HELP!!! About switched (coarse) tuning VCO

Hi all,
I am designing a VCO with band select function (by switching in/out caps).
But now I meet a problem about Kv linearity?

When the vco operates in lower bands, the Kv is kind of linear over the whole tuning range. But when I switched out caps to let the VCO operate at higher bands, the Kv becomes depending on Vtune and gradually flat out as the Vtune increase. (The curve looks like Transistor I-V curve). Those curves look exactly like Fig.13 in pp.1392 IEEEJSSC Volume: 23 , Issue: 6 , Dec. 1988

Please, could someone give me a hint or tell me any useful paper?

Thanks so much?

Please refer to Samuel M.Plermo 's thesis:
"A multi band ...",1999.I think it really helps.

BR

eric
5/17