Tank circuit oscillator design

[David Halliday]

Hi

I have been using a parallel tank oscillator (LC) circuit for transmitting a 10 khz's signal through water and then measuring the signal of a secondary inductor for measuring the conductivity of the water. But as the temperature of the coil reduces (when placed in water i.e 10C). The inductance changes and thus changes the oscillator frequency.

How can I compensate for the changes in temperature that the coil is seeing and keep the LC oscillator at 10khz ?
I have tried measuring by adjusting the input frequency but this causes measurement error. It would be ideal to keep the LC oscillator at a fixed frequency and amplitude ??

Any help or ideas greatly appreciated

Thankyou Dave

 

[OBW0549]

How can I compensate for the changes in temperature that the coil is seeing and keep the LC oscillator at 10khz ?

First, you would have to accurately characterize the frequency shift over temperature, then arrange to adjust the value of some other component (presumably the capacitor) in such a way that the change in inductance is balanced out to maintain a constant frequency.

Not an easy (or inexpensive) task.

I have tried measuring by adjusting the input frequency but this causes measurement error. It would be ideal to keep the LC oscillator at a fixed frequency and amplitude ??

Instead of using the inductor as a frequency-determining element in an oscillator, generate a 10 kHz signal through some other means (e.g., a 555 timer astable circuit, or a crystal oscillator and divider) and drive the inductor with that signal through some sort of driver circuit that gives a constant output amplitude.

What you are describing is similar, but not identical, to commercial electrodeless conductivity sensors which use a pair of toroidal inductors (one drive coil and one receive coil) coupled to one another by the conductivity of the fluid to be measured. This Rosemount whitepaper gives details; see Fig. 5 on page 3.

I'm not sure what kind of results your method would yield.

 

[duke37]

Why does the frequency matter?
If a transmitter torroid and similar receiver torroid are at the same temperature then they will track each other.