Modulation means to change a carrier in some way from one instant to another so as to convey information.
ASK means Amplitude Shift Keying. This means that the amplitude of a carrier is being shifted up and down following a serial bit stream. This can be received using a receiver with an AM detector. It can be transmitted using a transmitter with an Amplitude Modulator. The most common type of transmitter for this kind of modulation is the typical garage door opener, in which an internal oscillator is turned on and off to create the amplitude modulation. In this case the amount of modulation is 100% (On is 100%, Off is 0%).
Frequency shift keying means that the frequency of a carrier is shifted back and forth between two different frequencies following a serial bit stream. Usually the transmitter uses a Frequency Modulator which is fed by the serial bit stream. A frequency modulator is often just an oscillator with a varactor diode placed in the resonant circuit so as to allow a change of capacitance, and hence a change of frequency, when the input voltage changes from 0 to 1 or from 1 to 0. For receiving this kind of signal, an FM receiver is needed.
Phases shift keying means that the phase of a carrier is shifted following a serial bit stream. There are many kinds of phase shift keying and the main difference between them is the amount of phase that is changed. The simplest is binary phase shift keying, in which the phase is changed 180 degrees when the incoming bit stream changes from 1 to 0 or 0 to 1. The BPSK modulator in the transmitter is often just a mixer. Google BPSK for more details. The receiver might use an FM detector, or it might us a PLL type detector to receive such a signal. Many modern devices use phase shift keying, including cell phones. They usually demodulate the signal by mixing to In-phase and Quadrature (I&Q) baseband and then passing these into a DSP to demodulate numerically.
Differential Phase shift keying is a type of phase shift keying. The method of receiving is most commonly done in DSP as mentioned above. See this link for more details:
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