Switching tiny small capacitors to a VCO

[iig]

Because the varactors in question are inside the VCO IC that I have no way of
changing.

I think he means to use a varactor (or just a basic pin diode) in parallel
with the inductor. Varying the reverse bias will change the capacitance so
you can get 2pF or less, maybe down to 0.5pF depending on parasitics (the
inductor will also add parasitic capacitance).

 

[Joel Kolstad]

Hi Chris,

Which chip are you using? Is it a differential (symmetrical tank)
oscillator or a single-ended one? Is there any deliberate off-chip
capacitance across the inductor that you could absorb into the
off-capacitance of a switched capacitor bank?

ADF4360, differential, and no. The low-capacitance PIN diode idea (from
John Larkin) worked fine, so I'm going to stick with that. In fact, I'll
probably stick two of them in series to get even a litle lower capacitance
(they're 0.2pF each, so it doesn't take many until you're starting to fight
PCB parasitics... as-is I'll probably remove the copper directly under the
diodes).

---Joel