I frequently come across the term 'RF Signal' in circuits for ex in the below picture, please ignore the functionality of circuit not important.: 
I understand RF means Radio frequency but what I dont understand is why use this terminology for electrical signals. When I see RF I always visualize an Electro-magnetic wave. Then why is it used for electrical data? In the above example why use RF IN or RF OUT (I can to a certain degree understand why RF OUT) ? I know they are electrical input and output. I would like to know your understandings and hence expand my understanding.
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1\$\begingroup\$ Have you read the RF tag wiki? \$\endgroup\$– PlasmaHHCommented Nov 13, 2017 at 14:28
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\$\begingroup\$ No could you provide me a pointer? \$\endgroup\$– rsg1710Commented Nov 13, 2017 at 14:29
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\$\begingroup\$ @PlasmaHH No could you provide me a pointer? \$\endgroup\$– rsg1710Commented Nov 13, 2017 at 14:56
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\$\begingroup\$ This question is EXTREMELY vague because you're asking about a topic that could take two or more entire college courses to explain. It's not as easy as broadcasting a message and POOF your message appears to the receiver. To explain the process behind RF technology could take hours. There are many ways information gets to destination through the processes various modulation techniques ranging from analog to digital modulation techniques. Again, this question is too broad. It's a good question but it's not appropriate for this website. \$\endgroup\$– user103380Commented Nov 13, 2017 at 16:04
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\$\begingroup\$ For me, RF limits the context to amplifier or mixer or filter. Brain is prepared for IIP3, impedance matching, gain expressed as dB, compression point, noise figures etc. \$\endgroup\$– glen_geekCommented Nov 13, 2017 at 16:33
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1 Answer
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I can not tell you how this norm originated but what I can tell you is the benefits of this norm. Some of the points mentioned here are already mentioned in comments.
- When we term an analog electrical signal as RF signal, things that immediately pop up in mind are i) the signal is a high-frequency signal ii) the signal is either being transmitted or received at some later or former circuit stage and iii) the SNR for this signal is going to matters a lot. To satisfy/comply with these requirements for RF signal, we take into consideration RF parameters like IIP3, impedance mismatch, S parameters, noise figure and start analyzing the circuit through these parameters. So point is, terming a signal RF signal gives a lot of insight about the signal.
When I see RF I always visualize an Electro-magnetic wave. Then why is it used for electrical data?
- Exactly the point, it's there to force you to visualize it as an EM wave propagating through the channel(which consists of various circuit stages and conductive traces). So that you will treat it as wave and take care of parameters which matter.
Digital electrical signal will not be represented as rf signals, even if they are operating at a very high frequency (e.g. 2.4 GHz). This is because distortion in digital signals due to channel is not going to cause significant degradation in SNR and even with somewhat higher SNR degradation, it's possible to retrieve data properly in case of digital signals. So is not the case with RF signals(which are analog in nature). The factors which affect RF signals to high degree matters to digital signals or to any low-frequency electrical signal to less extent.
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\$\begingroup\$ That was a great answer gave a good understanding, and makes a lot of sense to use it as "RF Signal" as they special requirements and circuit can not be built just thinking it as analog signal. In circuits such as optical driver, they show input to the driver as RF, from the understanding of your answer can I say that the signal is actually coming from an antenna(receiver - we know that optical is backbone for RF)? \$\endgroup\$– rsg1710Commented Nov 22, 2017 at 9:32
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\$\begingroup\$ From the last part of answer and again applying it to optical comm circuits, say the circuit is driving data from one router to another, hence digital that's why in these circuits we do not refer to it as RF signal. I am probably repeating what you said, but just want to confirm my understanding. \$\endgroup\$– rsg1710Commented Nov 22, 2017 at 9:34