RF Modulation & the IQ Plane

A radio wave by itself is just a sine wave — it carries no information. Modulation is the process of stamping information onto that carrier wave by changing one of its three properties: amplitude, frequency, or phase.

Think of it like this: a carrier wave is like a blank piece of paper. Modulation is the act of writing on it. The receiver "reads" the changes to recover the original message.

These three techniques — AM (Amplitude Modulation), FM (Frequency Modulation), and PM (Phase Modulation) — are the building blocks of all radio. Modern digital radios combine amplitude AND phase changes simultaneously to carry far more bits per second. This is where the IQ plane comes in.

Any sine wave can be described as a rotating arrow (phasor) in a 2D plane. The arrow's length is the amplitude. The angle it makes is the phase. As time passes, the arrow spins. If we freeze a snapshot, we see exactly where the arrow is pointing — and that one point tells us everything about the signal at that instant.

The 2D plane that the phasor rotates in has two axes. RF engineers named them I (In-phase) and Q (Quadrature).

AM (left): The dot slides back and forth along the I axis only. The closer to the origin = smaller signal = "0". Further out = bigger signal = "1".

PM/PSK (middle): The dot stays exactly on the circle (fixed amplitude) but rotates to different angles. Each angle = a different symbol. QPSK uses 4 angles → 2 bits per symbol.

QAM (right): The dot can go ANYWHERE in the plane — different amplitudes AND different phases. 16-QAM has 16 dot positions → 4 bits per symbol. 256-QAM has 256 positions → 8 bits per symbol. More dots = more bits per second, but the dots must be further apart or the receiver makes mistakes!

Binary Phase Shift Keying (BPSK) is the simplest digital modulation. It uses just 2 points on the I axis — one for "0" and one for "1". 1 bit per symbol.

Quadrature Phase Shift Keying (QPSK) uses 4 points instead of 2, placing them 90° apart around the circle. Each point now represents 2 bits instead of 1 — doubling the data rate for the same bandwidth. This is the modulation used in GPS, many satellite links, and CDMA cellular.

Quadrature Amplitude Modulation (QAM) uses a grid of dot positions — both amplitude AND phase are varied. The more dots on the grid, the more bits per symbol. But — critically — the dots get closer together, so the radio needs much better signal quality (higher SNR) to reliably distinguish between them.

Each canvas below shows a constellation diagram. Every dot = one symbol = a pattern of bits. Hover or tap to see the bit count and spacing info. Notice how the dots get more crowded as the order goes up — that's why higher QAM needs better SNR!

EVM is the single most important number for measuring how "clean" a modulated signal is. It measures the distance between where the dot should be and where it actually landed — expressed as a percentage of the ideal symbol amplitude.

EVM is degraded by: noise (random scatter), PA nonlinearity (symbols push outward from their ideal positions), phase noise from the PLL/VCO (symbols rotate randomly), and IQ imbalance (constellation becomes distorted). Good RF hardware aims for EVM better than 2% at the transmitter output for high-order modulation.

PAPR stands for Peak-to-Average Power Ratio. It's a measure of how "spiky" a signal is. A simple sine wave has PAPR = 3 dB (the peak is √2 × the RMS). A complex QAM signal can have PAPR of 8–12 dB or more.

Why does this matter? Your power amplifier (PA) must be sized to handle the peak power — but it spends most of its time amplifying the much lower average power. This forces you to "back off" the PA from its maximum efficiency point to avoid clipping the peaks. Higher PAPR = more back-off = less efficient = more heat = shorter battery life.

In a real radio, the I and Q channels are created by two separate hardware paths — two mixers, two filters, two amplifiers. If these paths are not perfectly matched, you get IQ imbalance. Even a tiny mismatch distorts the constellation and degrades performance.

Orthogonal Frequency Division Multiplexing (OFDM) is the modulation technique behind WiFi (802.11a/g/n/ac/ax), 4G LTE, 5G NR, DAB radio, and digital TV. Instead of modulating one carrier, OFDM splits the data across hundreds or thousands of closely-spaced, independent sub-carriers.

The problem OFDM solves: In a multipath environment (indoors, urban), the signal bounces off walls and arrives at the receiver via multiple paths with different delays. This causes some frequencies to be boosted and others to be cancelled — "frequency-selective fading". A single wide carrier gets destroyed. But if you use thousands of narrow sub-carriers, only a few get affected at any moment — the rest are fine.

The "orthogonal" part is the genius: The sub-carriers are spaced exactly so that when you look at sub-carrier #5, all other sub-carriers are at their zero crossing. They overlap in the frequency domain but don't interfere with each other. This is achieved using the Fast Fourier Transform (FFT) — the entire OFDM symbol is generated as one IFFT operation at the transmitter and decoded with one FFT at the receiver.

Adjust the noise level and watch what happens to the constellation. Notice how BPSK survives far more noise than 64-QAM. This is exactly what a real receiver sees.