Thursday, February 07, 2008

Guard Bands

This was something I didn't even realize existed in radio for years. AM and FM Radio Frequencies exist in designated bands. This part is common knowledge. These have changed over time. The change isn't not common knowledge, but I've covered it before. Guard bands (Also called frequency guard bands) are virtually unknown outside of radio physics and FCC rule making notices. If you are math phobic, ship this post. the term guard bands is used in more applications than commercial AM and FM radio... I'll start slow.First a little review:
The FM radio band is from 88 to 108 MHz starting just above the VHF television Channel 6. Each FM station is assigned center frequency with 200 kHz separation. Then they also have a 75 kHz maximum deviation from the center frequency. That means it's not a solid bar of "power" in the RF band. it's allowed to vary slightly. This tolerance leaves 25 kHz above and below that range between that station and any adjacent station. These are deliberately left vacant. These are guard bands. their purpose is to minimize interaction between adjacent frequencies.

With AM radio the carrier frequencies are in the frequency range from 535-1605 kHz. The situation is much the same except the intervals are 10 kHz instead of 25kHz. In multiplexing the same senario exists and so do guard bands. Guard bands are commonly used in Frequency Division multiplexing (FDM) for example, but are also used in any data transmission method that relies on frequency modulation. Don't confuse this with the term "guard interval."

In data transmission that will come up as well, but that is aduration of time. These are used to ensure that individual transmissions on the same multiplexed frequency do not interfere with each other. Without the gap echoes and reflections can cause interference. More here.