Nuts & Volts - September 2005

Q&A

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Applications Manual (Figure 2). A more robust circuit can be found at www.uoguelph.ca/∼antoon/gad gets/pll/ pll.html

FM Modulation

Q. I have a question about FM transmission. In AM transmission, the sidebands give some of their power to the carrier. In FM transmission, sidebands steal some of the power from the carrier. And there are times when the carrier becomes zero. My question is, what is really going on here? Are you still transmitting signal or are you not because the carrier is zero? And, is there any significance to this when your carrier becomes zero?

Julius B.

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A. This is a difficult question to answer without going into a lot of math, but I’ll try. First, yes, you are still transmitting a signal even if the carrier goes to zero. That’s because the signal strength is contained in the sidebands. Unlike AM modulation — which imparts information by varying the power output of the carrier wave — FM modulation encodes information by changing the frequency of the carrier. FM sidebands are the frequencies — plus and minus — that exist on each side of the carrier frequency.

Sidebands are created when the carrier is modulated. In the US, the bandwidth is defined as

SEPTEMBER 2005

± 75 kHz — resulting in a bandwidth of 150 kHz. Well ... kinda. A 75 kHz FM broadcast signal requires a little more bandwidth for a host of reasons, including station spacing and the modulation index.

As in AM modulation, the sidebands are separated by the modulation frequency from the carrier. However, depending on the modulation index, the sidebands vary in amplitude. They appear, reach a maximum, then — at higher modulation indices — some sidebands disappear altogether. In fact, the carrier disappears at a modulation index of 2.4. This means that, if you apply a 31 kHz tone to an FM carrier and adjust the level of this tone to produce a deviation of 75 kHz, the carrier will actually null out.

No, the FM signal has not disappeared. All of its energy is now contained in sidebands spaced at 31 kHz increments from the carrier, i.e., 31 kHz, 62 kHz, 93 kHz, etc. They are, I must add, decoded equally in terms of power by the receiver because it

Figure 3

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