Before this:What is a radio wave?
Anatomy of a signal
Key takeaways A real signal is a carrier plus the sidebands that modulation spreads around it; together they occupy a width of spectrum called bandwidth. An SDR shows signals two ways: the spectrum view plots strength vs. frequency right now, while the waterfall plots the same thing over time, scrolling, with brightness for strength. Learning to read these — by a signal’s width, shape, and whether it’s steady or bursty — lets you recognise what’s on the air before decoding it.
Before we get into modulation, it helps to picture what a signal actually looks like on an SDR screen — because that’s how you’ll find and identify everything for the rest of the path.
Carrier and sidebands
A bare carrier — a steady wave at one frequency — appears as a single tall spike. But a carrier with no changes carries no information. The moment you modulate it (vary its amplitude, frequency, or phase), the energy spreads into sidebands on either side of the carrier. The richer or faster the information, the wider those sidebands — and the wider the signal.
So on screen, an idle carrier is a thin spike; an active, modulated signal is a wider shape. That width has a name.
What is bandwidth?
Bandwidth is how much spectrum a signal occupies, in hertz. It’s one of the most important numbers about any signal because it determines how much receiver bandwidth you need to capture the whole thing (a theme that returns in sample rate & Nyquist).
| Signal | Typical bandwidth |
|---|---|
| Narrowband FM voice | ~12.5 kHz |
| P25 / DMR digital voice | ~12.5 kHz |
| FM broadcast station | ~200 kHz |
| Wi-Fi channel | 20–80 MHz |
Wider isn’t “better” — it’s a trade. More bandwidth carries more data but uses more spectrum and demands more of your receiver and CPU.
This is why bandwidth is the first number to know about a signal: it sets how much receiver bandwidth you must capture. To decode a 12.5 kHz P25 channel you only need to filter out ~12.5 kHz around it — but to capture a whole FM broadcast station you need ~200 kHz, sixteen times as much. Capture too little and you clip the signal’s edges and lose data; the bandwidth tells you exactly how wide to open the window.
The spectrum view vs. the waterfall
SDRs give you two windows onto the same data:
- The spectrum view (or “FFT view”) plots signal strength against frequency at this instant. Peaks are signals; the wiggly baseline is the noise floor. It’s a live snapshot — great for how strong and how wide right now.
- The waterfall plots that same spectrum over time, scrolling downward (or up). Each horizontal line is one moment; brightness or colour shows strength. Because it keeps history, the waterfall makes intermittent signals, bursts, and repeating patterns jump out.
Reading signal strength and shape
On the spectrum, height above the noise floor is your SNR at a glance — the taller a peak stands over the baseline, the better it’ll decode. On the waterfall, that’s brightness.
Beyond strength, the shape and behaviour identify a signal:
- Width — narrow voice channel vs. wide broadcast.
- Continuous vs. bursty — a trunked control channel is a steady, fixed-width digital signal; individual voice calls flicker on and off.
- Structure — some signals have a distinctive look (evenly spaced tones, a flat digital “block,” etc.).
With a little practice the waterfall becomes a visual fingerprint: you’ll spot a control channel, a pager burst, or an FM station without decoding anything — exactly the skill you’ll use in finding systems.
Quick check: what does a waterfall show that a plain spectrum view doesn't?
Recap
- A signal is a carrier plus sidebands; their span is its bandwidth.
- Bandwidth sets how much receiver bandwidth you need to capture a signal.
- The spectrum view shows strength vs. frequency now; the waterfall adds time.
- Height/brightness ≈ SNR; width, burstiness, and structure identify the type.
Next: the classic analog ways to put a voice on a carrier — AM, FM, and SSB.
Frequently asked questions
What is the bandwidth of a signal?
Bandwidth is how wide a signal is in frequency — the span of spectrum it occupies, measured in hertz. A narrow FM voice channel might be ~12.5 kHz wide; an FM broadcast station ~200 kHz; Wi-Fi tens of megahertz. Wider signals can carry more data but take up more spectrum and need more receiver bandwidth to capture.
What is the difference between a spectrum view and a waterfall?
A spectrum view plots signal strength against frequency right now — a snapshot, like a row of peaks. A waterfall plots the same information over time, scrolling, so each horizontal line is one moment and brightness shows strength. The spectrum shows the instant; the waterfall shows history, which makes intermittent signals and patterns easy to spot.
What are sidebands?
When you modulate a carrier to add information, the energy spreads out into sidebands on either side of the carrier frequency. The width of those sidebands is essentially the signal’s bandwidth. A bare unmodulated carrier is a single spike; a modulated signal is a wider shape because of its sidebands.
How do I recognise a signal type on a waterfall?
By its shape and behaviour — width, whether it’s continuous or bursty, and any structure. A control channel is usually a steady, fixed-width digital signal; voice calls come and go; FM broadcast is wide and constant. With practice the waterfall becomes a visual fingerprint of what’s on the air.