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Transmitter & Receiver Operating region
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on Jan 28, 2015
on Jan 28, 2015 by mirek
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I have a doubt that why RF transmitter are non-linear in nature while receiver are linear in nature.
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Jan 28, 2015 9:26 AM
Of course, all things in nature are Non-Linear, it it just a matter of degree about how much.
For transmitter, it is commonly used at high power, in a strongly non-linear state for better power efficiency (strongly as in more than 1% distortion, maybe a two-tone level of -40 dBc or so) and so the non-linear behavior strongly affects the quality of the signal. There is a lot of effort in pre-distorting the signal such to compensate for the non-linear effect and allow a transmitter to operate in a non-linear mode but still produce a high quality signal.
For a receiver, it is commonly used at low power in a weakly non-linear state (weakly as in less than 0.01% distortion, maybe a two-tone level of -80 dBc or so), and so the non-linear behavior has almost no effect on the quality of the signal, relative to other defects such as noise and receiver flatness.
Even the filter in a radio, although passive, is non-linear. Google Passive Intermodulation (PIM) for an example.
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Jan 28, 2015 9:53 AM
The main reason is probably efficiency of the system (and if that big transmitters cost of operation).
From transmitter side: Let say you have FM transmitter in New York City that deliver 200KW of power. If you use A class (linear) power amp you only have 50% efficiency. That mean the you need 400KW/h of power. If you use different class of power amp that is non linear however deliver 90% efficiency you will need around 220KW. Now if the total cost per KW is $0.3, then cost of energy for the transmission for the first case will be 400*0.3= $120 per hour, or $2880 per day or $1,051,200 per year.
In case of 220KW you get 220*0.3= $66 per hour or $1,584 per day, or $578160 per year.
For receiver the power that you receive is very low, and the efficiency might be less important as total operational cost is vey low (look on your cell phone energy consumption, were most of the power is to drive CPU and display not the receiver). If the signal operate in linear mode less additional hardware need to filter some of the distortion unless the DSP post processing can do that. I am sure that vendors of phone system on chip probably when already or going there, to save every bit of power.
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