Usually I/Q sampling is performed on two signals with a 90° phase shift using separate ADCs. Suppose I have only a single (fast) ADC available to perform I/Q sampling, which approaches do exist? Is this an entirely stupid idea?
A bit of background: I've been toying with the idea of building an amateur radio HF receiver. Yesterday the Raspberry Pi Pico microcontroller board was released. However I was disappointed to learn that it has only a single 12 bit, 500kS/s ADC, making it impossible to implement a conventional I/Q baseband receiver with dual ADCs.
There are two approaches I found:
- Connect the in-phase and quadrature components to two different pins. Alternate between the components of the signal in the ADC. Then interpolate the signal to remove the phase shift between the samples.
- Mix the signal with a higher intermediate frequency and filter it in such a way that successive samples are equivalent to the I, Q, -I, -Q components of the signal at the desired frequency. See the drawing and explanation on the first page of this paper: "Ziomek, C., & Corredoura, P. (1995, May). Digital i/q demodulator. In Proceedings Particle Accelerator Conference (Vol. 4, pp. 2663-2665). IEEE." (PDF)