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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11FM makes a signal’s instantaneous frequency deviation follow the message; PM makes its phase deviation follow the message. The distinction affects how each waveform behaves, how engineers estimate its bandwidth, and what a receiver must do. Neither method has an automatic bandwidth or noise-performance advantage: those comparisons depend on the modulation settings, signal, and receiver.
What is the difference between FM and PM?
Frequency modulation (FM) and phase modulation (PM) are both forms of angle modulation. A constant-amplitude carrier can be written as s(t) = A cos(2πfct + φ(t)), where the changing phase term determines the carrier’s instantaneous frequency.
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| Comparison | FM | PM |
|---|---|---|
| Quantity controlled directly | The message controls instantaneous frequency deviation. | The message controls phase deviation. |
| Relationship to instantaneous frequency | Frequency deviation follows the message; phase contribution is its integral over time. | Frequency deviation follows the time derivative of the message. |
| Single-tone index | Modulation index β = Δf/fm, where Δf is peak frequency deviation and fm is the tone frequency. | Phase index h depends on peak message amplitude and phase sensitivity. |
| Receiver implementation | Requires angle demodulation; implementation depends on the receiver. | Analog PM is described as less common in part because receiving hardware is more complex (NTIA, 2021 edition revised 2023). |
| Noise comparison | No universal advantage established without matched signal, noise, and receiver conditions. | No universal disadvantage established without matched signal, noise, and receiver conditions. |
Why phase and frequency are connected
Instantaneous frequency is proportional to the time derivative of phase. FM and PM are therefore mathematically related, but they do not make the same quantity track the message directly. A constant message level held over an interval produces a constant frequency offset in FM, so phase ramps steadily. In PM, the held level produces a fixed phase displacement; frequency changes chiefly while the phase is moving.
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How do you calculate FM and PM bandwidth?
Bandwidth is determined by signal and modulation parameters, not by the label FM or PM alone. For an engineering estimate, the National Telecommunications and Information Administration (NTIA) gives these analog necessary-bandwidth expressions in its 2021 edition of the NTIA Manual of Regulations and Procedures for Federal Radio Frequency Management, revised 2023:
| Modulation | Necessary-bandwidth estimate | Parameters |
|---|---|---|
| FM | Bn = 2M + 2KD | M is the highest modulating frequency; D is peak frequency deviation; K is normally 1 in the fundamental Carson form. |
| PM | Bn = 2(h + 1)M | M is the highest modulating frequency; h is the phase modulation index. |
Applying the estimates
For a single-tone FM signal, the modulation index is β = Δf/fm. Use consistent frequency units for Δf, fm, and M. For PM, h is set by peak message amplitude and phase sensitivity; with a multi-frequency message, the frequency deviation produced also depends on how quickly that message changes.
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For example, NTIA lists M up to 15 kHz for broadcast-quality high-fidelity voice and music as a typical engineering category. It is not a universal limit for every broadcast or PM/FM system, and it is not enough by itself to calculate bandwidth: the relevant deviation or phase index is also needed.
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Carson’s rule is an approximate necessary-bandwidth rule, not a hard spectral cutoff. NTIA notes that K = 1 is the fundamental FM form and that another factor may be appropriate in measured cases. Actual occupied or necessary bandwidth depends on the applicable definition, system, filtering, and any subcarriers or composite modulation. State the signal assumptions whenever quoting a calculated value.
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Which has better noise immunity, FM or PM?
There is no supported universal FM-versus-PM noise winner. Both encode information in carrier angle rather than amplitude, and receivers can limit amplitude variation before angle demodulation. That is why FM is often described as resistant to amplitude noise; it does not prove that FM always outperforms PM.
A meaningful comparison needs the message spectrum, deviation or phase index, received carrier-to-noise ratio, receiver design and bandwidth, and a defined performance measure such as recovered-audio signal-to-noise ratio or bit error rate. The noise mechanism matters too: random additive noise, amplitude disturbances, adjacent-channel interference, and distortion from a nonideal demodulator can have different effects. NTIA documents PM receiver complexity, but the cited material does not provide a controlled numerical FM-versus-PM noise comparison.
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Where are FM and PM used?
Analog FM
NTIA’s handbook gives typical highest-modulating-frequency categories for commercial-quality voice, broadcast-quality high-fidelity voice and music, frequency-division multiplex, and standard-definition video links. These are technical examples in a government handbook, not evidence that every category remains common in current deployments. In a particular system, bandwidth and use depend on its specification and operating context.
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Analog PM
NTIA says analog PM is rarely used because receiving hardware is more complex. That is a practical distinction, not a claim that PM cannot carry information or that it is inherently inferior in every system.
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Digital relatives: FSK and PSK
Frequency-shift keying (FSK) and phase-shift keying (PSK) encode digital symbols using discrete frequency or phase states. They are related to frequency and phase modulation, respectively, but they are not synonyms for the analog FM and PM discussed here. The Loyola University Maryland EE 431 course material includes FSK and PSK examples in that digital context.
Quick Recap
How to choose the right comparison
- For waveform behavior, identify whether the message directly controls frequency deviation or phase deviation.
- For bandwidth, compare the actual message bandwidth and deviation or phase index under the same necessary/occupied-bandwidth definition.
- For noise, specify the noise or interference type, receiver, and performance measure rather than relying on a blanket ranking.
- For applications, distinguish analog FM/PM from digital FSK/PSK and check the requirements of the specific system.
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