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ADF4158 Recommended for New Designs
The ADF4158 is a 6.1 GHz, fractional-N frequency synthesizer with modulation and waveform generation capability. It contains a 25-bit fixed modulus, allowing...
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ADF4158 on Analog.com

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ADF4158 PLL strange 'moving' spurs



		

			

									
					    StephaneRey
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Mar 19, 2013 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hi there,
    
 
    I've made a PLL design going from 900 to 1000 Mhz using the ADF4158 and I'm experiencing strange results.
    
 
    The VCO is Minicircuit ROS-1000V (850-1050 Mhz), the loop filter is an active inverting low noise opamp (OP184). Components values from ADIsimPLL
    
 
    When generating 890 000 000 Hz with no modulation, I see my carrier and two side carriers at +/-315 KHz and -65dB approx. 
    
 
    But what is strange, I see two other carriers which are moving quite fastly, symetrically at each side of the main carrier and going into opposite direction like if it was starting from the carrier, and one is going down in frequency whereas the second is going high... and then it's comes back and return and so on...
    
 
    Replacing the REFin from a TCXO (with multiple harmonics) to an externa DDS function generator doesn"t change anything.
    
 
    If I change the frequency setting to for instance 890 000 005 Hz I change the speed of these moving carriers. If  I set 890 001 250, there is no longer these carriers.
    
 
    The power supply is supposed to be quite clean and nothing serious appears on the oscilloscope (noise of about 2-5mV in x1 probe). On each RF device there are 2 X7R caps (10pf, 100nF) following a bead ferrite. PCB layout has proper grounding.
    
 
    On the same board I have a CPU running at 18.668 MHz from a 4.9152 MHz crystal and I'm wondering if these carriers could be a beat frequency from a mixing between any of the CPU clocks and the PLL. Does anyone has heard about a such thing ? 
    
 
    Does anyone has already experienced a such behaviors on these 'moving' spurs. Any idea on how to investigate ? 
    
 
    Thanks
    
 
    Regards
    
 
    Stephane
    
									
			        
		
    

				
						
        

    
    Tags: 
                                            
                 Fractional-N PLL
                
                                                    
                 adf4158
                
                                                    
                 Phase Locked Loop (PLL) Synthesizer & Translation Loop
                
                                                    
                 rf and microwave
                
                                
            
    
    
    

		
    
			
						
									
									
						
						
			
			
			
			
		
    

        
        

					
    
			
    


    
















    
	
    
	
    
					



			








	
	
	
	



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                on Mar 28, 2013 9:15 AM
            
            
					
        
	
        

		
		
        
					
        For the reference I'm using a 10 MHz TCXO from Connor-Winfield (D75A). It has a LVCMOS output with about 2.5ns/v slew rate but harmonics every 10 MHz up to GHz ! The design allows an external reference to be used via a splitter between the TCXO which can be disabled and the external reference.
        
 
        I've then tried to used a DDS based function generator from Stanford Research (DS345) both with sine and squarre output as well as a GPS disciplined OCXO from HP (HP58503B)
        
 
        These 4 configurations have not changed significantly the phase noise and the TCXO (green) with the GPS (red) provides the best values. 
        
 
        The phase noise plot is automated thanks for the GPIB toolkit freeware that just controls the HP8562E. Even without the automated acquisition, I can see the noise on the carrier feet which is far above the HP8562E noise floor. I've no other mean to validate the measurement itself but I'm quite confident on the measurement result.
        
 
        I've checked power supplies on which efforts have been brought to ensure low noise. (ADP3300 and ADP3331 + bead inductors and capacitors on each RF component. Remaining noise is < 5mV.
        
 
        I'm going now to modify the loop filter according your comments.
        
			
        

		
			
	      

		
		
	
        
	
        
		
        
			
		
        
	
        
	
        
		
						
						
				
						
				
				
		
		
        
			
		
        
	
        

        

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                on Mar 28, 2013 9:15 AM
            
            
					
          
	
          

		
		
          
					
          For the reference I'm using a 10 MHz TCXO from Connor-Winfield (D75A). It has a LVCMOS output with about 2.5ns/v slew rate but harmonics every 10 MHz up to GHz ! The design allows an external reference to be used via a splitter between the TCXO which can be disabled and the external reference.
          
 
          I've then tried to used a DDS based function generator from Stanford Research (DS345) both with sine and squarre output as well as a GPS disciplined OCXO from HP (HP58503B)
          
 
          These 4 configurations have not changed significantly the phase noise and the TCXO (green) with the GPS (red) provides the best values. 
          
 
          The phase noise plot is automated thanks for the GPIB toolkit freeware that just controls the HP8562E. Even without the automated acquisition, I can see the noise on the carrier feet which is far above the HP8562E noise floor. I've no other mean to validate the measurement itself but I'm quite confident on the measurement result.
          
 
          I've checked power supplies on which efforts have been brought to ensure low noise. (ADP3300 and ADP3331 + bead inductors and capacitors on each RF component. Remaining noise is < 5mV.
          
 
          I'm going now to modify the loop filter according your comments.
          
			
          

		
			
	      

		
		
	
          
	
          
		
          
			
		
          
	
          
	
          
		
						
						
				
						
				
				
		
		
          
			
		
          
	
          

          

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