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LT5537 Production
The LT5537 is a wide dynamic range RF/IF detector, operational from below 10MHz to 1000MHz. The lower limit of the operating frequency range can be extended...
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LT5537 on Analog.com

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LT5537 behavior @10.7MHz

		
	
				

			

									
					    RoyKV
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Sep 8, 2024 
				
			
			
		
    
		
		                    	    
    
        	    
    Thread Summary
    
                The user is designing a matching network for the LT5537 at 10.7MHz and requested the S11/impedance data. The support engineer provided the input impedance model as a 1.94K resistor in parallel with a 2.59pF capacitor at 11MHz. The user also sought clarification on the 240Ω termination resistor used in the datasheet example at 200MHz, questioning its effectiveness in achieving maximum power transfer.
                        	    
    
                
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    Category: Datasheet/Specs
    
    		        		            		    
    Product Number:  LT5537  
    
    		        		    		    
    
		    		    
		    			
    
			    
    Hello support team,
    

    I am trying to design a matching network for the LT5537 from input of 330 ohm at 10.7MHz.
    

    Can you please provide data of its S11/impedance around this frequency?
    

    My intention is to use its differential input in order to get maximum sensitivity.
    

    Thanks!
    
			
    
			
									
		                             
                        

    
    
                                            
                 Datasheet/Specs
                
                                                    
                 lt5537
                
                                                    
                 10.7MHz
                
                                                    
                 RF Power Detectors
                
                                                    
                 Logarithmic Detectors
                
                                
            
    
    
    
		
    

				
				        
        		
    
			
						
									
												
			
			
                        
                                    
                        
                                                    
                        
            
			            
						
						
			
			
			
			
		
    

        
        
        
    
					
    
			
    


    
















    
	
    
	
    
					



			


				
	









	
      
	

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							FormerMember
					
      
		
      
		    
		    		    
			
				
									FormerMember
							        
			
			
			
    
            
						
                            
            
            				
            
                on Sep 9, 2024 11:13 PM
            
            
						
			            		
    
		
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    Hi RoyKV,
    

    At 11MHz, the LT5537's input impedance can be modeled as a 1.94K resistor in parallel with a 2.59pF capacitor.
    
			
    

		
			
	      

		
		
	
    
	
    
		
    
			
		
    
	
    
	
    
		
						
						
						
				
				
				
		
		
    
			
		
    
	
    

    


	

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                on Sep 10, 2024 12:06 PM
            
            
												
						in reply to FormerMember
					
										
			            		
      
		
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      Thanks,
      

      I found it difficult to understand the example made in the datasheet on page 9, when a termination resistor of 240Ω was connected in parallel to the AC coupled input of the LT5537 (figure 5).
      

      I understand the purpose of this resistor (matching the input impedance of the IC to the balun output of 200Ω for maximal power transfer).
      

      Here are my problems:
      

      1. Maximum power transfer requires that the load impedance is the complex conjugated of the source impedance. So in our case we need minimal imaginary part of the input impedance.
      

      2. I calculated that for 200MHz (the example frequency), the input impedance of the termination (neglecting C1 & C2) and the IC input is 2*(240Ω/2 || (1.73KΩ||1/(j*2*π*200MHz*1.45pF)) ~ 214Ω -j44Ω, so it is quite far from the expected 200Ω real part of the impedance. Thus a smaller termination resistor could have made a better job.
      

      What mistakes did I do?
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
				
				
				
				
		
		
      
			
		
      
	
      

      

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