Oscillators for soil moisture sensing

[raiffsf]

Hi,

I'm currently working on a soil moisture sensor based on capacitance.
Im looking for a circuit that can provide high excitation frequency (70MHz +) to the sensing capacitor.
I suppose frequency stability is important for that kind of measurement.

I'm studying some known LC Oscillators (Colpitts, Clapp).
And some ICs from Analog Devices and Texas Instruments like (AD5933 -> 100kHz, FDC2112-Q1 -> 10MHz) which send low frequencies to the sensor
However, those options don't quite help me achieve frequency stability (for better accuracy) and high frequency excitation signals (to compensate measurement erros intrinsic to the soil).
Can someone help me with ideas?
Is there a way to use crystal oscillators for this (a changing capacitive load)?

Thanks in advance for any help given !

 

[barry]

A crystal oscillator would certainly give you a stable output, but I suspect you are intending to use the capacitive sensor to change the oscillator frequency. Is that correct? Crystal oscillators are stable BECAUSE they don't stray far from their nominal frequency. You can definitely "pull" the frequency of a crystal oscillator, but not a lot.

Perhaps you can use a stable fixed oscillator and measure the reactance of the capacitance? Without knowing more about your application, it's hard to say anything. What is the nominal capacitance? How much does it vary? etc.

 

[raiffsf]

A crystal oscillator would certainly give you a stable output, but I suspect you are intending to use the capacitive sensor to change the oscillator frequency. Is that correct? Crystal oscillators are stable BECAUSE they don't stray far from their nominal frequency. You can definitely "pull" the frequency of a crystal oscillator, but not a lot.

Perhaps you can use a stable fixed oscillator and measure the reactance of the capacitance? Without knowing more about your application, it's hard to say anything. What is the nominal capacitance? How much does it vary? etc.

Thank you for your response.

I basically want to measure capacitance or impedance from a self-made soil moisture sensor (in the range of ~10pf to 300pf) and in order to do so my research led me to the conclusion that I need high excitation frequencies (tens of MHz). Do you know a circuit topology from which I can achieve this goal? It doesnt need to be a fixed frequency, if I can reliably measure capacitive reactance or capacitance on a high frequency range, I'm perfectly fine with it.

Thank you again for your time.

 

[KlausST]

Hi,

Why no RC oscillator, where the "soil capacitance" is part of the "C".
This gives an output frequency depending on soil capacitance. Frequency is easy and safe to transmit and easy to measure.

Klaus

 

[raiffsf]

Hi,

Why no RC oscillator, where the "soil capacitance" is part of the "C".
This gives an output frequency depending on soil capacitance. Frequency is easy and safe to transmit and easy to measure.

Klaus

Thank you for your response.

I can try that.
However, the only relationship between Capacitance and Frequency that I know of is from reactance. And to measure reactance I need to measure high frequencies voltages, currents and phases. Is there another way around this? To get from a frequencies do capacitances?

Thanks again for your time !

 

[barry]

Like Klaus said, an RC oscillator should work. What is your ‘frequency stability’ requirement? All you really need to do is measure the frequency out of the soil and compare that to the frequency in the soil. There won’t be any time or temperature stability issue.

 

[raiffsf]

Like Klaus said, an RC oscillator should work. What is your ‘frequency stability’ requirement? All you really need to do is measure the frequency out of the soil and compare that to the frequency in the soil. There won’t be any time or temperature stability issue.

I'm sorry, I didn't mean it as a requirement. I'm actually still assessing the issue.
Interesting idea.
I'm reading about frequency counters now.
But how do I convert excitation and resulting frequencies into capacitance values?
I know reactance gives me the relationship, but then I got to measure reactance in high frequencies, which is not so simple.

 

[KlausST]

Hi,

for an RC oscillator the output frequency is proportional to 1/(R x C).

The higner the C the lower the frequency.
in other words:
C is proportional to period time.

For sure there will be some influence:
* circuit
* stray capacitance
* stray inductance
* thermal drift (could be cancelled out according Barry´s idea)

***
I usually use just a comparator with push-pull-output, three equal resitors (bias and hysteresis) and the "R" and "C" to determine the frequency.
.. and like always: the "C" for power supply decoupling.
Simple and fairly stable over time, temperature and over supply voltage.


I recommend to use a good CMOS rail-to-rail Comparator with push-pull-output.
Not an OPAMP like the LM124. .. it will degrade performance

Klaus

 

[raiffsf]

Hi,

for an RC oscillator the output frequency is proportional to 1/(R x C).

The higner the C the lower the frequency.
in other words:
C is proportional to period time.

For sure there will be some influence:
* circuit
* stray capacitance
* stray inductance
* thermal drift (could be cancelled out according Barry´s idea)

***
I usually use just a comparator with push-pull-output, three equal resitors (bias and hysteresis) and the "R" and "C" to determine the frequency.
.. and like always: the "C" for power supply decoupling.
Simple and fairly stable over time, temperature and over supply voltage.


I recommend to use a good CMOS rail-to-rail Comparator with push-pull-output.
Not an OPAMP like the LM124. .. it will degrade performance

Klaus

Oh, I get it. I was making things harder than they actually are haha.
I'm going to try it and see the results from diferente perspectives, such as temperature, accuracy drift and non linearities (resistance and capacitance behave differently at high frequencies). Supply decoupling is a must.

Thank you all for the help. If someone wants to know how the project is progressing, you can send me a private message and I tell all about it.