Datasheet

AD9773
Rev. D | Page 39 of 60
INPUT
(REAL)
OUTPUT
(REAL)
OUTPUT
(IMAGINARY)
INPUT
(
IMAGINARY
)
90°
e
–jωt
= COSωt + jSINωt
02857-077
Figure 77. Implementation of a Complex Modulator
COMPLEX MODULATION AND IMAGE REJECTION
OF BASEBAND SIGNALS
In traditional transmit applications, a two-step upconversion is
done in which a baseband signal is modulated by one carrier to
an intermediate frequency (IF) and then modulated a second
time to the transmit frequency. Although this approach has
several benefits, a major drawback is that two images are
created near the transmit frequency. Only one image is needed,
the other being an exact duplicate. Unless the unwanted image
is filtered, typically with analog components, transmit power is
wasted and the usable bandwidth available in the system is
reduced.
A more efficient method of suppressing the unwanted image
can be achieved by using a complex modulator followed by a
quadrature modulator.
Figure 78 is a block diagram of a
quadrature modulator. Note that it is in fact the real output
half of a complex modulator. The complete upconversion can
actually be referred to as two complex upconversion stages,
the real output of which becomes the transmitted signal.
INPUT
(REAL)
OUTPUT
INPUT
(IMAGINARY)
90°
COSωt
SINωt
02857-078
Figure 78. Quadrature Modulator
The entire upconversion from baseband to transmit frequency is
represented graphically in
Figure 79. The resulting spectrum
shown in
Figure 79 represents the complex data consisting of the
baseband real and imaginary channels, now modulated onto
orthogonal (cosine and negative sine) carriers at the transmit
frequency. It is important to remember that in this application (two
baseband data channels) the image rejection is not dependent on
the data at either of the AD9773 input channels. In fact, image
rejection still occurs with either one or both of the AD9773 input
channels active. Note that by changing the sign of the sinusoidal
multiplying term in the complex modulator, the upper sideband
image could be suppressed while passing the lower one. This is
easily done in the AD9773 by selecting the e
+jωt
bit (Register 01h, Bit
1). In purely complex terms,
Figure 79 represents the two-stage
upconversion from complex baseband to carrier.