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How to shift the CV curve of varactor?

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eric0825

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Dear all:

The tuning voltage of my VCO circuit is from 1V~3V, however, due to the supply voltage limit, tuning voltage ranging from about 0~1.8V is required, how can i do?

Appreciate to ur help!
 

You can't modify the varactor CV curve, but the overall circuit circuit characteristic.
You need to redesign the tuning circuit so the varactor is directly coupled to the LC circuit and recalculate the LC values to cover the domain. You may try to minimize the voltage on the VCO LC tank so the AC voltage on the varactor is much lower than 1V and it wont drive it in conduction. This should solve it, since the highest frequency variation is at low control voltages.

If not, you need a different varactor to provide a broader range. Or use a switching capacitor voltage converter, to obtain 3V. Pay attention to filtering.
 

Sorry, maybe i should modify my title to "how to shift the tuning characteristic of my VCO"

The DC bias at the anode of the varactor is about 1.7V, so while i apply control voltage at cathode, it has to be ranging from 1V~3V to get the linear characteristic, if I can lower the DC voltage at the anode from 1.7V to about 0.8V, the tuning characteristic will "shift" to about 0.1V~2.2V for linear tuning.

However, because of the use of nMOS cross-coupled topology, the DC level at the varactor's anode is so high, is there anyway to solve this problem? I've tried complementary topology, but the phase noise performance is too bad....
 

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