what exactly is Residual carrier

Can someone explain what does "residual carrier" mean and how is that important to consider?
I googled and read wiki page but I am not satisfied much...

It could mean many things but without knowing the context we can't be sure.

Usually, it refers to a doubly balanced mixer. Under ideal balanced conditions it gives no output other than +/- products but if the balance is not perfect, some of the input components may be present in the output signal and this is referred to as "residual carrier".


Brian.

yeah, had a crude spread spectrum system once, and as one pseudo random modulated it (BPSK), you could see a tiny hump at the carrier frequency...LO leakage thru the mixer. I did finally figure out a cute way to remove it...but not so easy.

It gets more important for low level systems. Like you are piggybacking modulation on a satellite repeater, but your spread signal is under the thermal noise floor. You can share the repeater channels ONLY IF there are no discrete higher level spurs leaking out...like a tiny hump right at the carrier frequency!

It is related to amplitude modulation. With some modulation systems if the transmitter is not adjusted properly, it can not be modulated down to zero carrier, so there is residual carrier. With analogue TV, the AM video modulation must not modulate the carrier down to zero carrier, else the FM intercarrier sound will not work and you get a vision buzz on sound. So in this case there must be a 2% residual carrier.
Frank