Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I want to build a soil moisture measurement probe based on this idea:

enter image description here

An oscillator feeds a transmission line burried in a soil via a termination resistor R1, XOR gate acts as a phase shift detector - it will be high untill a reflection comes back from the transmission line. More water in soil - higher the dielectric constant, longer it takes for the reflection to come back, higher the voltage after R2C1 filter. I'm going to implement the transmission line as a trace on a PCB, i'm thinking about frequency of 80MHz and transmission line length of ~40cm.

My question is will an oscillator like CB3-3C-80M0000 be able to drive the transmission line or do I need some buffering? If so, what buffer would be suitable for speeds like these?

share|improve this question
To be honest, I don't know. But I assume you know about the oscillator's load capacitance. In which case, I'm not sure how you'd match the transmission line (especially when the purpose is to change the characteristics) and sensor capacitance to that of the oscillator. My best advice would be to buffer it for safety. But then maybe a neat alternative sensing trick would be to measure the oscillator frequency based on water level. of course I have no idea what else affects the oscillator. – lm317 Jan 25 '13 at 1:49

datasheet says output load (CL) up to 30 pF up to 80 MHz, then up to 15 pF. That seems to me a very small capacitance easily achieved with those long traces (40 cm) and a high epsilon-r medium. If you are already using an x-or gate, why not using another one to buffer it for free?

share|improve this answer
yes, I've put a buffer and don't regret about that :) – miceuz Feb 8 '13 at 7:36

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.