VHF oscillator circuit with large frequency range and large change in capacitance

Thread Starter

talikarng

Joined May 27, 2017
20
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.
 

Attachments

Dodgydave

Joined Jun 22, 2012
8,677
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&source=web&rct=j&url=http://pdf.datasheetcatalog.com/datasheet/RFMD/RF2506.pdf&ved=0ahUKEwjr29TD9szWAhUGAcAKHTnpAYwQFggqMAA&usg=AOvVaw0QSgBEZfkMH1Piul0taWL7
 

SLK001

Joined Nov 29, 2011
1,543
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.
 

Thread Starter

talikarng

Joined May 27, 2017
20
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).
 

Thread Starter

talikarng

Joined May 27, 2017
20
Thanks all for your thoughts.
I will change the course of my thinking and perhaps use the capacitor in a low pass filter instead.
 

Bordodynov

Joined May 20, 2015
2,469
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.
 
Top