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why does the receiver needs to add a carrier signal to the SSB signal?

I am new to ham radio and can't so much.

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The SSB signal needs to be converted to the audio range so that you can hear the modulation normally. The carrier that you inject acts as a frequency reference, defining the translation of each RF frequency to an audio frequency (after detection). If you are transmitting a 1 kHz tone at 14,200 kHz, upper sideband, you are sending a single frequency, 14,201 kHz. The receiver needs to inject 14,200 kHz "carrier" so that you will end up with 1 kHz audio after detection.

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You need to add a carrier in order to perform envelope detection, which is the standard detector in most applications.

If no carrier is added, the signal envelope is centered around 0, so the detected output is full-wave rectified. The carrier adds a DC component to avoid the zero crossing of the envelope.

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As Juancho already mentioned, adding a carrier to the signal is needed for envelope detection, one particular demodulation technique (which can be seen as locally converting the signal to an AM, or more precisely vestigial sideband, signal).

However, any SSB reciever, no matter how it is designed, must in some sense have a signal at the carrier frequency (or an intermediate frequency and the carrier frequency ± the IF). This is because the demodulator must, one way or another, translate the SSB signal from its original carrier frequency to zero (putting the “sideband” into audio frequencies), and in order to perform that translation, whether in electronics or in software, one must produce a signal at the frequency one is translating the signal by.

(At least, I haven't heard of any methods that don't have that as a component, and it's hard to see how something else could work — what defines the tuning frequency, if not a signal at that frequency or a known offset of it?)

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SSB is typically demodulated by a "product detector" which, as its name implies, multiplies the incoming RF signal by a carrier signal. If the carrier signal is at the frequency which was used to create the SSB signal in a "balanced modulator" - a multiplying frequency mixer - the product detector output is the demodulated sideband energy.

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As mentioned there are several way to demodulate SSB, but ALL of them that I am aware of need a "REFFERENCE" frequency (the carrier). How else will you differentiate between the various audio frequencies? How will you know if it is 100Hz or 1Khz or 10Khz? ONLY by refference to the carrier frequency can you properly demodulate SSB signals!

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