# Patent

## Title
Low-noise sigma-delta frequency synthesizer

## Abstract

A sample-and-hold (SAH) phase detector (PD) is clocked in such a way (using a reverse clocking mode) so as to avoid quantization noise increases due to folding that is generally associated with conventional charge pump based phase detectors. The PD is clocked with a clean clock (reference clock), rather than a divided clock. The SAH PD architecture additionally includes an integrated filtering function.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to sigma-delta (ΣΔ) loops, and more particularly to a sample and hold (SAH) phase detector (PD) architecture and method of clocking to avoid quantization noise folding in ΣΔ loop applications such as a ΣΔ frequency synthesizer.
2. Description of the Prior Art
Conventional charge pump based phase detectors suffer several problems when they are used in ΣΔ loop applications. These problems may include, for example, noise folding due to non-linearity, charge/discharge mismatch, and the like.
In view of the foregoing, it is highly desirable and advantageous to provide a PD architecture and a PD clocking scheme that avoids quantization noise increases due to noise folding. It would be further advantageous if the PD architecture could provide an integrated filtering function.
SUMMARY OF THE INVENTION
The present invention is directed to a sample-and-hold (SAH) phase detector (PD) that is clocked in such a way so as to avoid quantization noise increases due to folding that is generally associated with conventional charge pump based phase detectors. The PD is clocked with a clean clock (e.g. reference clock generated by TXCO), rather than a divided VCO clock. The SAH PD architecture additionally includes an integrated filtering function.
According to one embodiment, a method of clocking a sample-and-hold (SAH) phase detector (PD) comprises the steps of precharging a ramp node to a supply voltage in response to a first clock; discharging the precharged ramp node via a linear current source in response to a second clock; and sampling this ramp node in response to a third clock such that the output node is maintained at a desired output voltage level during the third clock, wherein the second clock commences in response to a voltage controlled oscillator (VCO) clock edge, and further wherein the third clock commences in response to a reference clock edge, such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using charge pump based phase detectors.
According to another embodiment, a method of clocking a sample-and-hold (SAH) phase detector (PD) comprises the steps of providing a stable reference clock operational to generate a reference clock edge and a voltage controlled oscillator (VCO) associated with a divider element operational to generate a VCO clock edge; and clocking the SAH PD in response to the reference buffer clock edge and the VCO clock edge such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using a charge pump based PD.
According to yet another embodiment of the present invention, a sigma-delta frequency synthesizer comprises a stable reference clock operational to generate a reference clock edge; a voltage controlled oscillator (VCO) responsive to a divider element to generate a VCO clock edge; and a sample-and-hold (SAH) phase detector (PD) responsive to the reference buffer clock edge and the VCO clock edge to hold a sampling node voltage at a desired output voltage level during a sampling pulse such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using a charge pump based PD.



BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:
FIG. 1 is a block diagram illustrating a well known type-I frequency synthesizer;
FIG. 2 illustrates a sample and hold (SAH) phase detector (PD) suitable for use with the frequency synthesizer shown in FIG. 1;
FIG. 3 is a clock diagram associated with the PD shown in FIG. 2;
FIG. 4 is a quantization noise simulation plot associated with the frequency synthesizer shown in FIG. 1 that employs the PD shown in FIG. 2 which is clocked as shown in FIG. 3; and
FIG. 5 illustrates another sample and hold (SAH) phase detector (PD) suitable for use with the frequency synthesizer shown in FIG. 1.



