**Application:**I'm using a 40 kHz ultrasonic transducer to measure distance (and later vibrations). The transducer is operated in continuous wave mode and the distance measurement, corresponding to ultrasonic signal phase, manifests itself as a transducer impedance variation. I want to measure 1um displacement, so in terms of 40kHz ultrasonic (in air) this corresponds to 0.05 degrees of phase.

**Measurement:**Using an impedance bridge with a reference, as close as possible to the transducers mean impedance together with an op-amp I am able to generate a signal proportional to +/-180 phase shift. My VNA measurements indicate a relationship of about 0.02 degrees of phase per 1um in my region of operation. With a little headroom, I would like to target a phase measurement capability of 0.01 degrees.

**Method 1:**I've looked at quadrature mixing (output signal with the reference, & reference -90 degrees : sine and cosine). Low pass filtering the results and dividing to get the tan of the phase angle. It seems like the long way around with the need for mixers/dividers/multipliers. Also I'm not quite sure how to generate the cosine signal. I dont fully trust an op-amp integrator. Is there a way to generate perfectly quadrature signals? I realise my measurement accuracy will depend on the accuracy of generated mixing signals.

**Method 2:**Next Ive been looking at finding mixing parts which can integrate whats mentioned in 1. The problem is that an overwhelming majority of these types of devices (perhaps all?) are designed for high frequency RF applications. I did find the AD8339 DC to 50 MHz, Quad I/Q Demodulator and Phase Shifter which looks very suitable. Still Im not given much in the datasheet about the phase measurement accuracy.

**Method 3:**I've looked briefly at a PLL solution. Again these PLL parts seem to all be targeting RF applications. At first glance it seems like I would only use the phase detector element, which seems never to have accuracy specified. I guess (not a PLL guru by any means) that the accuracy if the PLL is by virtue of the feedback rather than the highly accurate phase detector. Is there a clever way of putting my transducer inside the loop to generate an accurate phase measurement?

**Method 4:**Go digital, convert signals to square waves using a zero crossing detector and EXOR them and use a high frequency digital clock to count the pulse width.

Four alleys to explore then. I'm wondering if someone has the experience to advise me which one has the most potential. I dont have the resources to explore all four, but my feeling is to keep it analogue, and I have no experience with PLLs. Is there another solution I have not considered.

Thanks