:
A local oscillator (LO) generator for driving a bank of mixers that provides precise phase relationship between multi-phase LO outputs comprises a shift register using slave-master-slave flip-flop elements. The LO generator can be used over a wide operating frequency range. The LO generator is suitable for driving a multi-phase LO clock input to a harmonic suppression mixer. A pattern generator produces a pattern signal and a reclocking signal that determines the frequency of the LO signals and the phase delay between the LO output phases.

Description:
RELATED APPLICATIONS 
   This application claims priority from U.S. provisional application No. 60/636,584 filed Dec. 16, 2004 entitled “Phase-accurate multi-phase wide-band Radio Frequency Local Oscillator generator”, incorporated herein by reference. 
   U.S. patent application Ser. No. 11/078,050, filed Mar. 11, 2005 entitled “Harmonic suppression mixer and tuner”, incorporated herein by reference. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates to radio frequency local oscillator generators used with multiple-mixer tuners. 
   2. Prior Art 
   Superheterodyne receivers use one or more local oscillators (LO) to convert an input frequency to an intermediate frequency before the signal is demodulated. In an ideal receiver, these frequency conversions would not distort the input signal and all the information in the input signal would be recovered. In actual application, the local oscillator can add distortions that can limit the receiver&#39;s ability to recover the signal information. The principle local oscillator degradation is random phase variations known as phase noise. 
   The ideal local oscillator (LO) generator for implementing multiple output phases for use in applications such as the commonly assigned U.S. patent application Ser. No. 11/078,050, filed Mar. 11, 2005 entitled “Harmonic suppression mixer and tuner”, incorporated herein by reference, must have low phase noise and there must be a minimum phase mismatch between the multiple outputs. Ideally, the LO generator would use minimum chip area and minimum power. 
   The minimized phase mismatch between the multiple outputs is required in many applications because of the need for high image rejection and high harmonic rejection in the mixing and tuning process. Multiple frequency division ratios are also needed to provide frequency agility in the tuner. 
   U.S. Pat. No. 6,856,208, issued to Lee et al. on Feb. 15, 2005, entitled “Multi-phase Oscillator and Multi-phase Oscillation Signal Generation Method”, incorporated herein by reference, describes a multi-phase oscillator that can be a single phase oscillator, a 180 degree phase difference oscillator or a multiple phase difference oscillator. 
   In applications such as multi-mixer tuners, described in co-pending U.S. patent application Ser. No. 11/078,050, filed Mar. 11, 2005 entitled “Harmonic suppression mixer and tuner”, multiple frequency division ratios are needed in order to generate a wide range of LO output frequencies from a limited input clock frequency range. 
   The multi-mixer architecture needs multiple LO clock phases to actively reject higher harmonics of the LO clock signal. To do this with high suppression ratios, the low phase mismatch requirement is of the utmost importance. 
   To reduce overall cost, chip area must be kept as low as possible; furthermore, a more compact layout will result in a lower uncertainty on phase accuracy. 
     FIG. 1  shows a solution known in the prior art. The slave latch not only provides the LO signal to the mixer, but is also loaded by the master latch in the next flip-flop. This results in a deterioration of the signal slope and phase accuracy. To minimize the deterioration due to capacitive and other loading, an emitter follower or source follower can be included at the output of the slave latch or the bias current of the latch can be increased. 
   The master input of flip-flop  2  in  FIG. 1  is directly connected to the output of flip-flop  1 . Any feed-through of the master clock to the input of the master latch of flip-flop  2 , results in a ripple on the LO signal going to the mixer. This can result in LO feed-through and down conversion of unwanted RF signals at harmonics of the LO signal. Both effects directly lead to deteriorated harmonic rejection performance of the mixer architecture referred to above. 
   The propagation delay from the slave output of flip-flop  1  to the master input of flip-flop  2  has to be low enough to meet the setup-time requirement of the slave latch at high frequencies. If this requirement is not met, the phase accuracy of the output signal is deteriorated considerably. The optional emitter/source follower included to solve the load-related issue described above adds additional delay. This delay can be minimized by increasing its bias current, but results in increased circuit power consumption. 
   The physical layout of the circuit as shown in  FIG. 1  poses considerable challenges that need to be solved in order to avoid adverse effects on the performance. Clock lines and slave output lines can couple resulting in unwanted clock feed-through. Additionally, the connection between slave output and master input needs to be kept short to avoid extra loading due to parasitics. The interconnection scheme used at the slave output may be complicated and therefore, layout asymmetry will be unavoidable. If a differential signaling implementation is used, this asymmetry will have a negative influence on the final phase accuracy. 
   Even by substantially increasing current consumption, not all of these issues can be solved. 
