Switching tiny small capacitors to a VCO

Joel Kolstad · 2007-01-05T00:06:56+00:00

I have a VCO that using varactor tuning internally along with a "hang inductor on this pin" approach to set the center frequency. In my case, that center frequency is around 400MHz and the inductor turns out to be ~22nH (working backwards from the data sheet, internally the varactors are between 5.17pF and 7.44pF as you change the tuning voltage).I'd like to extend the tuning range of the VCO, and figured that, as the varactors have a range of ~2.27pF, if I could switch in a ~2pF or so capacitor in parallel with the inductor, the tuning range would be extended. I soldered down a 2pF cap and verified that the VCO range does drop as predicted, so the only problem is: How do you switch in something so small as a 2pF cap? Most devices I'm aware of -- MOSFETs, a discrete varactor diode, etc. -- tend to have 10x as much parasitic capacitance and therefore appear as a short to the 2pF cap without providing any significant switching action.Another problem is that the oscillator's output is ~1.5Vpp (centered at ground due to the inductor), so I expect that things like a PIN diode would just self-bias and also turn into a pretty good short.Any ideas?Thanks,---Joel Kolstad

Jan 05, 2007 21 Replies

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iig 19 years ago

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).

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Joel Kolstad 19 years ago

Hi Chris,

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

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