1. Amplitude Modulation
Modulation is the process of varying a higher frequency carrier wave to transmit information. Though it is theoretically possible to transmit baseband signals (or information) without modulating it, it is far more efficient to send data by modulating it onto a higher frequency "carrier wave." Higher frequency waves require smaller antennas, use the available bandwidth more efficiently, and are flexible enough to carry different types of data. AM radio stations transmit audio signals, which range from 20 Hz to 20 kHz, using carrier waves that range from 500 kHz to 1.7 MHz. If we were to transmit audio signals directly we would need an antenna that is around 10,000 km! Modulation techniques can be broadly divided into analog modulation and digital modulation. Amplitude modulation (AM) is one form of analog modulation.
Figure 1. Basic Stages of AM
2. Mathematical Background
The carrier signal is generally a high-frequency sine wave. There are three parameters of a sine wave that can be varied: amplitude, frequency, and phase. Any of these can be modulated, or varied, to transmit information. A sine wave can be mathematically described by a sine or cosine function with amplitude Ac, frequency fc, and phase φ.
Figure 2. Carrier Wave
The carrier signal is modulated by varying its amplitude in proportion to the message, or baseband, signal. The message signal can be represented by
and the carrier signal can be represented by
To make the equations simpler, assume that there is no phase difference between the carrier signal and the message signal and thus φ = 0.
The modulated signal can be represented by multiplying the carrier signal and the summation of 1 and the message signal, as shown below.
With some basic trigonometric manipulation, the above waveform can be written as
3. Types of AM Modulation
As described in the previous section, the modulated signal has waves at three frequencies: fc, fc – fb and fc + fb. Transmitting at all three frequencies wastes power and bandwidth. To avoid that problem use a filter to remove one of the sidebands (usually the lower sideband, fc – fb). Use a highpass filter to remove the lower sideband signal; this process is single sideband (SSB) modulation.
However, by removing one of the sidebands we lose some of the original power of the modulated signal. To maximize the power transmitted, transmit both the lower and the upper sideband. This process is double sideband (DSB) modulation. The following figure illustrates DSB.
Figure 3. Frequency Domain View of Double Sideband – Full Carrier
One of the components of the modulated signal is the pure carrier wave. Because the carrier wave does not have any information, we can remove the carrier wave component from the signal before we transmit it. This process is called single sideband/double sideband – suppressed carrier (SSB-SC, DSB-SC) modulation. However, we need the carrier when demodulating the signal. Special circuits can extract information about the carrier from one of the sidebands; these circuits are used when demodulating SSB-SC or DSB-SC signals.
We can also use amplitude modulation to send digital data. Quadrature amplitude modulation (QAM) uses four predetermined amplitude levels to determine digital bits.
4. Reality Check
Although understanding AM is helpful to understand modulation, it is not the most efficient or useful way to modulate a signal. Simple AM is slow and requires too much power. Because most communication today is digital, far more complex methods are used. Generally, phase shift keying (PSK)—a type of phase modulation—is used to transmit digital data.
5. Related Products
NI PXIe-5663 6.6 GHz RF Vector Signal Analyzer
The National Instruments PXIe-5663 is a modular 6.6 GHz RF vector signal analyzer with 50 MHz of instantaneous bandwidth optimized for automated test.
NI PXIe-5673 6.6 GHz RF Vector Signal Generator
The National Instruments PXIe-5673 is a 4-slot 6.6 GHz RF vector signal generator that delivers signal generation from 85 MHz to 6.6 GHz, 100 MHz of instantaneous bandwidth, and up to 512 MB of memory.
NI PXI-5660 2.7 GHz RF Vector Signal Analyzer
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NI PXI-5671 2.7 GHz RF Vector Signal Generator
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NI PXI-5652 6.6 GHz RF and Microwave Signal Generator
The National Instruments PXI-5652 6.6 GHz RF and microwave signal generator is continuous-wave with modulation capability. It is excellent for setting up stimulus response applications with RF signal analyzers.
NI RF Switches
The National Instruments RF switch modules are ideal for expanding the channel count or increasing the flexibility of systems with signal bandwidths greater than 10 MHz to bandwidths as high as 26.5 GHz.
National Instruments LabVIEW is an industry-leading graphical software tool for designing test, measurement, and automation systems.
NI Modulation Toolkit
The National Instruments Modulation Toolkit extends the built-in analysis capability of LabVIEW with functions and tools for signal generation, analysis, visualization, and processing of standard and custom digital and analog modulation formats.
For the complete list of tutorials, return to the NI Measurement Fundamentals main page, or for more RF tutorials, refer to the NI RF Fundamentals main subpage. For more information about National Instruments RF products, visit www.ni.com/rf.