Is there an oscillator design whose output can be varied across a wide range of frequencies, using a tuning capacitor which has a wide range of capacitance?
It does not have to be stable in its output, just enough that I can read the output frequency.
Background: I plan to measure soil moisture levels using a small piece of insulated PCB as a sensor (approx. 10pF in air, going up to 800pF in water) and as a tuning capacitor. I would like the oscillator frequency output to vary between 200MHz (in air) down to about 100MHz (in water). These frequencies are needed to overcome the effect of minerals in the soil.
I tried to use a Clapp-Gouriet oscillator (2017-06-20_J113.asc) which worked on breadboard (much lower frequency than expected because of the breadboard). But when I made a custom PCB,I found that with a small sensor the oscillations died out as I dipped the sensor in water.
I have also tried a different design (simulation only), but cannot decrease the values of the capacitive voltage divider far enough to achieve a high frequency.
It does not have to be stable in its output, just enough that I can read the output frequency.
Background: I plan to measure soil moisture levels using a small piece of insulated PCB as a sensor (approx. 10pF in air, going up to 800pF in water) and as a tuning capacitor. I would like the oscillator frequency output to vary between 200MHz (in air) down to about 100MHz (in water). These frequencies are needed to overcome the effect of minerals in the soil.
I tried to use a Clapp-Gouriet oscillator (2017-06-20_J113.asc) which worked on breadboard (much lower frequency than expected because of the breadboard). But when I made a custom PCB,I found that with a small sensor the oscillations died out as I dipped the sensor in water.
I have also tried a different design (simulation only), but cannot decrease the values of the capacitive voltage divider far enough to achieve a high frequency.
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