What does the term keying mean in PSK?

Answer 1

Grab a seat and we'll kick it about PSK. PSK is phase shift keying. It's a modulation scheme, a way to put digital information onto a carrier wave. Let's do a quickie review and then go right to the answer. Ready? Let's jump. Let's say we need to send a digital signal. Our signal, the digital string, is just a series of "on's" and "off's" that isn't much different from something like, say, Morse Code. The telegraph code could be looked at as a binary code. It's just short and long pulses, not unlike our binary on's and off's in the digital domain. So how do we get that information onto a carrier wave? One way is to shift the phase of the carrier signal to modulate it, to add our signal to it. Let's look at the carrier wave. We have a carrier humming along at (probably) some microwave frequency. If we want to send an 'on' bit, we slow down the carrier just a tad, and for a tiny interval of time. If we want to send an 'off' bit, we speed the carrier up just a tad for a tiny interval of time. The slowing down or speeding up of the carrier in PSK is the keying. Wouldn't it be nice to know what effect this has on the other end of the transmission? Let's look. On the receiving end, we generate the original carrier frequency (our "beat" frequency), and then we "beat it against" the incoming signal. (Our generated signal in the receiver is held tightly "dead on" the carrier frequency.) When we beat the two signals together, if they are the same frequency, there is no "differential" signal generated. If the transmitter is slowing the carrier down a tad or speeding it up a tad, our detectors in the receiver will "see" the difference between the incoming signal and the signal that the receiver is beating against it. The differences are logged as on's and off's by the receiver, and the digital data is then reassembled to recreate the original pulse string. Piece of cake. Oh, and got a link for ya.