Why bother with MOSFETs? You can get varactor diodes used for tuning
AM broadcast receivers with that much capacitance.

What is the capacitance tolerance for such varactors and what is the
temperature coefficient ? These can be quite a problem in any
oscillator circuits.

The tolerance of AM BCB varactors is extremely tight,
since the front end has to track with the LO in the radio.
Certainly far tighter tolerance than unspecified MOSFETs.

The tempco of those varactors is very low. Probably better than
any inductor you are going to resonate it with. In the low
100's of PPM.

To get such huge capacitance, the control voltage must be quite low.

No it's not. The tuning voltage range is at least 10 volts and often
higher. At least as high as the breakdown voltage of the gate
oxide in a MOSFET.

varactor capacitance, thus, you would have to use two pairs of back to
back (4) varactors of 1500 pF each to get a total 1500 pF capacitance.

You can get several thousand pF in one diode from Toko. 1500 pF is
by no means the limit.


.. I guess that the
larger varactors are quite expensive, so this would also limit the
number of units.

No they're not expensive. Consumer electronics manufacturers
buy them by the millions at low prices.

Paul OH3LWR


Rick N6RK