Hi
I woder if anybody could help me, I have got question.
Q, How to calculate availabe bandwidth for guardband. if 3Mhz bandwidth is given for use by radio stations each full duplex channel of 28Khz bandwidth
Thanks
Rafi-muqaddar
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Hi
Thanks all those repiled. Question to aburr. How to calculate number of channels in this case.
Can we calulate number of channels by doing this:
Total bandwidth= 3MHz or 3000KHz
we know that from the question each radio station would employ a full duplex channel of 28KHz bandwidth
Hence:
n= 3000/28
n=107
Is it correct
Thanks
Rafi-muqaddar
Is it correct
no
To get n you must divide 3 MHz by the width of each channel including the guard band
The total channel width will be 28KHz (times 2 if each "duplex" takes 28 KHz ) plus the guard band.
You cannot find both n and guardband. The bigger the guard band the fewer the channels in a given total bandwidth.
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by: grg99Posted on 2006-01-05 at 09:53:55ID: 15620767
We tried to answer this question before.
The size of the guardbands depends on MANY technical and economic and environmental variables. Here are some of them:
(1) The guarbands have to be wide enough so the receiver doesnt hear much or any of of the adjacent channels.
(2) Now receiver filters are not very expensive anymore, so sharp filters are not a big concern, BUT the shaerper the
filter, the more phase-shift there is near the edges. So you don't want a filter with extremely steep edges if
you need a low phase shift data stream. There is a basic tradeoff beween adjacent channel rejection and phase shift
in channel. You usually have to come up with some compromise.
(3) Also guardbands are partially there to get you away from adjacent channel distortion products. If a transmitter isnt
super-pure in its modulation scheme, harmonics of the modulating frequency can cause extra sidebands, widening
the transmitter bandwidth. Also in phase and frequency modulation, the theoretical sidebands go off forever in both
directions, so you have to know how wide the transmitter output filter is.
(4) Also in the real world transmitters and receivers drift somewhat in frequency, due to crystal aging, temperature, and other factors.
You need extra guardbands to allow for these drifts. For example, a taxi radio working in Alaska is going to undergo a temperature range from -50C to +30C every year. Even with a 10 parts per million per degree xtal, that's a lot of drift, not to mention drift from aging, potholes, and component drift.
(5) Finally in most jurisdictions the federal radio agency sets up frequency spacings for each frequecy band, AND the signal bandwidths, AND the adjacent channel splatter, AND the guardbands, otherwise all would be chaos.
So you may not even have a choice, you'll have to use the mandated channel spacings, which will determine your transmitter bandwidth, once you decide how much of a guardband you can engineer. Or if the signal badwidth is legislated, you have even less choice.
Hope this helps.