User waveforms, constellations, and
fi
lters
The points lie on a circle of constant amplitude. The radius of the circle is 32767.
Thus, we can compute the IQ coordinates as shown in the following table.
Table 12: QPSK constellation point computations
Symbol
Formula
Value
Hex values
0
32767 (cos(
π
/4), sin(
π
/4))
(23170, 23170)
(5A82, 5A82)
1
32767 (cos(3
π
/4), sin(3
π
/4))
(–23170,23170)
(A57E, 5A82)
2
32767 (cos(7
π
/4), sin(7
π
/4))
(23170,–23170)
(5A82, A57E)
3
32767 (cos(5
π
/4), sin(5
π
/4))
(–23170,–23170)
(A57E, A57E)
Note that we need only de
fi
ne 4 constellation points. All others may be set to
zero, since they will not occur for 2-bit waveforms. Furthermore, since the
constellation does not change from symbol to symbol, we should zero out all
symbol set RAM as well.
Combining all this information together we can synthesize the following
command to download the constellation:
WRTC 2, 0, #42560<5A 82 5A 82 A5 7E 5A 82 5A 82 A5 7E A5 7E A5
7E…><NL>
The
fi
rst parameter indicates this is a 2-bit constellation. The second parameter
indicates that the IQ values are not staggered. The third parameter indicates that
we are transmitting 2560 binary bytes. The portion inside the brackets shows
the
fi
rst 16 bytes of the transmission. These bytes represent our 4 constellation
points. The following 2544 bytes are all zero. The brackets are not part of the
transmission. Finally, all commands must be terminated with a semicolon, a
carriage return <CR>, or a new-line <NL>. This command is no exception. Thus,
a <NL>, which has the hexadecimal value 0x0A, follows the 2560 binary bytes.
User
fi
lters
NOTE.
You can use the TGS File Assistant utility software to download supported
fi
le types of user constellations to the generator when it is connected to the
Ethernet through a LAN connection. (See page 58, Using the
TSG4100A Series RF Signal Generators User Manual
69
