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.

 

There won't be a circuit for such a wide range in frequency, you will have to do it in steps, like 100- 130, 130-160, etc...

There is a chip RF2506, that can go from 10 - 1000Mhz, this uses a Varicap diode, but you could omit that an use your capacitor instead, but the capitance range of the diode is 1.8 to 20pF, ...

Datasheet...
https://www.google.co.uk/url?sa=t&s...pAYwQFggqMAA&usg=AOvVaw0QSgBEZfkMH1Piul0taWL7

 

I believe your original problem was that the water was conductive and shorted out the capacitor. While it's true that pure water is non-conductive, "real" water isn't, so your oscillator was shorted out.

 

Widest tuning range for one stage that I have seen that can be made with common parts:
Series-LC-tank VCO breaks tuning-range · records. Louis Vlemincq, Belgacom, Evere, Belgium <= Link

More conventional:
Check out the range of Band 5 in the table of the schematic below.

 

I believe your original problem was that the water was conductive and shorted out the capacitor. While it's true that pure water is non-conductive, "real" water isn't, so your oscillator was shorted out.

I forgot to mention that the sensors are insulated behind a layer of epoxy. (I originally had the same thought as you, so I went and insulated them).

 

Thanks all for your thoughts.
I will change the course of my thinking and perhaps use the capacitor in a low pass filter instead.

 

See

 

View attachment 136297 See

Thanks for that.
Is this a form of Colpitts Oscillator?

 

No. I came up with this scheme myself. I used it for a wireless microphone. Inductance I had a short-cut length of a coaxial cable 10 cm long. The frequency was 100 MHz.