ASK vs FSK vs PSK: Digital Modulation Techniques Compared

This page explores the differences between ASK (Amplitude Shift Keying), FSK (Frequency Shift Keying), and PSK (Phase Shift Keying) modulation techniques, all of which are fundamental digital modulation methods.

Unlike analog modulation, digital modulation techniques use a digital binary input as the modulating signal, combined with an RF carrier. The output of this process is the modulated signal.

ASK (Amplitude Shift Keying)

ASK, short for Amplitude Shift Keying, is a digital modulation technique where the amplitude of the RF carrier signal is varied according to the baseband digital input signal.

Fig.1 ASK Modulation

As shown in the figure above, a binary ‘1’ is represented by a carrier signal with a certain amplitude, while a binary ‘0’ is represented by a carrier with zero amplitude (i.e., no carrier).

ASK modulation can be represented by the following equations:

• s(t) = A2 * cos(2 * π * fc * t) for Binary Logic-1
• s(t) = A1 * cos(2 * π * fc * t) for Binary Logic-0

Where A2 > A1. This is known as ON-OFF signalling.

The bandwidth requirement for ASK is:

BW = 2/Tb = 2 * Rb

Often, in ASK modulation, binary ‘1’ is represented by a carrier with amplitude A2, and binary ‘0’ is represented by a carrier with amplitude A1, where A2 is greater than A1.

A specific form of ASK, where no carrier is transmitted during the transmission of logic zero, is known as OOK modulation (On-Off Keying modulation), as demonstrated in Figure 1.

• Disadvantages of ASK: High probability of error (Pe) and lower SNR.
• Lowest noise immunity.
• Bandwidth-efficient but has lower power efficiency.

FSK (Frequency Shift Keying)

FSK, short for Frequency Shift Keying, is another digital modulation technique. In FSK, the frequency of the RF carrier is varied according to the baseband digital input signal.

Fig.2 FSK Binary

The figure above illustrates FSK modulation. Binary ‘1’ and ‘0’ are represented by two different carrier frequencies. Binary ‘1’ is represented by a high frequency ‘f1’ and binary ‘0’ is represented by a low frequency ‘f2’.

Binary FSK can be represented by the following equations:

• s(t) = A * cos(2 * π * f1 * t) for Binary 1
• s(t) = A * cos(2 * π * f2 * t) for Binary 0

In FSK modulation, NRZ (Non-Return-to-Zero) signaling is typically used.

The bandwidth requirement for FSK is:

BW = 2 * Rb + (f1 - f2)

• Advantages of FSK: Lower Pe and higher SNR compared to ASK.
• Widely used in modem design and development.
• Increased immunity to noise.
• Disadvantage: Requires a larger bandwidth compared to other modulation types.

To overcome the drawbacks of BFSK (Two-level Binary FSK), multiple FSK modulation techniques with more than two frequencies have been developed. In MFSK (Multiple FSK), more than one bit is represented by each signal element.

PSK (Phase Shift Keying)

PSK, short for Phase Shift Keying, is a digital modulation technique where the phase of the RF carrier is changed based on the digital input.

Fig.3 PSK

The figure above depicts Binary Phase Shift Keying (BPSK), a common type of PSK. Binary ‘1’ is represented by a 180-degree phase shift of the carrier, while binary ‘0’ is represented by a 0-degree phase of the RF carrier.

Binary PSK can be represented by the following equations:

• If s(t) = A * cos(2 * π * fc * t) for Binary 1
• Then s(t) = A * cos(2 * π * fc * t + π) for Binary 0

In PSK modulation, NRZ signaling is used.

The bandwidth requirement for PSK is:

BW = 2 * Rb = 2 * Bit rate

• Advantages of PSK: Lower probability of error and higher SNR compared to ASK.
• Power-efficient.
• Disadvantage: Lower bandwidth efficiency.
• Widely used in wireless transmission.

Variants of basic PSK and ASK modulations include QAM (Quadrature Amplitude Modulation), 16-QAM, 64-QAM, and so on.