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values of phase shift (none or 180°), and there are many
versions of phase- and amplitude-shift keying systems that
use more intermediate values. The most common are
called quadrature modulation because they are created by
modulating a pair of carriers that have the same frequency,
but have a 90° phase difference. One carrier (I) is the
in-phase carrier and the other (Q) is the quadrature carrier.
The simplest form of quadrate modulation is to
modulate each carrier independently by digital signals that
shift the phase by 0 or 180°; then, combine the modulated
carriers into one signal. Figure 3 shows a typical 4-PSK
waveform with each carrier’s phase being reversed by
the digital signals and then added together to create four
different possible phase shifts: 0°, 90°, 180°, and 270°.
Because there are two carriers — each being in one of
two states (reversed or not reversed) — that creates four
possible combinations. It is interesting to note that if the
carriers are amplitude-modulated with signals of +1 and -1
(which is the same as a 180° phase shift), then the output
signal for this QAM signal is exactly the same as for QPSK.
A number of amateur radio modes use QPSK. The
popular VHF/UHF D-STAR system uses 4-PSK; there is
a QPSK31 variation of PSK31; and there is a recently
introduced digital voice mode suitable for use on the HF
bands called FreeDV. Regulatory limitations on symbol rate
and bandwidth have limited the data rate of digital modes
on amateur radio in the US, but this is currently under
evaluation by the FCC.
Quadrature modulation is often described and
measured by using a constellation diagram. Figure 4A
shows a constellation diagram for the QPSK signal in
Figure 3. Each of the two carriers (I and Q) has their
own axis, along with their phase shifts (0 or 180°). The
resulting four combinations are shown as dots. Depending
on the digital data input to the two modulators for I and
Q, the resulting modulated signal is described by one of
Tuning In to Digital Modes
You can get an idea of what these signals actually sound
like from samples of signals that can be played back online.
The ARRL’s “Modes and Systems” web page ( www.arrl.
org/modes-systems) includes many explanatory articles
in its “Digital Data Modes” section. Look for the link to “Digital
Another such website is on the HF Radio website at
hfradio.org.uk/html/digital_modes.html. Click and
If you have a general-coverage receiver and can hook
it up to a PC, the popular Flgigi software package (www.
w1hkj.com/beginners.html) is completely free and can
decode more than 100 modes!
n FIGURE 4. (A) Constellation diagram showing four
states for QPSK; (B) shows the effects on the signal of
adding noise and distortion.
n FIGURE 3. QPSK is created from two independent
carriers with the same frequency, but a 90° phase shift.
The carriers are modulated by the digital signals, then
recombined to make the output signal. (Graphic courtesy of
Splash at wikipedia.org.)
n FIGURE 4. Alternate + Noise