Amplitude Modulation vs. Frequency Modulation for Communication

August 23, 2021

Cadence System Analysis

Key Takeaways

Wireless and wired communication with a single carrier frequency use modulation to transfer information.
AM and FM are two common methods of modulation that require similar layout practices to ensure signal integrity.
Other types of modulation include phase modulation and pulse modulation, and some modulation formats combine characteristics of multiple methods.

FM radio

Young people may not remember this thing called FM radio, where you got to listen to the newest pop songs over a blur of static. Before that was AM radio, which was even worse in terms of sound quality. Frequency modulation and amplitude modulation are still with us to this day, but now these modulation methods are used to send digital data at high speeds over a high frequency carrier wave. Technologies like 802.11 standards (a.k.a. WiFi) use amplitude modulation while other radio technologies continue to use digitally modulated frequency modulation.

At the system level, it’s interesting to see the evolution between amplitude modulation vs. frequency modulation as well as the move to all-digital modulation schemes. While these two broad methods of modulation are most well-known among electronics enthusiasts, they are not the most common methods used in wired or wireless communications. Instead, when you peel back industry standards on wireless communication protocols, you’ll find that standardized modulation techniques are digitized multilevel signaling schemes involving either amplitude or phase modulation with time division or frequency division multiplexing.

Amplitude Modulation vs. Frequency Modulation

Modulation refers to the technique of applying variations to some operating characteristic of a sinusoidal signal (amplitude, frequency, or phase). This opens up three possible modulation schemes: amplitude, frequency, and phase modulation. Amplitude modulation (AM) is the earliest form of modulation that was adapted for civilian use, and it remained the dominant form of modulation in civilian radio broadcasting until the late 1970s. After that time, frequency modulation (FM) became dominant and AM radio began its slow decline.

Analog vs. Digital Modulation

In electronics, there are many other types of modulation beyond simple AM and FM modulation. The various acronyms for the range of possible modulation schemes are divided into digital and analog modulation. When most books refer to AM or FM, they are generally referring to analog modulation, where the information-carrying signal is a continuous sinusoid or linear ramp superimposed on a sinusoidal carrier signal. Today, mainstream modulation has gone digital, where signal characteristics are varied between distinct states in order to represent digital data.

The image below shows a summary of digital and analog modulation schemes. The primary sinusoid is the carrier signal with some definite frequency, while the superimposed analog or digital signal is the modulating signal. The modulating signal encodes the desired information onto the carrier, which could be a digital bitstream (digital modulation) or an analog signal (analog modulation).

Comparison of digital modulation (left) and analog modulation (right, AM shown only)

Effectively, the only difference between these schemes is the nature of the modulating signal (either analog or digital). The use of these two types of modulating signals has led to the development of multiple modulation formats used in various communications protocols.

Types of Digital Modulation

While we can’t cover all the possible types of modulation, we’ve summarized some of the important modulation formats in the table below. This table shows the type of modulation as AM, FM, or as phase modulation (PM).

Name	Type	Description
Amplitude shift keying (ASK)	AM	Comes in binary (BASK) or M-ary forms
Amplitude phase shift keying (APSK)	AM+PM	Allows 2 digital bitstreams to be encoded on 1 carrier
M-ary quadrature amplitude modulation (M-QAM)	AM	Encodes M-bit data stream onto a carrier
Frequency shift keying (FSK)	FM	Comes in binary (BFSK) or M-ary forms
Phase shift keying (PSK)	PM	Comes in binary (BPSK) or M-ary forms
Pulse code modulation	Varies	Encodes data as a stream of pulses with varying repetition rate, period, or amplitude

Design Advice for Modulated Signals

Today, you don’t really get to choose between amplitude modulation vs. frequency modulation in a communications system. Whether you’re operating in a licensed or unlicensed band, the type of modulation you use will depend on the wireless standard or signaling standard you need to use in your system. Generally, unless you’re building some unique equipment to operate in the ISM band, modulation is used to transmit digital data over the air. Even AM radio went all-digital in October 2020.

The standardization of communication protocols is largely beneficial for system designers, as it cuts down the overall development effort required to build communications capabilities into a new system. Chipsets supporting every wireless or wired protocol can be found on the market in standard packaging and processor vendors provide firmware/software libraries needed to use these protocols in a new system.

Modern signaling standards specify multiple pieces of information for board and systems designers, including:

Modulation format and carrier frequency
Data/signal transfer characteristics: packet size and format, unit interval (UI), maximum data rate
Interconnect impedance requirements
Channel operating margin (a.k.a. loss budget in fiber optics)
Linearity and distortion requirements (IIP-3/OIP-3 limit)

If you’re designing a system that will not be using one of the many standardized wired or RF communications standards, you’ll be operating in an unlicensed band (such as ISM) and you’ll have more freedom to choose your modulation format. Software-defined radio (SDR) is one emergent area where designers have the freedom to choose the modulation format and carrier frequency they use in RF communications, and there are some open-source SDR packages you can use to help speed up system deployment.

Software-defined radio architecture using an NCO for heterodyne reception in a receiver. [Image credit: Curtis-Wright]

Finally, modulated signals can experience losses during propagation on an interconnect and interaction with a receiver. Obviously, transmission lines should be designed to the required impedance, but channel evaluation needs to be performed with S-parameter simulations and measurements. The best simulation tools for PCB layout evaluation will generate simulation data directly from your physical design to help you evaluate channels for AM and FM signals.

Cadence’s PCB design and analysis software can help you determine the appropriate system design choices for amplitude modulation vs. frequency modulation in channel design. Once you’ve generated your simulation data, you can export the data for use in any modeling application to analyze signal behavior. When you use Cadence’s software suite, you’ll also have access to a range of simulation features you can use in signal integrity analysis, giving you everything you need to evaluate your system’s functionality.

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