What is a QAM display

A constellation diagram plots a quadrature amplitude modulation (QAM) signal’s in-phase and quadrature components.

The EE World article “Should I use a spectrum, signal, or vector network analyzer?” in part 3 mentioned that vector-signal analyzers (VSAs) can display modulation-domain and frequency-domain information. Other instruments incorporating digital signal processing (DSP) capabilities, including oscilloscopes, can provide insights into digitally modulated signals.

Could you provide an example?

Yes, we’ll use quadrature amplitude modulation as an example. QAM uses two carriers of equal frequency but phase-shifted by 90°, as shown in Figure 1. One carrier is “in phase,” represented by the letter I, and the other is the “quadrature” carrier, represented by the letter Q. In the figure, we’ll arbitrarily assign the blue cosine wave as I and the red sine wave as Q. A vital characteristic of these I/Q carriers, because of the 90° phase shift, is that they are orthogonal, and each can be modulated independently of the other.

QAM display — Figure 1. A QAM signal includes I (blue) and Q (red) components.

How do we modulate these carriers?

The article “QAM: The Basics and Beyond” from CWNP, a vendor-neutral wireless LAN certification organization, provides a good overview of the modulation process. But from a test-and-measurement perspective, we’re more interested in evaluating an already modulated carrier. Figure 2 assumes the input is an I/Q modulated signal from a remote transmitter.

How do we analyze that signal?

Typically, we’ll have an analog downconversion stage, and then we digitize the signal and send it to two digital mixers. Note that the local oscillators of these mixers are out of phase by 90° or p/2 radians. This phase shift allows us to separate the I and Q components, which, with some additional processing, we can send to the display.

What exactly are we seeing on the display?

We plotted Q vs. I. Note the solid dot in the upper right quadrant. If you look at the tick marks on the axes, you’ll see that the dot resides at I = 1 and Q = 3 in rectangular coordinates. We can also locate the dot in polar coordinates; it has a vector length of 3.16 and an angle of 71.6°.

What about the circles?

The 15 circles represent other possible valid values for I and Q. Altogether, we have 16 valid locations and call this configuration 16-QAM. The complete diagram is a constellation diagram.

Are there other QAM configurations?

Yes. I’m using 16-QAM as an example because it demonstrates the principles involved with diagrams that fit on the screen, but higher QAM levels are common. Wi-Fi 7, for example, uses 4096-QAM—also called 4K QAM.

How is information conveyed using QAM?

The dot and circles in Figure 2 each represent a unique symbol, with all possible 16-QAM symbols shown in Figure 3. Each symbol represents a unique bit sequence, and QAM conveys information by transmitting successive symbols.