The term “quadrature” has many meanings, but in the context of DSP and SDR it refers to two waves that are 90 degrees out of phase.
- By being 90 degrees out of phase they become orthogonal, and there’s a lot of cool stuff you can do with orthogonal functions. For the sake of simplicity, we use sine and cosine as our two sine waves that are 90 degrees out of phase.
- We call the cos() the “in phase” component, hence the name I, and the sin() is the 90 degrees out of phase or “quadrature” component, hence Q. Although if you accidentally mix it up and assign Q to the cos() and I to the sin(), it won’t make a difference for most situations.
Using this I and Q approach, we can transmit any magnitude and phase we want, using a circuit that looks something like this:
The important takeaways are that when we add the cos() and sin(), we get another pure sine wave of the same frequency but with a different phase and amplitude. Also, the phase shifts as we slowly remove or add one of the two parts (the amplitude also changes).
The “utility” of this behavior is that we can control the phase and amplitude of a resulting sine wave by adjusting the amplitudes I and Q (we don’t have to adjust the phase of the cosine or sine). For example, we could adjust I and Q in a way that keeps the amplitude constant and makes the phase whatever we want.
- As a transmitter this ability is extremely useful because we know that we need to transmit a sinusoidal signal in order for it to fly through the air as an electromagnetic wave. And it’s much easier to adjust two amplitudes and perform an addition operation compared to adjusting an amplitude and a phase. It also allows us to represent baseband signals more conveniently, keeping them agnostic of the carrier.
https://pysdr.org/content/sampling.html#