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AD9516-0 Recommended for New Designs
The AD9516-0 provides a multi-output clock distribution function with subpicosecond jitter performance, along with an on-chip PLL and VCO. The on-chip...
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AD9516-0 on Analog.com

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AD9516-0 with passive loop filter and external VCO



		

			

									
					    giuliano.gabrielli
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 30, 2016 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Dear All,
    
 
    I need a technical support about the following issue involving the AD9516-0.
    
 
    I am using the AD9516 with external 820-870MHz VCO and passive loop filter. The PLL has been programmed to work at 860MHz and in this condition the VCO works at about 3V of tuning voltage.
    
 
    Internally the 860MHz is divided by 4, downto 215MHz, and distributed to 3 outputs.
    
 
    The Ad9516-0 VCP is set to +5V and CPRSET is set to 5.1Kohm.
    
 
    I meet the following problem:
    
 
    At ambient temperature the PLL is correctly locked, but the PLL loses lock when the temperature drops downto 0°C about. During the unlock condition, measuring the cp output voltage, I cannot measure more than 2.9-3V.
    
 
    At page 19 of the datasheet there is a graph where the behaviour of Charge Pump current is reported, versus voltage: it shows that the AD9516 Charge Pump seems able to supply current (4.5mA when the CPRSET is set to 5.1Kohm) to loop filter until voltage as high as 4.5V. Therefore this voltage should be enough to grant phase lock to the VCO with 3V of tuning voltage.
    
 
    Is this interpretation of the graph  correct? Where is the critical point? Is the PLL AD9516 able to stand tuning voltage as high as 3V or more? Is it possible that there is a problem in the component? Could it be a loop filter issue? Is there an internal register that could help me to solve this critical behaviour?
    
 
    I know that I could solve the problem foreseeing an active loop filter in place of the passive loop filter but I would like to avoid it....
    
 
    Thanks in advance for the support.
    
 
    Giuliano
    
									
			        
		
    

				
				
		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    


	
    
		
	
    















    
	
    
	
    
					



			








	
	
	
	



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													0
							
			
				
									
							        
			
			
			
        
            
			                            
                        
                on Jun 24, 2016 5:51 PM
            
            
					
        
	
        

		
		
        
					
        Hi yi2016wang,
        
 
        The AD9516 looks correctly supplied.
        
 
        I have modified the loop filter setting C1 (capacitance to gnd) at 470pF, and the series C2-R
        (to gnd) at 33nF – 1K. I have set the 10h register to 2C: in this way, cp
        current is at 1.8mA. According to this setup, the loop bandwidth is about
        30KHz.
        
 
        At ambient temperature, the PLL locks correctly at 860MHz (output
        215MHz= 860MHz / 4) (Vtune = 3V about); it also locks to 870MHz (output
        217.5MHz= 870MHz / 4) (Vtune = 4V about). Decreasing temperature downto 0°C the
        PLL loses lock and the Charge Pump output cannot exceed 2.9-3V; actually at
        860MHz setup the PLL is in a condition near to lock; at 870MHz setup the output
        frequency does not exceed 862-863MHz.
        
 
          
        
 
        I have set the 10h register to 0C, 4C, 7C: in this way, cp current is at
        0.6mA to 4.8mA. According to this setup, the loop bandwidth change in the range
        15KHz to 70KHz about with a degraded phase margin. For all the condition I
        observe a correct behavior at ambient temperature but unlock at low
        temperature.
        
 
          
        
 
        I have set the 10h register to 24h: in this way, cp current is set to
        1.8mA and in Pump Up mode. In this condition at all temperature the cp output
        is set to +5V (the same of Vcp) and the VCO output is at 880MHz about.
        Therefore in Pump Up mode the AD9516 works correctly and it is correctly
        powered; in Operative mode (10h register at 2C) the AD9516 does not seem
        working well  decreasing temperature.
        
 
          
        
 
        As I already wrote, what I see is like the cp clips or saturate to a
        voltage that is about 4.2V at ambient temperature and decreases reducing
        temperature, down to 3V at 0°C. This decreasing is gradual in temperature. 
        
 
          
        
 
        I wonder if it is possible in Analog laboratory on a AD9516 with
        external VCO demo board to repeat a temperature test in order to verify if the
        cp can be critical at low temperature. 
        
 
          
        
 
        In any case I am solving the problem changing the VCO with a 850-900MHz
        one, that operates at 860MHz with a Vtune in the range 1.5-2V.
        
 
          
        
 
        Let me know your comments and update.
        
 
          
        
 
        Thanks in advance
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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													0
							
			
				
									
							        
			
			
			
          
            
			                            
                        
                on Jun 24, 2016 5:51 PM
            
            
					
          
	
          

		
		
          
					
          Hi yi2016wang,
          
 
          The AD9516 looks correctly supplied.
          
 
          I have modified the loop filter setting C1 (capacitance to gnd) at 470pF, and the series C2-R
          (to gnd) at 33nF – 1K. I have set the 10h register to 2C: in this way, cp
          current is at 1.8mA. According to this setup, the loop bandwidth is about
          30KHz.
          
 
          At ambient temperature, the PLL locks correctly at 860MHz (output
          215MHz= 860MHz / 4) (Vtune = 3V about); it also locks to 870MHz (output
          217.5MHz= 870MHz / 4) (Vtune = 4V about). Decreasing temperature downto 0°C the
          PLL loses lock and the Charge Pump output cannot exceed 2.9-3V; actually at
          860MHz setup the PLL is in a condition near to lock; at 870MHz setup the output
          frequency does not exceed 862-863MHz.
          
 
            
          
 
          I have set the 10h register to 0C, 4C, 7C: in this way, cp current is at
          0.6mA to 4.8mA. According to this setup, the loop bandwidth change in the range
          15KHz to 70KHz about with a degraded phase margin. For all the condition I
          observe a correct behavior at ambient temperature but unlock at low
          temperature.
          
 
            
          
 
          I have set the 10h register to 24h: in this way, cp current is set to
          1.8mA and in Pump Up mode. In this condition at all temperature the cp output
          is set to +5V (the same of Vcp) and the VCO output is at 880MHz about.
          Therefore in Pump Up mode the AD9516 works correctly and it is correctly
          powered; in Operative mode (10h register at 2C) the AD9516 does not seem
          working well  decreasing temperature.
          
 
            
          
 
          As I already wrote, what I see is like the cp clips or saturate to a
          voltage that is about 4.2V at ambient temperature and decreases reducing
          temperature, down to 3V at 0°C. This decreasing is gradual in temperature. 
          
 
            
          
 
          I wonder if it is possible in Analog laboratory on a AD9516 with
          external VCO demo board to repeat a temperature test in order to verify if the
          cp can be critical at low temperature. 
          
 
            
          
 
          In any case I am solving the problem changing the VCO with a 850-900MHz
          one, that operates at 860MHz with a Vtune in the range 1.5-2V.
          
 
            
          
 
          Let me know your comments and update.
          
 
            
          
 
          Thanks in advance
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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