AM and FM
A low-frequency audio signal can't simply be broadcast through the air by itself — it needs to be carried by a high-frequency wave suited to radio transmission. Modulation is the process of impressing information onto that carrier wave, and the two classic schemes for doing it, amplitude and frequency modulation, close out this module by tying the whole idea of communication systems back to a single pair of waveforms.
Carrier and information signals
Two distinct signals are involved in any modulated transmission.
- The carrier signal is a high-frequency sinusoidal wave, generated purely so it can be transmitted efficiently through the chosen medium — for example, a specific radio-frequency oscillation chosen to suit a broadcast band. On its own, an unmodulated carrier has constant amplitude and constant frequency, and carries no information whatsoever.
- The information signal (sometimes called the modulating signal) is the original, typically much lower-frequency signal that actually needs to be communicated — an audio waveform, for instance.
Modulation is the process of varying some property of the carrier — its amplitude or its frequency — in step with the instantaneous value of the information signal, so that the information is impressed onto the carrier and can travel with it through the transmission medium.
Amplitude modulation (AM)
In amplitude modulation, the carrier's amplitude is varied in proportion to the instantaneous value of the information signal, while the carrier's frequency is held constant throughout. The resulting waveform looks like a rapidly-oscillating carrier wave whose overall size — its envelope — swells and shrinks, tracing out the shape of the slower information signal.
Reading such a trace: the carrier frequency is found by counting the rapid oscillations within the envelope (the fast wiggles), while the information frequency is found from how quickly the overall envelope itself rises and falls — a much slower repeating pattern layered on top of the fast carrier oscillations.
Frequency modulation (FM)
In frequency modulation, the carrier's frequency is varied in step with the instantaneous value of the information signal, while the carrier's amplitude is held constant throughout. The resulting waveform maintains a constant overall height, but its cycles visibly bunch closer together when the information signal is large, and spread further apart when the information signal is small (or negative).
Reading such a trace: the carrier frequency corresponds to the average spacing of the cycles, while the information frequency is read from how quickly the bunching-and-spreading pattern itself repeats — again, a slower pattern superimposed on the faster carrier oscillation, but expressed through spacing rather than amplitude.
Bandwidth of AM and FM signals
Modulating a carrier doesn't just shift its frequency — it spreads the transmitted signal across a whole band of frequencies around the carrier, called the bandwidth. Both schemes have a named formula for this bandwidth.
Comparing the two results shows a genuine practical trade-off: for a comparable information signal, FM generally requires a considerably wider bandwidth than AM, because the frequency deviation term $\Delta f$ adds directly to the bandwidth in a way that AM's formula simply doesn't include.
Bandwidth and channel data capacity
A wider bandwidth generally allows a higher data-carrying capacity: more frequency space means more information can be encoded and transmitted per unit time. This is exactly why FM broadcasts are typically capable of higher-fidelity audio than AM broadcasts — FM's wider bandwidth per station allows it to carry a richer, more detailed audio signal, at the practical cost of using up more of the available frequency spectrum per station than an equivalent AM broadcast would.
It's worth noting that these amplitude- and frequency-modulation principles apply regardless of whether the original information signal was analogue to begin with, or had already been converted into a digital form — as covered in the analogue-to-digital conversion and pulse code modulation content earlier in this module. Whether an analogue voltage or a stream of digital pulses is being modulated onto the carrier, the basic idea of varying the carrier's amplitude or frequency in step with the information signal doesn't change; only what's doing the modulating changes.
AM varies the carrier's amplitude and keeps frequency constant; bandwidth $=2f_M$. FM varies the carrier's frequency and keeps amplitude constant; bandwidth $=2(\Delta f + f_M)$. FM's bandwidth is generally larger because of the added deviation term $\Delta f$, and that extra bandwidth is exactly what gives FM its typically higher audio quality.
Key takeaways
- The carrier is a high-frequency wave carrying no information alone; the information signal is the original, typically lower-frequency signal to be communicated.
- In AM, the carrier's amplitude varies with the information signal while frequency stays constant, producing an envelope that traces the information signal's shape.
- In FM, the carrier's frequency varies with the information signal while amplitude stays constant, producing cycles that bunch and spread.
- On a trace, carrier frequency is read from the fast oscillation and information frequency from the slower envelope (AM) or bunching pattern (FM).
- AM bandwidth $=2f_M$; FM bandwidth $=2(\Delta f+f_M)$, where $f_M$ is the maximum modulating frequency and $\Delta f$ is the frequency deviation.
- FM typically needs a wider bandwidth than AM, but that wider bandwidth generally supports a higher data/information-carrying capacity and better fidelity.
- These modulation principles apply the same way whether the original information was analogue or already digitised.
- Across this whole module: semiconductor devices provided the physical building blocks, signal processing turned analogue data into robust digital form, resonance and the ideal op-amp gave frequency-selective and amplifying foundations, op-amp circuits built practical amplifiers and converters, digital logic let systems make decisions and count, and this final communications topic shows how all of it combines to actually move a signal from one place to another.