Side-by-side comparison
| Parameter | FM | PM Modulation |
|---|---|---|
| Modulated parameter | Instantaneous frequency: f_i = f_c + k_f × m(t) | Instantaneous phase: φ(t) = k_p × m(t) |
| Modulation index (sinusoidal) | m_f = Δf / f_m (ratio of deviation to message freq) | m_p = k_p × A_m (independent of message frequency) |
| Bandwidth (Carson's rule) | BW = 2(Δf + f_m) = 2 f_m(m_f + 1) | BW = 2(m_p + 1) f_m — same formula but m_p differs |
| Effect of message frequency on index | m_f increases as f_m decreases (Δf fixed) | m_p independent of f_m for single tone |
| Noise immunity | Better for low-frequency audio (commercial FM) | Better noise immunity in digital phase-shift schemes |
| Pre-emphasis/De-emphasis | 75 μs pre-emphasis in broadcast FM (50 μs in India) | Not typically used; phase is inherently differentiated |
| Demodulator | PLL, Foster-Seeley, ratio detector | Phase comparator, Costas loop |
| Typical IC / system | NE566 VCO for FM; commercial FM 88–108 MHz | MC145151 PLL synthesiser; BPSK/QPSK modems |
| Spectral efficiency | Lower for voice — wideband by design | Higher in digital form (QPSK = 2 bits/symbol) |
| Relationship between FM and PM | FM of m(t) = PM of integral of m(t) | PM of m(t) = FM of derivative of m(t) |
Key differences
FM index m_f = Δf/f_m means the index rises as message frequency drops — a 50 Hz bass note with 75 kHz deviation gives m_f = 1500, producing enormous bandwidth compared to a 10 kHz treble note. PM index m_p = k_p × A_m is frequency-independent, making PM closer to digital phase-shift keying used in 4G modems. The mathematical duality is tight: FM of m(t) is identical to PM of ∫m(t) dt, which is why many transmitters integrate the audio before a PM modulator to produce FM. Carson's rule BW = 2(Δf + f_m) applies to both but gives different numbers because the indexes differ.
When to use FM
Use FM when transmitting analog audio over a radio link requiring high noise immunity — commercial VHF broadcast at 88–108 MHz with 75 kHz deviation and 50 μs pre-emphasis is the standard Indian example.
When to use PM Modulation
Use PM (or its digital descendant QPSK) when transmitting digital data where spectral efficiency matters — a QPSK modem encodes 2 bits per symbol and is the basis of UMTS and LTE uplink.
Recommendation
For communication systems exams where the signal is analog audio, choose FM as your working example and use Carson's rule with 75 kHz deviation and 15 kHz audio. Switch to PM conceptually only when digital modulation or PLL synthesis questions appear.
Exam tip: Examiners expect you to calculate bandwidth using Carson's rule for both FM and PM given a sinusoidal message, and to explain why FM index changes with message frequency but PM index does not — this distinction appears in nearly every university question paper.
Interview tip: Placement interviewers at telecom companies (Ericsson, Nokia hiring through campus) ask you to relate FM and PM mathematically — state clearly that FM = PM of integrated signal and derive it briefly on the whiteboard.