Circuit to retain only the positive frequency components in a signal?

[crixus]

sanjaysan, the negative frequency components are redundant, in a sense. Consider your time domain signal, $cos(2\pi \omega t)$. The angular frequency, $\omega$, could be either positive or negative, and the resulting wave would look the same in the time domain. That ambiguity leads to the two-sided, symmetric spectrum.

You can move to a one-sided spectrum if you wish, with no loss of generality, but that's just a mathematical trick. You don't need to design any real, physical device to discard the negative frequencies; they're all in your head from the beginning!

- Warren

Then how do you explain the recovery of baseband signal from single sided passband signal. Suppose we have only upper sideband of a signal then in recovery of message signal the mirrored band of the signal contributes to form the spectrum of the message signal. What do you think happens physically here...

 

[sophiecentaur]

Look at the block diagram of an ssb receiver. A local oscillator at the original carrier frequency will mix with the sideband and produce a baseband signal - and other mixing products, of course but they will be at non-baseband frequencies and their power level is not relevant.

If you're concerned about the SNR of the demodulated signal then it would be 3dB lower than when both sidebands are demodulated because the noise bandwidth would be half but the demodulated signal would be half the level - giving 3dB net loss. BUT there was 3dB less power transmitted for a start so there would is overall disadvantage (as long as the transmitter can be made efficient.

It is important to get the Maths and the Physical World reconciled properly. Like I said earlier. All the answers to the calculations should start off with "The real part of".