RF based soil moisture detection with capacitance based probes

[dr pepper]

Is there a design reason why you mentioned 100Mc's, Vhf sounds really high for this.
Unless the soil has a capacitance in the order of a few pF then it will be difficult to measure the difference in soil moisture as a lot of the Rf would pass through.
Might be worth sticking a capacitance meters probes into some soil samples and finding out what your dealing with then choose the frequency, up to a few mc's its simple to make a sine wave with a 1 transistor xtal osc, then you could make a capacitance bridge with the soil as one of the caps, ideally you'd make the cap in series with the soil the same as the soils capacitance mid range, then you'll get a good measurement voltage or power change.
Instead of a return loss bridge you could use an Rf power detector module, you can get them on ebay cheaply.
Stainless steel is avaliable in different grades, 316 doesnt rust easily, Dc though would generate electrolysis and eat one of the electrodes.
Pcb board track unless made of some exotic material would be at risk of corroding/oxidizing or some way of chemically dissapearing in soil even with being connected to anything, with Rf you could have said probes insulated in plastic, - think I've seen that somewhere.
You considered a Tdr or guided wave radar?

 

[earckens]

Capacitance meters work in the kHz range, for soil moisture I need to be in the 100MHz range, I refer to the links in my first post: this has to do with the sensitivity for soil minerals at lower frequencies (ie minerals skew capacitive measurement results at lower frequencies).
The purpose of the design is to use an oscillator and have a rf-power detector module (AD8307) give the result (details in first post).
I know this is in actual use in commercial soil moisture sensors, so that is why I dig my teeth in this project.
The capacitive sensing probes will not be exposed to soil, hence no oxidation risk.

 

[dr pepper]

Ok then.
I found a 110mc osc with sine wave o/p, a bit more routing might turn up other freq's;
https://www.ebay.ca/itm/1PCS-TCXO-0...843259?hash=item4d5a06497b:g:PNsAAOSwONBZGqId
A mimic or transistor amp ought to give you enough output clout.

 

[earckens]

Thks, but this is a plain crystal? Do you have a design suggestion for a schematic that goes with this component?

 

[earckens]

Same as this? **broken link removed**

 

[ronsimpson]

The divider pin #4 receives either +5V, 0V, or hanging in the air as per the data sheet; but still no correct divisions by 2 or by 4. Any clues maybe?

The demo board works with 11 different parts. There are many different ways to make the board. I see the jumpers for 1/2/4. I want to know if the voltage on the pin is changing when the jumpers are changes. Do not trust the jumpers. Do not trust that the right parts are on the board.

 

[earckens]

The demo board works with 11 different parts. There are many different ways to make the board. I see the jumpers for 1/2/4. I want to know if the voltage on the pin is changing when the jumpers are changes. Do not trust the jumpers. Do not trust that the right parts are on the board.

The pdf is for the demo board, but I have a plain TLC integrated circuit for which I made my own board with 1x 0.1uF cap, 1x 10k and 1x 10k pot.

 

[ronsimpson]

Would it be because of the stray capacitances of the probes, the non-adjusted load impedance and the oscilloscope working on its bordercapacities (the Rigol has been upgraded with new firmware to the 100MHz version) causing any square wave output to be distorted to a smaller PtP level sine wave?

With a 100mhz scope:
It will reproduce a 100mhz sign wave smaller than it is by 2db.
A 200mhz sign wave will be much much smaller.
A 400mhz sign wave will be so small you can't find it.
A 100mhz square wave is a mixture of harmonics that are very fast. Because the scope can not reproduce 1000mhz signals they are removed.
What happens when you take a square wave and remove all the high harmonics? You get a sign wave.
I do not know your scope. Most 100mhz scopes can see 75mhz accurately. At 100mhz the signal will look a little smaller than it really is.

This same things happen with audio amplifiers. "20hz to 20khz -3db" So at 20hz and 20khz the signal is smaller. This is the very limit.

 

[dr pepper]

Nope the osc is a metal can package, a mimic is a wideband Rf amp often used for this kind of thing, a gali55 is the kind of thing I was thinking of.