     FIG. 2  shows another prior art solution where the input of flip-flop  2  is driven with the master latch output of flip-flop  1 . This requires the clock signals of flip-flop  1  and flip-flop  2  to be inverted with respect to each other to avoid digital race conditions. For example, flip-flop  1  would be clocked at the rising edge and flip-flop  2  would be clocked at the falling edge of the clock signal. In fact, using the master of flip-flop  1  as a reference, the input latch of flip-flop  2  then can be considered a slave latch since it is clocked on the same clock edge as the slave of flip-flop  1 . 
   A major negative side effect of this clocking scheme is that the resulting output signal phase accuracy becomes dependent on the duty cycle of the clock signal. Furthermore, the propagation delay of the master latch will increase due to increased loading. This can be alleviated but requires increasing the master latch bias current or adding an emitter or source follower. 
   SUMMARY OF INVENTION 
   The present invention generates a multi-phase local oscillator (LO) signal with a clocked shift register that uses a slave-master-slave flip-flop configuration in each stage of the shift register. The shift register is driven by a reclocking signal produced by a programmable divider. This divider-pattern generator produces an input signal of a particular phase and frequency to the first stage of the shift register. The input slave latch of each stage accepts a data signal, the slave latch is coupled to a master latch that is clocked on an alternate edge of the reclocking signal, then coupled to an output slave latch. The LO phase signal at each stage is output from the output slave latch. The output of the intermediate master latch of a succeeding register stage is used to drive the input of the input slave latch in the following register stage. By coupling the intermediate latch output to the next stage, loading of the LO output is avoided. 
   In order to reduce the phase mismatch between the clock signals, the output slave latch of all stages is used to align the LO signals in phase. The number of shift register stages used is equal to the highest number of required LO phases. 
   The flip-flops are arranged in a shift register fashion. Thus, a flip-flop output provides a signal to the mixer circuit and also drives the input of the next flip-flop. To avoid deterioration of the signal going to the mixer circuit, especially degrading the slope of the signal, the output driving the next flip-flop is taken from the master latch inside the slave-master-slave flip-flop circuit. 
   In order to avoid digital race conditions when driving a flip-flop input from a master latch coming from a previous flip-flop while keeping the output phase accuracy independent of the clock duty cycle, each flip-flop is given an additional slave latch at the input of the flip-flop. This additional slave latch then drives the following master-slave section of the flip-flop. 
   The input signal to the first flip-flop in the shift register and the clock signal are set up to provide the wanted combination of the final output frequency and the number of different phases to the mixer circuit as LO signal. 
   The present invention local oscillator is suitable for use with a multi-mixer harmonic suppression mixer and tuner. The digital LO signals drive switching mixers of the tuner. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a prior art shift register interconnection. 
       FIG. 2  shows an alternative prior art shift register interconnection. 
       FIG. 3  shows a multi-phase LO generator with clock divider, pattern generator, and shift register according to the present invention. 
       FIG. 4  shows a slave-master-slave flip-flop interconnection according to the present invention. 
       FIG. 5  shows an exemplary ECL transistor schematic of edge-triggered slave-master-slave D-flip-flop for use with the present invention. 
       FIG. 6  shows an example of waveforms output from the multi-phase LO generator of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a detailed block diagram of a multi-phase LO generator consisting of a pattern generator  320  and a reclocking shift register  330 . The pattern generator  320  provides the input signal  306  to the first flip-flop  332  of the shift register  330 . Each flip-flop in the shift register is identical and the detail of the flip-flop is described below. The pattern generator  320  additionally divides the input frequency (Fin)  310  by programmable divider  322  to supply the reclocking signal  308  to the clock input of all the flip-flops in the shift register  330 . Different types of flip-flops can be used, but in this embodiment and as an example, D-type flip-flops are used. The flip-flops herein will be referred to as “DFF”. The flip-flops have a slave-master-slave configuration with the master latch and the second slave latch providing outputs. In this embodiment, a shift register with 8 flip-flops is described. Other embodiments could vary in the number of flip-flops used in the shift register, in the presence or absence of the pattern generator circuit or in the topology of the pattern generator circuit, and in type of circuit to realize the flip-flop functionality. 
   In the pattern generator circuit  320 , the input frequency  310  (Fin) is divided down to the desired LO frequency  306  by two-stage divider  324 . This signal is reclocked at frequency  308  by the eight flip-flop shift register. To describe the functionality, three frequency division ratios are considered:
         Rclk is the frequency division ratio of the input frequency  310  (Fin) and the reclocking frequency  308  (Fclk), where Rclk=1, 2 or 4.   Rphase is the frequency division ratio of the reclocking frequency  308  (Fclk) and the input frequency  306  (FLO) of the first DFF, where Rphase=4, 8 or 16.   Rdiv is the total frequency division ratio of the input frequency  310  (Fin) and the LO frequency  306  (FLO). It is equal to Rphase*Rclk, which results in Rdiv=4, 8, 16, 32 or 64.       

