PPM

Send precise timing signals for optical fiber communications or radio control. Position modulation avoids amplitude distortion over extremely long paths.

Core Concept

Pulse position modulation keeps both the pulse amplitude and the pulse width absolutely constant. The message signal simply shifts the exact position of the pulse within its allocated time slot. A higher voltage delays the pulse further.

Circuits often generate PPM by triggering a monostable multivibrator from the falling edges of a PWM waveform. Because the pulses are extremely narrow this technique consumes very little average power.

The receiver measures the precise arrival time of each pulse against a master clock. Any amplitude noise gets completely ignored. However timing jitter severely degrades the demodulated audio.

Key Equations

The time shift is linearly proportional to the modulating signal. dt = k * m(t) represents this precise deviation.

The constant k defines the modulation sensitivity in microseconds per Volt. The unmodulated pulse sits exactly in the center of the time frame.

Given:\nMessage voltage m(t) = 3 V\nSensitivity (k) = 2 us/V\n\nWhy this formula:\nThe position shift is the product of voltage and modulator sensitivity.\n\nFormula:\ndt = k * m(t)\n\nSubstitution:\ndt = 2 * 3\n\nCalculation:\ndt = 6\n\nFinal Answer:\n6 us

Exam Tip: PPM requires perfect synchronization between the transmitter and the receiver. Without a shared common clock the absolute position shift is completely meaningless.

Key Properties

• Average transmitted power remains perfectly constant regardless of signal amplitude.
• It exhibits excellent immunity to amplitude variations and non-linear distortion.
• Receiver complexity is significantly higher due to strict synchronization demands.
• It operates optimally in optical links where peak power is limited.
• Bandwidth requirements are massive due to the very narrow pulse widths.

Quick Revision

• Amplitude and width are fixed.
• Only timing carries information.
• Constant power transmission.
• Excellent noise immunity.
• Requires complex clock recovery.
• Exam trap: Thinking PPM and PWM have the same power efficiency. PPM is vastly superior for saving power because the constant pulses are extremely narrow.