best analog circuit to generate 50 to 70 MHz square wave

Thread Starter

micromet

Joined Sep 10, 2016
9
I need to generate a square wave around 70 megaHz for a capacitance soil moisture sensor I'm working on.
The plan is to run the square wave through a low pass filter, where the soil sensor will act as the capacitor.
Then route the signal through a near unity gain op amp, then to a peak detection circuit. I'm running on a single supply 3.3V DC.
Below is a circuit from this paper
https://www.researchgate.net/public...domain_capacitance_based_soil_moisture_sensor

They use a carrier frequency from the ucontroller, but I want to use much higher freq

Any help to get me started would be greatly appreciated .
upload_2018-8-4_12-37-17.png
 

crutschow

Joined Mar 14, 2008
26,312
A 70 MHz square-wave will radiate a great deal of harmonic EFI, which will not make the FCC happy.
How long are the leads carrying this 70MHz.
Why such a high frequency?
And why a square-wave?
 

olphart

Joined Sep 22, 2012
78
A 70MHz square wave in analog? Why?
This 579-MX575ABC70M0000 from Mouser is $2.50 in stock.
Crutschow's right: shielded cables, impedance matching, termination, all that RF stuff & considerations.
RF is voodoo to me, so Good Hunting... <<<)))
 

Thread Starter

micromet

Joined Sep 10, 2016
9
A 70 MHz square-wave will radiate a great deal of harmonic EFI, which will not make the FCC happy.
How long are the leads carrying this 70MHz.
Why such a high frequency?
And why a square-wave?
Good point on the EFI.
  1. The leads will be < 1 meter and deployed in agricultural areas. The probe will be buried in the soil.
  2. We need something over 50MHZ to reduce sensitivity to salts in the soil - there has been quite a bit of research on this (see link to paper below).
  3. We don't need a square wave - we should be able to use other wave forms.

Here is a paper on the freq effect
Kizito et al., 2008
F. Kizito, et al.Frequency, electrical conductivity and temperature analysis of a low-cost capacitance soil moisture sensor
J. Hydrol., 352 (3–4) (2008), pp. 367-378
 

danadak

Joined Mar 10, 2018
4,057
In the link you psoted I see this commentor -

... The fixed frequency square-wave of the four frequencies (250 kHz, 125 kHz, 83.3 kHz and 62.5 kHz) are transmitted to measure the soil capacitance in which the probe sensors are connected to an Arduino microcontroller and a simple compensation formula has been derived and published by (Oates et al., 2017b). The following compensation formulae were derived: ...
So is it 70 Mhz or 70 Khz you are looking for ?

Is there an accuracy spec for the frequency needed ? Also power
level, impedance to drive (I gather the cap, if so caps ~ value)....

Regards, Dana.
 

Thread Starter

micromet

Joined Sep 10, 2016
9
In the link you psoted I see this commentor -



So is it 70 Mhz or 70 Khz you are looking for ?

Is there an accuracy spec for the frequency needed ? Also power
level, impedance to drive (I gather the cap, if so caps ~ value)....

Regards, Dana.
 

Alec_t

Joined Sep 17, 2013
11,862
1 Schmitt inverter gate +
1 resistor +
1 capacitor

would make a conventional square-wave oscillator (albeit for 70kHz rather than 70MHz).
 

Thread Starter

micromet

Joined Sep 10, 2016
9
Its 70 Mhz, my initial work was done with the PWM on the Arduino Timer 0 pin - up to 250 Khz with the register control.
However, I want to implement a much higher freq to avoid the salinity effects in the soil.

I think the accuracy spec can be quite low given my objectives -
perhaps +/- 5% on the duty cycle - the jitter could be > 1% also.
 

Thread Starter

micromet

Joined Sep 10, 2016
9
Here is a quote from . Kizito, et al.Frequency, electrical conductivity and temperature analysis of a low-cost capacitance soil moisture sensor
J. Hydrol., 352 (3–4) (2008), pp. 367-378

"Using a measurement frequency of 70 MHz, a single calibration curve was determined for a range of mineral soils, independent of soil salinity, suggesting there might be no need for a soil specific calibration."
 
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