In-phase and quadrature components
The two amplitude-modulated sinusoids are known as the in-phase (I) and quadrature (Q) components, which describes their relationships with the amplitude- and phase-modulated carrier.
In an angle modulation application, with carrier frequency f, φ is also a time-variant function, giving:[1]: eqs.
But when A(t) and φ(t) are slowly varying functions compared to 2πft, the assumption of orthogonality is a common one.
A stream of information about how to amplitude-modulate the I and Q phases of a sine wave is known as the I/Q data.
[6] By just amplitude-modulating these two 90°-out-of-phase sine waves and adding them, it is possible to produce the effect of arbitrarily modulating some carrier: amplitude and phase.
So I/Q data is a complete representation of how a carrier is modulated: amplitude, phase and frequency.
Some sources treat I/Q as a complex number;[1] with the I and Q components corresponding to the real and imaginary parts.
Others treat it as distinct pairs of values, as a 2D vector, or as separate streams.
The data rate of I/Q is largely independent to the frequency of the signal being modulated.
Designs such as the Digital down converter allow the input signal to be represented as streams of IQ data, likely for further processing and symbol extraction in a DSP.
[3] Since I/Q allows the representation of the modulation separate to the actual carrier frequency, it is possible to represent a capture of all the radio traffic in some RF band or section thereof, with a reasonable amount of data, irrespective of the frequency being monitored.
[8] And similarly a vector signal analyser can provide a stream of I/Q data in its output.
The term alternating current applies to a voltage vs. time function that is sinusoidal with a frequency f. When it is applied to a typical (linear time-invariant) circuit or device, it causes a current that is also sinusoidal.
The input sinusoidal voltage is usually defined to have zero phase, meaning that it is arbitrarily chosen as a convenient time reference.
When φ happens to be such that the in-phase component is zero, the current and voltage sinusoids are said to be in quadrature, which means they are orthogonal to each other.
The phase modulation ( φ ( t ), not shown) is a non-linearly increasing function from 0 to π /2 over the interval 0 < t < 16. The two amplitude-modulated components are known as the in-phase component (I, thin blue, decreasing) and the quadrature component (Q, thin red, increasing).
LO is the local oscillator - the carrier sine wave being modulated
I(t) and Q(t) are the time-series data for the in-phase and quadrature components.
S is the signal