Hi, I am just starting learning about radio and there something I don't quite understand when it comes to data/bandwidth limits.


Lets say I transmit a signal at exactly 1000000hz (1Mhz) and encode 1 and 0 as full power or half power (amplitude modulation) for each herz to get 1 million bits/sec (1Mbit) and then do the same at a frequency of 1000002hz and again at 1000004hz etc. all the way up to 2000000hz so I have half a million separate signals (with 1hz spacing) at between 1Mbit & 2Mbit data-rate per signal. my total data rate would be something like 500000K Mbit or (500Gbit). Clearly in the real world this is not possible and I would like to know why. Is it because I would need half a million transmitters and receivers or is it possible to listen to or transmit all frequencies at once with a single antenna using oversampling with software defined radio. I get the feeling i'm missing something here so If somebody could fill me in it would be most welcome.




Thanks

 

Hello,

The way you want to do it is not possible.
Modulating a signal will give sidebands.

Have a look how a VDSL signal is made:
http://en.wikipedia.org/wiki/VDSL2

Bertus

 

I did a bit more reading about sidebands and AM and as I understand it sidebands are only created with traditional analogue voice AM not digital On-off keying Amplitude-shift keying (ASK) as used for digital data where a carrier wave is simply turned on (to represent 1) or kept off (representing 0) rather than actively modulated by another frequency that would create sidebands. If someone could explain why the example if my first post can not work I would be most grateful.

 

Hello,

An ASK signal WILL create a lot of sidebands.
Have a look at the following page:
http://www.maximintegrated.com/en/app-notes/index.mvp/id/3587

Bertus

 

Thanks for the link, this webpage was also a good read>http://www.w8ji.com/cw_bandwidth_described.htm


I now understand that the more you change a carrier wave per unit of time the wider the sidebands will be and the more bandwidth consumed which explains the relationship between data rate and bandwidth.


I still can't get my head around exactly "how" sideband frequencys are created when a carrier is modulated, ie. how the change of a signal creates two new signals either side of the carrier. I'm trying to picture it in my head but i can't.


Thanks for your advice.

 

Hello,

The squarewave is made from adding a lot of odd harmincs of sinuses.
That way the odd harmonics of the base frequency will come back in the modulated signal.

Here is a tutorial how a basic AM modulation works:
http://www.williamson-labs.com/480_am.htm

This is a link from the EDUCYPEDIA page on analog modulation techniques:
http://educypedia.karadimov.info/electronics/modulation.htm

About the question if it is possible to receive a lot of frequencies at the same time, I would say yes.
Have a look at the following page where you can "see" the signals in a broad spectrum using a web SDR receiver:
http://websdr.ewi.utwente.nl:8901/
You can "tune" the receiver using mouse or typing in the numbers in the field below the waterfall.

Bertus