A low cost, printed microwave based level sensor with integrated oscillator readout circuitry

This paper presents an extremely low cost, tube conformable, printed T-resonator based microwave level sensor, whose resonance frequency shifts by changing the level of fluids inside the tube. Printed T-resonator forms the frequency selective element of the tunable oscillator. Unlike typical band-pass resonators, T-resonator has a band-notch characteristics because of which it has been integrated with an unstable amplifying unit having negative resistance in the desired frequency range. Magnitude and phase of input reflection coefficient of the transistor has been optimized over the desired frequency range. Phase flattening technique has been introduced to maximize the frequency shift of the oscillator. With the help of this technique, we were able to enhance the percentage tuning of the oscillator manifolds which resulted into a level sensor with higher sensitivity. The interface level of fluids (oil and water in our case) causes a relative change in oscillation frequency by more than 50% compared to maximum frequency shift of 8% reported earlier with dielectric tunable oscillators.