While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a system block diagram illustrating a typical type-I frequency synthesizer architecture 100. The present inventors recognized the design of the phase detector (PD) 102 for ΣΔ high performance frequency synthesizers is key to low noise applications. Regarding frequency synthesizer architecture 100, the ΣΔ modulator 104 generates the fractional channel for the divider 106 by randomly changing the divider 106 ratio between different values. For a ΣΔ modulator using one known architecture, for example, the divider 106 changes between a maximum of N+4 and a minimum of N−3 wherein N is an integer of value 4 or greater. These variations are problematic however, in that they cause what is commonly referred to as quantization noise which can increase the total output noise of the frequency synthesizer 100.
The present inventors also recognized that type-II phase locked loops (PLLs) using charge pump based phase detectors are problematic in that they can suffer high quantization noise due to the non-linearity of the PD and the mismatch between the up and down currents. The synthesizer architectures discussed herein with reference to the particular embodiments are then most preferably using type-I PLLs. These type-I PLLs, according to the most preferred embodiments, are based on sample and hold (SAH) phase detectors (PDs) that do not rely on charge pumps. Such SAH PDs provide good linearity without suffering the problems associated with current mismatch as in the conventional PDs based on charge pumps. The most preferred embodiments are now discussed herein below with reference to FIGS. 2-5 to describe a clocking scheme for a SAH PD that avoids the problems of quantization noise folding while providing inherent filtering characteristics.
FIG. 2 illustrates a simplified circuit structure for a sample and hold (SAH) phase detector (PD) 102. Operation of this PD commences when the node Vs is initially precharged to VDD through the clock φp. When the reference edge, depicted as (PD_R in FIG. 1), coming from the reference buffer reaches the PD 102, it generates a clock φr to discharge the node Vs via a linear current source 108. When the VCO edge, depicted as (PD_V in FIG. 1), reaches the PD 102, it stops φr and generates a small pulse φs to sample the voltage on the node Vs and hold it at Vo. Since the clock PD_V is jittery due to the division variation with time, the output Vo is affected by this jitter; and hence total output noise increases.
Recognizing the foregoing adverse quantization noise characteristics associated with a SAH PD, the present inventors implemented a reversed clocking scheme to substantially reduce the noise impact. With continued reference to FIG. 2, this reversed clocking scheme advantageously generates the phase φr by the clock PD_V rather than by the clock PD_R; and further generates the phase φs by the clock PD_R rather than by the clock PD_V. This change was found by the present inventors to yield a very clean sampling pulse φs since it is now generated via the temperature compensated crystal oscillator (TCXO) buffer contained within VCO Calibration block 110. Because a crystal oscillator and the reference buffer inherently have low phase noise, the output Vo is much cleaner.
FIG. 3 is a clock diagram associated with the SAH PD architecture shown in FIG. 2 according to one embodiment of the present invention that employs the reversed clocking scheme discussed herein before.
FIG. 4 is a quantization noise plot simulation associated with the frequency synthesizer 100 shown in FIG. 1 that employs the SAH PD shown in FIG. 2 which is reverse clocked as shown in FIG. 3. The quantization noise is plotted using the reverse clocking scheme 200 and also using the conventional clocking scheme 202 that is known in the art. The quantization noise plots clearly demonstrate that the noise folding is avoided in the new reverse clocking scheme 200 compared to the conventional clocking scheme 202.
Equation (1) below represents the theoretical quantization noise of the frequency synthesizer 100 where no noise folding is occurring.




  
    
      
        
          
            ξ
            ⁡
            
              (
              f
              )
            
          
          =
          
            
              
                F
                ref
              
              
                12
                ⁢
                
                  f
                  2
                
              
            
            ⁢
            
              
                (
                
                  2
                  ⁢
                  
                    sin
                    ⁡
                    
                      (
                      
                        
                          π
                          ⁢
                          
                            
                          
                          ⁢
                          f
                        
                        
                          F
                          ref
                        
                      
                      )
                    
                  
                
                )
              
              6
            
            ⁢
            
              
                
                
                  H
                  ⁡
                  
                    (
                    f
                    )
                  
                
                
              
              2
            
          
        
        ,
      
    
    
      
        (
        1
        )
      
    
  




where Fref is the reference clock (e.g. 26 MHz), ƒ is the frequency offset, and H(ƒ) is the transfer function of the loop from the divider 106 control point. This theoretical quantization noise is depicted by the continuous arcuate plot 204 in FIG. 4.

The present inventors further discovered another advantage associated with the SAH PD using the reverse clocking scheme is provided in association with the inherent pole caused by the charge sharing between the two capacitors Cs and Ci seen in FIG. 2. This inherent pole is represented by the time constant shown as equation (2) below.




  
    
      
        
          
            C
            i
          
          +
          
            
              C
              s
            
            /
            2
          
        
        
          
            C
            s
          
          ⁢
          
            F
            ref
          
        
      
    
    
      
        (
        2
        )
      
    
  




This means that the PLL extra free pole is determined by capacitor ratio and is independent of process variations. This pole can then be accurately controlled simply via correct capacitor sizing and selecting a desired capacitor ratio.

If more filtering is desired for the loop, than the SAH PD can drive an RC network 300 such as shown in FIG. 5 without additional buffering. FIG. 5 illustrates a sample and hold (SAH) phase detector (PD) suitable for use with the frequency synthesizer 100 shown in FIG. 1 according to another embodiment of the present invention. The first pole for the SAH PD depicted in FIG. 5 is given in equation (3) below as




  
    
      
        
          
            
              
                C
                s
              
              /
              2
            
            +
            
              C
              2
            
            +
            
              C
              3
            
          
          
            
              C
              s
            
            ⁢
            
              F
              s
            
          
        
        +
        
          
            R
            2
          
          ⁡
          
            (
            
              
                C
                2
              
              +
              
                C
                3
              
            
            )
          
        
        +
        
          
            R
            3
          
          ⁢
          
            
              C
              3
            
            .
          
        
      
    
    
      
        (
        3
        )
      
    
  




By making the switched capacitor term Cs more dominant, overall response is then less sensitive to process variations.

In summary explanation, a sample-and-hold (SAH) phase detector (PD) is clocked in such a way (using a reverse clocking mode) so as to avoid quantization noise increases due to folding that are generally associated with conventional charge pump based phase detectors. The PD is clocked with a clean clock (reference clock), rather than a divided clock. The SAH PD architecture additionally inherently includes an integrated filtering function.
In view of the above, it can be seen the present invention presents a significant advancement in the art of phase detector clocking schemes and associated methods. This invention has been described in considerable detail in order to provide those skilled in the ΣΔ frequency synthesizer art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.

