I am currently using the ADF4360-9 in a narrow band FM application (25KHz channel spacing). I am running the VCO at ~320MHz and taking the O/P from the divide by 2 O/P to give operation at ~160MHz. Because it is not possible to select the VCO operating band I am having to measure the Vtune voltage during production testing and reject units which are too close to the band edges (having characterised operation over temperature etc), otherwise we risk the PLL/VCO starting up in a different band and having incorrect modulation levels. Ideally I need a method of setting the Vtune voltage to be mid band during production testing - Any ideas on how this could be achieved would be appreciated.
My current thought is to add a variable capacitor in series with say 2pF across one of the inductors to give an adjustment range of ~1.1pF to 1.67pF, and adding 1.5pF across the other inductor balance this up. Due to height restrictions it is not possible to use a variable inductor.
Is there any reason why this may approach may cause a problem?