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?