## Claims

1. A method of clocking a sample-and-hold (SAH) phase detector (PD), the method comprising the steps of:
precharging a ramp node to a supply voltage in response to a first clock;
discharging the precharged ramp node via a linear current source in response to a second clock; and
sampling the discharging precharged ramp node in response to a third clock such that the ramp node is maintained at a desired output voltage level during the third clock, wherein the second clock commences in response to a voltage controlled oscillator (VCO) clock edge, and further wherein the third clock commences in response to a reference clock edge, such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using charge pump based phase detectors.

2. The method according to claim 1 wherein the SAH PD is not associated with a charge pump.

3. The method according to claim 1 wherein the SAH PD is associated with a sigma-delta frequency synthesizer comprising a type-I phase locked loop (PLL).

4. The method according to claim 1 wherein the SAH PD is associated with a sigma-delta frequency synthesizer comprising:
a temperature compensated crystal oscillator (TCXO) configured to generate the reference clock edge; and
a voltage controlled oscillator (VCO) associated with a divider element configured to generate the VCO clock edge.

5. A method of clocking a sample-and-hold (SAH) phase detector (PD), the method comprising the steps of:
providing a stable reference clock operational to generate a reference clock edge and a voltage controlled oscillator (VCO) associated with a divider element operational to generate a VCO clock edge;
clocking the SAH PD in response to the reference clock edge and the VCO clock edge such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using a charge pump based PD;
wherein a φr and a φs phase clock, used for clocking the SAH PD, are generated based on a first lead edge of the VCO clock edge and a second lead edge of the reference clock edge, respectively; and φs is for sampling a voltage and holding the voltage on a capacitor circuit; and φr is for discharging the capacitor circuit so the voltage is no longer held.

6. The method according to claim 5 wherein the stable reference clock comprises a temperature compensated crystal oscillator (TCXO).

7. A method of clocking a sample-and-hold (SAH) phase detector (PD), the method comprising the steps of:
providing a stable reference clock operational to generate a reference clock edge and a voltage controlled oscillator (VCO) associated with a divider element operational to generate a VCO clock edge; and
clocking the SAH PD in response to the reference clock edge and the VCO clock edge such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using a charge pump based PD;

and wherein the step of clocking the SAH PD comprises the steps of:
precharging a ramp node to a supply voltage in response to a first clock;
discharging the precharged ramp node via a linear current source in response to a second clock; and
sampling the discharging precharged ramp node in response to a third clock such that the ramp node is maintained at a desired output voltage level during the third clock, wherein the second clock commences in response to the reference clock edge.

8. The method according to claim 7 wherein the stable reference clock comprises a temperature compensated crystal oscillator (TCXO).

9. A sigma-delta frequency synthesizer comprising:
a stable reference clock operational to generate a reference clock edge;
a voltage controlled oscillator (VCO) responsive to a divider element to generate a VCO clock edge; and
a sample-and-hold (SAH) phase detector (PD) responsive to the reference clock edge and the VCO clock edge to hold a sampling node voltage at a desired output voltage level during a sampling pulse such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using a charge pump based PD; and
wherein the SAH PD comprises a switched capacitor circuit operational to provide one additional phased locked loop (PLL) pole due to charge sharing between two SAH PD capacitors.

10. The sigma-delta frequency synthesizer according to claim 9 further comprising a sigma-delta modulator operational to randomly change a divider ratio associated with the divider element to generate a fractional channel for the divider element.

11. The sigma-delta frequency synthesizer according to claim 9, wherein the stable reference clock comprises a temperature compensated crystal oscillator (TCXO).

12. The sigma-delta frequency synthesizer according to claim 9, wherein the additional PLL pole is represented by (Ci+Cs/2)/CsFref, where Ci and Cs are capacitive elements associated with the SAH PD and Fref is the reference clock.

13. The sigma-delta frequency synthesizer according to claim 9, wherein the SAH PD further comprises an RC ladder network filter driven directly by the switched capacitor circuit to provide a desired increased level of filtering.

14. A sigma-delta frequency synthesizer comprising:
a stable reference clock operational to generate a reference clock edge:
a voltage controlled oscillator (VCO) responsive to a divider element to generate a VCO clock edge; and
a sample-and-hold (SAH) phase detector (PD) responsive to the reference clock edge and the VCO clock edge to hold a sampling node voltage at a desired output voltage level during a sampling pulse such that quantization noise folding associated with the SAH PD is substantially reduced below levels achievable using a charge pump based PD, wherein the SAH PD comprises:
a sampling node;
a linear current source;
a precharging clock circuit operational to precharge the sampling node to a supply voltage level;
a discharging clock circuit operational to discharge the precharged sampling node via the linear current source; and
a sampling clock circuit operational to sample the discharging precharged sampling node such that the sampling node is maintained at a desired output voltage level during a sampling clock period, wherein the precharging clock commences in response to the VCO clock edge, and further wherein the sampling clock period commences in response to the reference clock edge.

15. The sigma-delta frequency synthesizer according to claim 14 further comprising a sigma-delta modulator operational to randomly change a divider ratio associated with the divider element to generate a fractional channel for the divider element.

16. The sigma-delta frequency synthesizer according to claim 14, wherein the stable reference clock comprises a temperature compensated crystal oscillator (TCXO).