Q.The signal is affected by noise in a communication system

Step 1. Noise is any undesirable electrical signal that interferes with and attenuates the transmitted signal; it may be man-made (vehicles, welding machines, motors) or natural (lightning, solar/stellar radiation).

Step 2. The transmitter and modulator are where the signal is generated and shaped -- these stages are internal, shielded circuitry and are not exposed to the external disturbances that constitute noise.

Step 3. The communication channel is the medium (wireline or wireless/free space) carrying the signal between transmitter and receiver, and it is while travelling through this open medium that the signal picks up external man-made and natural noise.

Step 4. Eliminating the others: (a) and (b) occur before the signal has left the transmitter and are not where external noise enters; (d) the receiver processes a signal that has already picked up noise in the channel -- it does not itself introduce the noise.

Recall noise's definition -- an undesirable signal picked up while the transmitted signal travels through the communication channel/medium.

• Thinking the receiver 'causes' the noise, rather than merely receiving a signal that already picked up noise in the channel.
• Confusing internal circuit stages (transmitter, modulator) with the external transmission medium where interference actually enters.