AD9775-EBZ Analog Devices Inc, AD9775-EBZ Datasheet - Page 37

• Manufacturer Part Number: AD9775-EBZ
• Description: 14Bit 160 MSPS Dual TxDAC+ D/A Converter
• Manufacturer: Analog Devices Inc
• Series: TxDAC+®r

Specifications of AD9775-EBZ

• Number Of Dac's: 2
• Number Of Bits: 14
• Outputs And Type: 2, Differential
• Sampling Rate (per Second): 160M
• Data Interface: Parallel
• Settling Time: 11ns
• Dac Type: Current
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD9775
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
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
IMAGINARY
IMAGINARY CHANNEL (IN)
REAL CHANNEL (IN)
REAL
A
B
DC
DC
1
2
F
F
C
Q
= COMPLEX MODULATION FREQUENCY
= QUADRATURE MODULATION FREQUENCY
QUADRATURE
MODULATOR
Figure 79. Two-Stage Upconversion and Resulting Image Rejection
MODULATOR
COMPLEX
OUT
Rev. E | Page 37 of 56
–A/4 – B/4J
–B/2J
–A/2J
–F
IMAGINARY CHANNEL (OUT)
A/4 + B/4J A/4 – B/4J
–F
A/2
B/2
–F
–F
–F
A/2 + B/2J
C
Q
REAL CHANNEL (OUT)
1
C
C
C
– F
C
–F
–F
–F
Q
Q
Q
2
A/4 – B/4J
–F
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 AD9775 input channels.
In fact, image rejection still occurs with either one or both of
the AD9775 input channels active. Note that by changing the
sign of the sinusoidal multiplying term in the complex
modulator, the upper sideband image could have been
suppressed while passing the lower one. This is easily done in
the AD9775 by selecting the e
purely complex terms, Figure 79 represents the two-stage
upconversion from complex baseband to carrier.
Q
REJECTED IMAGES
+ F
C
B/2J
A/2J
A/2
B/2
F
F
–F
F
C
C
C
C
(IMAGINARY)
A/4 + B/4J A/4 – B/4J
A/4 + B/4J –A/4 + B/4J
F
Q
– F
(REAL)
TO QUADRATURE
MODULATOR
INPUT
INPUT
C
Figure 78. Quadrature Modulator
F
F
F
Q
Q
Q
SINωt
A/2 – B/2J
F
Q
+ F
C
90°
+jωt
bit (Register 0x01, Bit 1). In
COSωt
OUTPUT
AD9775