   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Relationship between phases and division ratios 
             
           
        
         
             
                 
               Division ratios 
               Number of phases 
             
           
        
         
             
                 
               Rclk 
               Rphase 
               Rdiv 
               (nφ) (Modes) 
             
             
                 
                 
             
           
        
         
             
                 
               1 
               4 
               4 
               2 
             
             
                 
               1 
               8 
               8 
               4 
             
             
                 
               1 
               16 
               16 
               8 
             
             
                 
               2 
               16 
               32 
               8 
             
             
                 
               4 
               16 
               64 
               8 
             
             
                 
                 
             
           
        
       
     
   
   The division ratio Rphase sets the number of output phases of the LO generator. Using division ratios of 4 up to 16, the output of the shift register bank will provide two, four or eight different phases. An overview of the different modes is given in Table 1. The number of phases is not dependent on the value of Rclk, since both input signal and reclocking signal are divided by this ratio. For values of Rdiv higher than sixteen, the input frequency is divided down by a factor Rclk before splitting the signal between the flip-flop input path and the reclocking path. The modes with division ratios of 32 and 64 therefore still have a resolution of π/8 ( 1/16th of a period), resulting in eight phases at the output of the shift register. 
   Output  334  (the master latch output) of each flip-flop is used as input for the next flip-flop. In order to avoid extra, and possibly asymmetric, loading on the sensitive slave output  336 , an extra latch clocked on the slave clock phase is inserted as the input stage of each DFF, and the output to drive the next stage is taken from the master latch of the previous DFF. To minimize the loading effect on the master latch, an emitter follower or source follower is used to drive the two slaves. 
   In this description, the circuit referred to as “flip-flop” could be any circuit that realizes the following basic functionality with all signals considered “digital” (either high or low):
         The circuit has a signal input and a clock input, and a signal output;   During clock phase  1  the signal input has no direct influence on the output signal and the output signal is held at its previous value (hold phase);   Only during clock phase  2 , the output is allowed to change so its value at the end of phase  2  is identical to the input value at the end of the preceding clock phase  1 .       

     FIG. 4  shows the details of the shift register flip-flops, using slave-master-slave configuration, and the interconnection of two consecutive flip-flops  401  and  402  each having two slave latches. Master and slave latches are active or triggered on opposite phases or phase edges of the clock signal  404  (Clk). Clk  404  is driven by Fclk  308  in  FIG. 3 . Slave latch  406  at the input of each flip-flop drives the master latch  408  inside the same flip-flop. Master latch  408  drives the second slave latch  410  inside the same flip-flop and also the first slave latch  414  of the following flip-flop  402  in the shift register. The total number of flip-flops interconnected in this way is set by the number of desired output signals. Signal  412  is the input signal of the first flip-flop in the shift register. Using this kind of interconnection, the output slave is not loaded by the next flip-flop and edge transitions can be faster and better defined in time and feed-through of the master clock to the slave output is avoided. Out 1  and Out 2  are examples of LO phases that drive a mixer bank. 
   The connection of the clock line  404  limits the ratio of the re-clock frequency and the LO frequency (Fclk/FLO) to a minimum of 4. In this clocking scheme, the output signal phase accuracy is not sensitive to the duty cycle of the clock signal. 
   Using an alternative clocking scheme, the minimum ratio of Fclk/FLO can be decreased to 2. More specifically this can be done by using both phases or edges of the clocking signal. For example, all odd numbered flip-flops would be clocked at the positive phase or rising clock edge and all even numbered flip-flops would be clocked at the negative phase or falling edge. The edge or phase that clocks the master latch is considered the edge or phase that clocks the flip-flop. Other configurations could also be used to realize the same minimum ratio of Fclk/FLO. This alternative clocking scheme is sensitive to duty cycle. 
     FIG. 5  shows an example of a detailed implementation of an edge-triggered double-slave flip-flop using emitter coupled logic (ECL). Other implementations could differ in the type of event that triggers the flip-flop (clock edge, clock level, etc), in the type of technology used (bipolar, 
   BiCMOS, CMOS, GAS, etc.), or in the type of logic or in any detail of the used circuit topology to realize the double-slave flip-flop functionality. 
     FIG. 6  shows an example of waveforms output from the multi-phase LO generator of the present invention. Each LO waveform is identical in shape and output with a different phase from the outputs of the flip-flops in the shift register. The pattern generator determines the shape of the waveforms and the reclocking signal determines the phase difference, or delay, between each LO signal. The staggered-phase LO waveforms drive a multi-mixer tuner, where each mixer can apply a different weighting to the RF input signal to result in a stepped synthesized sinusoidal waveform as the effective local oscillator signal for the mixer. The clock harmonics of the effective LO signal are far above the fundemental of the sinusoid as shown in the spectrum plot. Images of the tuned RF signal produced by the high harmonic frequencies can be filtered out leaving only the desired signal produced by the first harmonic, or fundamental, frequency of the sinusoid.