ASK, FSK and PSK modulation

These are the basic digital-to-analog modulation schemes: the binary data changes one parameter (amplitude, frequency or phase) of a sine carrier.

ASK (Amplitude Shift Keying)

• Expression: s(t) = A·m(t)·cos(2πfct), with m(t) = 1 for bit 1 and 0 for bit 0 (on-off keying).
• Generation: a product modulator (multiplier), or a switch that passes the carrier when the bit is 1.
• It is sensitive to noise; bandwidth B = (1 + r)·Rb.

FSK (Frequency Shift Keying)

• Expression: s(t) = A cos(2πf₁t) for bit 1, A cos(2πf₂t) for bit 0.
• Generation: two oscillators (f₁, f₂) with an electronic switch driven by the data, or a voltage-controlled oscillator (VCO) driven by the bit stream.
• Better noise immunity than ASK; needs more bandwidth, B = (1 + d)·Rb + (f₂ - f₁), where d is between 0 and 1.

PSK (Phase Shift Keying)

• Expression (BPSK): s(t) = +A cos(2πfct) for bit 1 and s(t) = -A cos(2πfct) = A cos(2πfct + π) for bit 0 (phase 0° or 180°).
• Generation: a balanced modulator (multiplier) with the carrier and the bipolar data signal (+1/-1).
• Best noise performance of the three; needs a coherent receiver (carrier recovery). QPSK sends 2 bits per symbol.

Example: data 1 0 1 1 0

Data :  1      0      1      1      0
ASK  : /\/\/\ ______ /\/\/\ /\/\/\ ______
FSK  : /\/\/\ /-\_/-\_ /\/\/\ /\/\/\ /-\_/-\_
PSK  : /\/\/\ \/\/\/ /\/\/\ /\/\/\ \/\/\/
(FSK: more cycles for 1; PSK: 180-degree flip for 0)

Comparison