A Back to Basics Column from November 2018
From the Canadian Basic Amateur Radio Question Bank
Back To Basics is a regular column in the SARC Communicator Newsletter, available at: The Communicator Digital Edition: Amateur Radio Newsletter (ve7sar.blogspot.com)
It is a subject that is important because of the interference overmodulation can cause...
This month we’ll look at percentage of modulation and overmodulation. In all the exams I have administered, this topic is always covered. It’s important because it has the ability to cause significant issues on the air. The impact of this is highlighted by the fact that it is repeated a half-dozen times in the Canadian Basic Question Bank with slightly different wording, for example:.
The maximum percentage of modulation permitted in the use of radiotelephony by an amateur station is:
A. 100 percent
B. 50 percent
C. 75 percent
D. 90 percent
When you transmit a signal, you do so over what’s called a carrier frequency. This is a relatively constant oscillation, usually in the radio frequency band, that gets modulated (altered) by the signal. In terms of radio use, the modulation is generally (but not always) a waveform produced by the human voice, music or other audible means.
For example, either the amplitude or the frequency of the carrier gets modified (or “modulated”) by the signal, hence “AM” – (Amplitude Modulation) and “FM” – (Frequency Modulation).
When this modulation is so large that the carrier signal clips (distorts, in the case of AM) or the frequency changes to such a degree that it goes beyond the range that the receiver can pick it up or overlaps other carrier frequencies (in the case of FM), the signal is said to be overmodulated.
Likewise, if the signal is of such small amplitude or frequency variation that it cannot be picked up or adequately amplified by the receiver (because of background noise and/or the strength of the carrier frequency), it is said to be undermodulated.
Overmodulation is the condition that prevails in telecommunication when the instantaneous level of the modulating signal exceeds the value necessary to produce 100% modulation of the carrier. In the sense of this definition, it is almost always considered a fault condition. In layman's terms, the signal is going "off the scale". Overmodulation results in spurious emissions by the modulated carrier, and distortion of the recovered modulating signal. This means that the envelope of the output waveform is distorted.
In the image, an amplitude modulated sine wave:
- At 0% unmodulated [top left], the sine envelope is not visible at all;
- Less than 100% modulation [top right] depth is normal AM use;
- At 100% modulation depth [bottom left], the sine envelope touch at y=0. Maximum modulation that can be retrieved with an envelope detector without distortion;
- At greater than 100% modulation depth [bottom right], "overmodulation" occurs and the original sine wave can no longer be detected with an envelope detector.
Therefore, the answer to our sample question at the top of this article is A. 100 percent.
~ John VE7TI
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