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LTC5598 Last Time Buy
The LTC5598 is a direct I/Q modulator designed for high performance wireless applications, including wireless infrastructure. It allows direct modulation...
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LTC5598 on Analog.com

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Minimum output frequency for LTC5598

		
	
				

			

									
					    mwb
					
							

            
            			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on May 9, 2025 
				
			
			
		
    
		
		                     
				
        		
    
		    
		    		    
    
		      		            		        
    Category: Hardware
    
    		        		            		    
    Product Number:  LTC5598  
    
    		        		    		    
    
		    		    
		    			
    
			    
    We currently use the LTC5598 for 5MHz-100MHz operation. We actually use it down to 2MHz but not regularly but I am looking to make 2MHz the bottom limit for our application. The chip's minimum frequency is specified in the datasheet to be 5MHz. The issue with going down to 2MHz is there is another 8dB loss from 5MHz to 2MHz. This is fine for us as we can calibrate it out, we have a variable gain block before the mixer. My question is: aside from the loss, are there any major issues with me using the LTC5598 down to 2MHz? I see data in the datasheet for output impedance down to 0.1MHz so I assume it was of interest to others at some point. 
    

    

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      •  Analog Employees  
      
            
						
                            
            
            				
            
                on May 21, 2025 11:04 AM
            
            
						
			            		
      
		
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      As you go down in frequency, other specification also degrade. OIP3 and OP1dB degrade so putting more gain before the IQ Mod will not necessarily get your output power to where you want it to be (this depends on how close your output power is to the specified OP1dB level). Noise floor also degrades and that cannot be calibrated away. The quadrature accuracy of the LO phase splitter (i.e the image rejection spec) also degrades but that can be calibrated out if your signal is  narrow-band. 
      

      So if you can live with the gain roll-off, degraded noise, p1dB and OIP3 and can calibrate for the degraded image rejection (i.e. adjust the relative amplitude and phase of the I and Q baseband signals), the part will work ok at 2 MHz.  
      
			
      

		
			
	      

		
		
	
      
	
      
		
      
			
		
      
	
      
	
      
		
						
						
						
				
				
				
		
		
      
			
		
      
	
      

      

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