I need to coarse control the frequency of oscillation on a parallel resonant tank. The frequency band is in the 500kHz-1Mhz range, so the small capacitance of varactors doesn't suit the task.
Another way of doing it is by switching a capacitor branch with a MOSFET
The problem is that the tank is floating, thus the source is not fixed biased and that makes it difficult to control the conduction on the MOSFET.
I've not considered adding a PLL or another stage because it's the coil where I need the output to be and the current in the tank is already pretty high.
A second problem is that even if I can switch the capacitor on and off, the amplitude of oscillation will vary according to the impedance change, and I need to get as stable amplitude as possible.
¿Are there any solutions for this specific arrangement or is it better to use a grounded tank so I can switch the then properly biased MOSFET?
Thanks in advance!
- Note the values in the schematic are somewhat random