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Sunday, 23 May 2021

Single Sideband Amplitude Modulation

Single Sideband Amplitude Modulation

The general AM equation shows that when a carrier is amplitude-modulated by a single sine wave, the resulting signal consists of three frequencies — the original carrier frequency, the USB (fc + fm) and the LSB (fc — fm). Out of this, the carrier itself does not carry any information, and hence the power transmitted in the carrier signal will have no use. The two sidebands carry the same information. We need only one sideband at the receiver for the demodulation of the AM signal. We can improve the efficiency of the transmission by transmitting only one sideband (upper or lower one can be used). The resulting signal is referred to as single sideband SSB. As we know, the power transmitted,


PT = PCARR (1+ m2/2) = PCARR + PCARR m2/2


That is about 2/3rd of the power is transmitted by the carrier, which is a wastage. For the highest modulation index (m = 1), the efficiency of transmission is 33%. Under these conditions, 67% of power is transmitted by carrier, and this much power is waste. For values of m < 1, the efficiency is less than 33%. If the carrier is suppressed, only the side band power remains. As this is only PCARR m2/2 , a 2/3rd saving is effected at 100% modulation, and even more is saved as depth of modulation is reduced. If one of the sidebands is now also suppressed, the remaining power is PCARR m2/4 a further saving of 50% can be achieved over the carrier suppressed AM.


Single-sideband transmission (SSB) is a method of transmitting signals based on amplitude modulation in which only one sideband is transmitted. Essentially, the carrier and one sideband of an AM signal are removed, leaving only the other sideband. Assuming both sidebands are symmetric, no information is lost in the process. The required signal bandwidth is reduced and, since the final RF amplification is concentrated in a single sideband, effective power output is greater than normal AM. The carrier and removed sideband account for well over half of the power output of an AM transmitter.


To decode the signal at the receiving end the original AM mode is synthesized by adding a carrier signal to the lone sideband. The signal can then be demodulated as a standard AM signal. Because the synthesized carrier is locally generated, it of much higher quality than a transmitted one, which contributes to a higher quality received signal. An SSB signal cannot be demodulated by standard AM receivers because of the lack of a reference carrier signal. The major advantages of SSB over normal AM are


1. The power saving - Since the carrier is not transmitted, there is a reduction by 50% of the transmitted power. In AM, at 100% modulation, 1/2 of the power is comprised of the carrier; with the remaining (half) power in both sidebands.


2. Because in SSB, only one sideband is transmitted, there is a further power reduction by 50%.


3. Since SSB has only one sideband, the bandwidth required is only half than that required for normal AM.

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