-3db refers to half power. But why only half power is taken as reference and not 75% or some thing else?
Jason:
Since dB refers to amplitude and is a measure of power how are you using it relative to band width?
dB is a logarithmic function every increase of 10 is an order of magnitude change. As a result a change of plus three results in a doubling and a minus three halves the value.
For more information look up logarithms using Google or another search engine.
Niel Leon
engineering.com
Thank you, Niel.
In digital communication, the information is transmitted as bits(1/0).
Consider that ‘1’ is being transmitted:
If due to noise there is a decrease of more than 50% of the amplitude, then it will be interpreted as ‘0’ by limiter. There it fails in correctly regenerating the input signal.
So 50% is taken as the reference. Hence the half-power
-3dB also refers to the bandwidth
Deepthi:
To me having a threshold of signal strength or amplitude difference between a valid 1 and a signal strength that is interpreted as a zero (less than 1/2 power) is different than talking about bandwidth.
The only place where this might be different is the determination of the frequency or maximum speed the signal can change (a function of bandwidth) where the system impedance prevents the receiver from interpreting between the zero signal and the 1 signal.
Niel
Hi Niel,
Thanks for your reply.
I placed the above explanation by considering that the bandwidth refers to the range of frequencies in which most of the signal information is concentrated.
Can you please let me know if this is incorrect?
One more question:
You mentioned that my explanation is applicable to determine the maximum speed with which the signal can change.
Does speed effect the signal strength? Can you please place more explanation regarding this too?
Thanks,
Deepthi.
Deepthi:
Because of inductive nature of long wires there is a speed limit to how fast the signal can change. The longer the length of the wire slower this speed limit. This is one of the reasons why we went to optical fibers for long transmission lines and that there is a distance that you can be away from the telecom switching station for high speed access.
Coaxial cables have a higher speed limit (frequency limit).
Niel