DataCard CP60 Service Manual page 45

Signal Modulation
The concept of signal modulation arises from the relationship between frequency
and required antenna size. Namely, the antenna needs to be on the same order as
the wavelength of the signal. The lower the frequency--the longer the
wavelength--the larger the antenna required to transmit or receive the signal.
An audio signal for example, would require an antenna of approximately
300 Km in length. This wouldn't fit very well in the trunk of your car for
listening to your favorite music, and a boom box would be totally out of the
question.
Instead, the lower frequency (the data signal) is combined with a higher
frequency (the carrier). The higher frequency is more easily transmitted and
received. Combining two signals together is called modulation.
There are many different types of signal modulation, but they all boil down to
three basic parameters—altering the amplitude, frequency and/or phase of the
carrier signal.
You are probably familiar with the terms Amplitude Modulation and Frequency
Modulation because these are used by most consumer radios. We know them as
AM and FM radios. Because of their nature, digital signals can utilize many
variations on these basic modulation schemes.
The following diagrams show a couple of the basic modulation schemes as well
as the variants used by digital signals. The AM and FM modulation diagrams are
shown with analog data signals, but phase-shift modulation is easiest to show
with a digital signal.
CP60 and CP60 Plus Service Manual 2-33