
TAGS 61
Types of Keying
The term “keying” comes from the days of telegraphy, when an operator pressed a manual
key to make long and short tones. Keying describes which attributes of an analog carrier—
the analog carrier can be a wave or a field—may be modulated to represent the ones and
zeros of a digital message. There are three main types of keying:
Amplitude-shift keying (ASK)
A type of keying that sends digital data over analog carriers by changing the amplitude
of a wave in time with the data stream
Frequency-shift keying (FSK)
A type of keying that sends changes through the frequency of the wave (or how often a
wave crest comes along)
Phase-shift keying (PSK)
A type of keying that sends changes through the distance by which the waves lead or
follow a reference point in time
Encoding
Encoding determines the way the tag and reader will interpret changes in the analog car-
rier to represent digital data. Thus, an encoding is an agreement between sender and
receiver regarding what the keyed changes mean. Morse Code, one example of an encod-
ing, uses long tones to represent “dashes” and short tones to represent “dots.” If we substi-
tuted “0” for “dash” and “1” for “dot,” Morse encoding would work for sending
information over a serial bus.
It might seem odd to think of RFID as devices communicating over a serial bus, but even
though the components are wireless rather than wired, most tags and readers ...