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ADF4110 RFin A and B

Question asked by eyetube on Dec 7, 2010
Latest reply on Dec 9, 2010 by eyetube

I am designing a frequency synthesizer to operate in two relatively narrow band (8%) ranges, one near 50 MHz and the other near 70 MHz.  Each will have a seperate VCO.  I have a breadboard version working now with an ADF4001.  Performance is satisfactory but I need a smaller step size (5 KHz) than I can get with that part.  I guess that the ADF4110 is the right choice, but it has a low frequency limit of 50 MHz.  Am I cutting it too close?  Is there a better part for this application?

 

Also, I am using a relay in the breadboard to switch the VCO outputs to the RFin A pin.  This works OK, but it occured to me that I could connect the 50 MHz VCO to the RFin A pin and the 70 MHz VCO to the RFin B pin.  Since the unused VCO is shut down when the other is activated, the "unused" pin on the 4110 would be left with a 50 Ohm termination rather than a 100 pF bypass as recommended in the data sheet.  This would eliminate the cost and complexity of a relay or diode switching.  Do you see a problem with this approach?

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