# read a frequency of 150 MHz

#### Marwa Tayahi

Joined Apr 16, 2018
19
Hi, I have a capacitive sensor that measures soil moisture using electrodes. the electrodes are part of the reaction chain of a series LC oscillator. during the injection of the electrodes in the ground the capacitance is changed which makes vary the frequency of the oscillator, then the frequency will be transmitted towards arduino (reading of frequency and treatment) then arduino sends the treatment towards a card SD12 via bluetooth. I am a beginner in programming with arduino .. can I read a frequency of 150 MHz or I have to use a frequency divider knowing that the output of the oscillator is sinusoidal. below the diagram of the oscillator with its sinusoidal output

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Joined Mar 10, 2018
4,057
You would have to signal condition it and pre scale it before
you could input tro Arduino for sure.

A number of vendors have amp/pre-scaler parts you can use.
Board layout very critical for handling 150 Mhz. "Google RF layout"
for tips and techniques.

Analog Devices, Richardson RF, Microsemi, ON Semi....

Regards, Dana.

#### Sensacell

Joined Jun 19, 2012
3,180
Why 150 Mhz?

That's a very high frequency for a capacitive sensor?
Why not use a lower frequency where you have none of the nasty RF design problems to contend with?

#### Picbuster

Joined Dec 2, 2013
1,041
First of all define the frequency shift as function of the soil change.
next step is to make an oscillator creating a difference (osc-150 Mhz)
This might deliver a by arduino measureable frequency when not possible then trigger a PLL at the measured frequency.
dividing the frequency is not my first choice.

Picbuster

#### Marwa Tayahi

Joined Apr 16, 2018
19
yes to minimize the effect of the conductivity (ratio conductivity / frequency) of the soil on the measurement, but that's not the problem, my problem is how can I read this frequency on arduino with a frequency divider?

#### jpanhalt

Joined Jan 18, 2008
11,087
I would follow the path outlined by Picbbuster. That is, treat it as an FM signal and demodulate it.

The AAC textbook has a description: https://www.allaboutcircuits.com/te...emodulation/how-to-demodulate-an-fm-waveform/

There are lots of chips to chose from for doing that. Here is one example from TI: http://www.ti.com/lit/an/scha002a/scha002a.pdf Some of that is a bit complex, but go to page 17 to see a simple example.

Curious, why did you pick 150 MHz? Are you transmitting the signal via air or wire?

#### bertus

Joined Apr 5, 2008
22,143

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#### Sensacell

Joined Jun 19, 2012
3,180

#### Kjeldgaard

Joined Apr 7, 2016
476
I still need to answer why it should be around 150 MHz?

Everything becomes cumbersome and quite expensive.

#### jpanhalt

Joined Jan 18, 2008
11,087
If you simply divide down, you are also dividing the signal by the same amount. Wouldn't it be better to subtract the carrier as is done in FM radio?