Reading Ronald Quan's nice book 'Build your own transistor radios' inspired me to improve the detection circuitry on the ultrasonic Fabry-Perot (UFP). My first experiments had diode-detected the amplitude of the transmitted ultrasound signal through the semi-transparent mirrors. This approach is fine, though I was worried about the linearity of the diode, and it relates well to the optical case where intensity of transmitted light is usually detected.
However, a defining characteristic of any type of wave is phase and if we purely measure amplitude or intensity we ignore this important information. We can do better by using a frequency mixer to determine both the amplitude and phase, or equivalently the in-phase and quadrature components of the transmitted wave. To see that these measures are equivalent, recall that a sinusoid with a phase shift can be decomposed into a sin and cos with specific amplitudes.
Below are schematic drawings of the two detection approaches and over the next couple of posts I will show how an IQ measurement is implemented in the UFP by using the Tayloe detector, a commonly used detector in software defined radios.