 

[earckens]

Good post. Thks for this refresher on my student era RF knowledge.
The scope is capable 100MHz max but indeed that is for sine, not square, and with attenuation.
And given I use no BNC nor proper grounding nor ultrashort leads...

 

[earckens]

I did some further research, and considering I did the measurements with 20cm long unshielded leads with no impedance matching or whatever it is even a surprise I measure anything at all. So, 1. the oscillator most probably is working as designed and 2. I need to get proper BNC connectors, RG58 cable and terminators before doing any further measurements.
To be continued (because this still is part of my setup for a RF based capacitive moisture measurement sensor).

 

[earckens]

The LTC6905 oscillator is working; I can adjust it to between 80MHz and to where my scope does not follow anymore, beyond 160MHz. I used 50 ohm termination, 50 ohm coax and less than a few mm of wire between the IC output and the input to its BNC connector.
The dipswitch is to set the division ratio per datasheet.
The supposedly square wave is void of its harmonics, still a nice sort of sine wave to be seen.
Next step now is to build an oscillator loss bridge with a capacitive sensor, and connect this to the AD8307 DC to 500MHz logarithmic amplifier which arrived last week.
First I have to make some sensor with capacitive properties, probably two long thin strips of pcb with waterproof coating.

 

[dr pepper]

I have a chinese cheap component tester that has a freq generator on it not too different to that, I've used it a couple of times its Ok.
Terminations and coax is amuch better approach.
For a one off you can get cheap pre made loss bridges from china.

 

[earckens]

I have a chinese cheap component tester that has a freq generator on it not too different to that, I've used it a couple of times its Ok.
Terminations and coax is amuch better approach.
For a one off you can get cheap pre made loss bridges from china.

Hi, can you give a link to these Chinese loss bridges? Thank you.

 

[dr pepper]

**broken link removed**

 

[earckens]

Thanks but that is not what I can use. I need a symmetric input: https://qsl.net/n9zia/rlb/rlb.png or see my first post.
And I already built the 100MHz oscillator for this circuit; the AD8307 I have, and then a few resistors and ceramic caps.

 

[dr pepper]

Sorry its been a while since that post I forget, old age.
You'll have to build that methinks.
You will most likely be able to measure smaller differences to 50 ohm with that circuit, and I can see for your system that being important.
Also probably better to use some low temp coeffecient resistors too, the only ones I can think of are wirewound, and they aint much use.

 

[earckens]

Some updates on this project progress.
1. Since the oscillator works as desired I now want to engage the detection part: enter AD8307.
2. There are a few commericial sensors available using this technology (http://www.vegetronix.com/Products/VH400/, http://www.decagon.com/en/soils/volumetric-water-content-sensors/10hs-large-volume-vwc/, ..). Research on university level is being conducted on this sensor technology (**broken link removed** etc..). All advanced sensors for soil moisture measurement seem to use RF capacitance probes.

Enter my question: so far I found two types of loss bridges setups are being used, see below; what is the difference in application between both setups ?
And which one would be best suited to measure variations in probe capacitance and hence variations in dissipated power in the sensor in various moisture environmentals?

This is a standard loss bridge setup where the imbalance between the reference and the probe causes an imbalance in signal to the AD8307; it seems to me the issue here is to determine a correct reference value for the reference impedance. Or what is your opinion here?

And here is the circuit used in one of the commercial units ( http://www.tindie.com/products/Power_Modules/fdr-100mhz-plant-soil-sensor-mineral-transparency/ )(R=470R, C=10nF); IC1=AD8307, IC2=LTC6905; the oscillator 100MHz (or 80MHz, depending on the source) is fed into R2:

I think that in the commercial unit the dissipated power is being measured, while in the first loss bridges the variation in impedance between the probe and a reference is measured; however, which method would be more appropriate, and which method would be more sensitive? Or how to qualify the differences between these two circuits?

The probe (antenna) I intend to use in testing would be two 2 mm wide 10cm long copper strips placed alongside each other with a 1mm spacing in between, and insulated from the environment (waterproof sealing).