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TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine
— 03-04/2013
—
TELE-audiovision.com
FEATURE
Phase Shifts in Digital TV
Jacek
As satellite signal analyzers become more and more
affordable, many satellite enthusiasts decide to buy
and use them. When they start playing with their new
instruments, they sometimes encounter terms not so
obvious to everybody. Transponder frequency, symbol
rate, FEC or polarization are commonly used and most
of the users have no problem in apprehending their
meaning. But I/Q vectors can be a puzzle for some of
the fans. You can see “I/Q Normal” and “I/Q Inverted”
(or “I/Q Swapped”) options in some analyzer screens.
What does it mean? In fact, it is not anything complex
and we will explain it in a simple way in this feature
article.
Phase
shift
Symbol
45°
00
135°
01
225°
11
315°
10
Let’s consider the simplest
form of modulation used in
satellite TV – QPSK. In this
modulation, the sinusoidal
signal amplitude remains
unchanged but its phase can
change at regular intervals.
For example, if we have a
transponder broadcasting
with a symbol rate of 27.5
Ms/sec, its phase can change
27.5 million times in a sec-
ond. Or we can say that one
symbol lasts for
sec (about 36 nanoseconds).
There are four phase shifts
allowed in QPSK what cor-
responds to four different
symbols.
In the figures below
(graph.1-3), you can see an
example of a QPSK modulat-
ed carrier with all four possi-
ble phase shifts in the order:
45°, 135°, 225°, 315°.
In this example, there are
1...,132,133,134,135,136,137,138,139,140,141 143,144,145,146,147,148,149,150,151,152,...228