Preliminary
5-95
RF9678
Rev A4 010622
5
Application Notes
Quadrature modulator performance can be correlated to a set of specifications known as Carrier and Sideband Suppres-
sion. In addition, Sideband Suppression can be correlated with the amplitude and phase balance of the In-Phase (I) and
Quadrature (Q) signals and Carrier Suppression can be correlated to the DC offset between the I and Q signals (see Fig-
ure 1). For a more thorough discussion of the theory and mathematics behind these specifications refer to RF Micro
Devices application note AN0001.
Effects of Carrier Suppression and Sideband Suppression on W-CDMA (QPSK) Modulation
W-CDMA signals may be displayed on a vector signal analyzer as a collection of points called a constellation. Each point
in the constellation is called a symbol and is representative of a bit sequence. In QPSK modulation, there are four sym-
bols and each symbol is representative of two data bits (see Figure 2). The I and Q signals are added together to create
a vector of precise phase and amplitude. The vector is then sampled at a rate called the symbol rate and it's position at
these intervals corresponds to the target symbol locations. Errors in the phase and amplitude of the I and Q signals will
translate to errors in the vector's phase and amplitude. This phase and amplitude error will result in a displacement of the
vector from it's target symbol point. A measurement of this error is called the Error Vector Magnitude (EVM) and it repre-
sents the magnitude of the displacement of the actual vector from it's target location.
QPSK constellation points exist on a circle of constant radius around the origin. Amplitude errors result in symbol points
being displaced either inside or outside of their target locations on this circle. Phase errors result in symbol points being
displaced on an arc either to the left or right of their target location. Finally, DC offset errors cause the origin to shift,
resulting in a constant I and Q offset of all target points (see Figure 3).
Σ
In-Phase Signal
Quadrature Signal
LO Signal
RF Output Signal
0°
90°
Figure 1. Quadrature Modulator Block Diagram
Ref Lvl
0dBm
0dBm
Ref Lvl
0dBm
0dBm
A
-1.875
1.875
REAL
CF
380 MHz
Meas Signal
Constellation
Constellation
IMAG
T1
EXT
SR
3.84 MHz
Demod
QPSK
-1.5
1.5
Date:
6.FEB.2001
01:20:38
Figure 2. W-CDMA (QPSK) Constellation
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