Schmitt Trigger fixed 100Mhz oscilator with DC out

[vitoa]

Hi, I need to develop a circuit that oscilates at fixed 100MHz between points A and B
Points A and B have atached a variable capacitor from 10pF to 500pF.
Capacitor variable need to be proportional to DC out at fixed 100MHz between probe A and B.

Capacitance meter at 100Mhz to DC out proportional.

Check this picture circuit to better understand idea.



Low power alternative nand IC would be great any voltage between 2,7V and 5V and low current consuption.
Could anyone help to choose components value, or if need any crystal to give fixed frequency.

Any similar circuit on this circuit simulator online
www.circuitlab.com

 

[Audioguru]

It looks like you copied my improved Plants Watering Watcher circuit but added a variable capacitor to where the wet soil was. The variable capacitor ruins the PWM output when the dampness of the soil (its resistance) between A and B changes the duty cycle of the PWM then the filtered output on your copy produces a variable DC voltage. Point A in my circuit is about 4kHz which is MUCH lower than the 100MHz you want.

You want to make a capacitance meter? Not with this circuit. This circuit is a resistance meter. Here is my original improved Plants Watering Watcher:

 

[vitoa]

Hi Sir, I found this circuit interesting to test using soil probe variable from 10pF to 500pF. It's not a copy, but a reference. I used it because found on websearchs.. Nice to meet the developer.

My circuit needs to oscillate at 100MHz. I've done some similar concept in the past using 74LCX06 hex inverter and crystal but no success. Even built PCB..

I'm goin to use this probe for sand measure. Please check my capacitance measures.
**broken link removed**

Is possible to run your circuit at 100MHz?
By the way I found your circuit in a chinease website..



You circuit can provide DC out is very nice. I need analog out DC from variable capacitor probe at fixed frequêncy of 1000MHz

Regards

 

[Audioguru]

100MHz is 25 thousand times higher than the 4kHz in my circuit.
Google Images shows thousands of Capacitance Meter Circuits, here is one similar to my circuit: https://www.redcircuits.com/Page144.htm

 

[dick_freebird]

Schmitt triggers (of the CMOS or TTL logic type) tend
to have fairly sloppy hysteresis bands that vary with
almost everything. So you will not see a stable 100MHz
out of them, if indeed 100MHz can be had at all. And
an op amb / comparator w/ hysteresis based oscillator
is unlikely to come close to 100MHz.

I'd step back and think about the stability and amplitude
requirements (as opposed to wishes and notions) first.
Also whether you really want a square(ish) wave output
which a logic type bang-band oscillator would produce,
with all the harmonics and ground / supply spikes (for
RF applications that mix in the signal, this would be icky).

 

[vitoa]

Thanks for all information

For 100Mhz needs new concept. So need to switch to return loss bridge technic.

Check it here
https://www.edaboard.com/threads/353365/#post1512594

 

[Audioguru]

Why do you need very high accuracy for soil moisture? Do plants have complex brains?
Or do you want to conserve water for plants because you are in a desert?

 

[vitoa]

The ideia is an open source low cost high precison soil sensor for all.
The good ones in market go from $100 - $200 ... So open sorce wins , but needs time and cooperation.

Developing is a challenge, so why not to do it?

I'm developing a precison soil sensor but I'm lazy so I want simple and efficient stuff.
Return loss bridge method is probably the best. LTC6905 (17MHz to 170MhZ Oscillator) 2euros plus AD8307 (logaritmic instrumentational amplifier)
20cents . plus 5 external components R C
PCB soil probe four layer 4euros (cooper strips inside PCB to avoid corrosion, long time potted) , oscillator/detector/analog out ,two layer pcb 1,5euro.

For now about aprox 8euros to have performance of a $100 or $200.
I'm doing some calculations on return loss bridge. Matching impedance to soil probe.

Analog out direct to arduino raspberry esp8266 or any other microcontroller with ADC.



All knowledge is welcome.