Amplitude Modulation Basics With UNI-T UTG932E Signal Generator

May 18, 2023

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Waves Working Together: Carrier Wave, Modulation Signal

The two fundamental concepts in RF amplitude modulation are the carrier wave and modulation signal. Both work in parallel and need to be clearly defined in the context of AM signaling.

• Carrier – A wave that cycles in a set frequency over time tends to be very fast when compared to the modulating signal. AM broadcasts in the United States take place in the 1000 kHz ballpark, as defined here by the FCC.
• Modulation Signal – A signal that increases and decreases in magnitude according to transmitted information. This wave is not uniform over time and is typically much slower than the carrier wave.

When the carrier signal is modulated with input data, what results is a varying signal envelope that can then be interpreted, i.e. demodulated, by receiving equipment. A wave on top of a wave, one might say.

While a carrier could be nominally thought of as sinusoidal, this can take the form of other wave types as well. As demonstrated below, when viewing at a large time scale, the difference in carrier waves may be difficult or impossible to ascertain.

UNI-T UTG932E Signal Setup

Setting up amplitude modulation on the UNI-T UTG932E:

• Optionally reset everything to defaults: Utility > System > Page Down > Preset > Ok
• Hit Mode > AM > CH1 to activate amplitude modulation on the default settings

With this default output, the signal generator produces a carrier wave of 1 kHz, with a modulation signal of 100 Hz. Therefore, there are 10 carrier waves contained inside each modulated signal envelope. Zoom out on the monitoring oscilloscope, and the carrier waves begin to fade into the background, while the larger modulated envelope is still visible.

(Image Credit: Jeremy Cook / Caption: Square wave modulation of a square wave carrier at 80% depth)

To modify the carrier wave, hit the Wave button, and change the frequency. If this is increased, the smaller waves in the modulated envelope visually fill out. All things being equal, the smoothness of your signal is therefore directly related to the ratio of the carrier wave frequency to the modulation wave frequency.

You can also change the type of carrier wave in this interface. Depending on your output frequency and zoom level, the carrier wave shape may not appear to make a huge difference when observed on an oscilloscope.

To turn AM modulation off and only output the carrier, hit the mode button once to select the modulation wave option, and again to turn it off. The green light will go out, and you’ll be left with the unmodulated carrier wave. This interface methodology can be confusing at first, but works quite well once mastered.

Signal Modulation: A Vast Rabbit Hole Complex

(Image Credit: Jeremy Cook)

For those on the outside of the RF world, signal modulation can seem like a dark art, or a black box that you depend on to work without questioning its nature. While it’s certainly a vast subject, the concept of changing the amplitude of a carrier wave with a modulating signal is a good starting point on this learning journey.

Reading about amplitude modulation is one thing, but being able to see and modify this type of wave on a signal generator in real-time is a fantastic way to learn. Once ready, you can then change the modulation wave type to FM (frequency modulation), FSK (frequency-shift keying), and beyond. Be sure to check out my review of the UNI-T UTG932E signal generator linked above, as well as my separate piece on the OWON HDS2102S handheld oscilloscope used here to monitor its operation.