# termination of a parallel wire differential transmission line as delay line on variable dielectric constant medium

Well this has quite a long title, lets explain the situation I'm currently working on the design of a sensor to measure the dielectric permittivity of a liquid medium, the idea is to use a ring oscillator and a delay line (the mentioned transmission line)

problems arise however due to the transmission line effects, the bandwidth of the drivers is around 750mhz (the actual oscillation frequency should be around 70-150M) and the separation between the conductors in the line must be reasonably high so as the fields to propagate outside the pcb and actually change the propagation time, in turn changing the frequency of the oscillator

the problem in question is that the impedance of the given wave guide actually changes when the medium changes, so the termination will be only suitable for a given range of dielectric permittivity, then it will start bouncing significantly, that wouldn't be much of an issue except that if the signal bounces from the tx side into rx with enough strength it can flip the inverter and the frequency would not be linear with propagation time anymore

so the question would be, is there some way of terminating the line that somehow compensates for this?

and also what about the transition zone from narrow to wide wave guide? there is not enough space to make a long tapered adapter, what method do you suggest to minimize reflection there?

• Is the method you propose somehow more suitable for your needs compared to using a Colpitts oscillator and detecting change of frequency thus inferring change of permittivity? Commented Jun 18, 2019 at 12:09
• yes, its a ring oscillator essentially, one advantage i see vs a colpitts is that you need no inductors, which saturation and inductance may vary significantly with temperature, also this can be made only with digital circuitry, making it cheaper and possible to implement on an fpga Commented Jun 18, 2019 at 12:12
• Point well made. Commented Jun 18, 2019 at 12:27
• Instead of trying to measure the propagation time through the line, you may want to 'more directly' measure the capacity between two conductors. I.e. create a kind of capacitor from two conductors and an oscillator based on that capacitor. As the dielectric between the conductors changes, the capacity changes and thus the oscillator's frequency. Commented Jun 18, 2019 at 12:28
• indeed jimmy, that is an other approach im pursuing, however not for this design in particular, the idea for that one would be to use a pll an inductor and the sensor element and track resonance, that can also be implemented digitally Commented Jun 18, 2019 at 12:51