AD9775EB AD [Analog Devices], AD9775EB Datasheet - Page 31

• Manufacturer Part Number: AD9775EB
• Description: 14-Bit, 160 MSPS 2X/4X/8X Interpolating Dual TxDAC+ D/A Converter
• Manufacturer: AD [Analog Devices]

The entire upconversion, from baseband to transmit frequency,
is represented graphically in Figure 33. The resulting spectrum
shown in Figure 33 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 AD9775 input channels.
REV. 0
IMAGINARY
IMAGINARY CHANNEL (IN)
REAL CHANNEL (IN)
REAL
Figure 33. Two-Stage Upconversion and Resulting Image Rejection
A
B
DC
DC
QUADRATURE
MODULATOR
*F C = COMPLEX MODULATION FREQUENCY
*F Q = QUADRATURE MODULATION FREQUENCY
MODULATOR
COMPLEX
OUT
–31–
–B/2J
–A/2J
–F
IMAGINARY CHANNEL (OUT)
–A/4 – B/4J
–F
–F
–F
B/2
–F
A/2
A/4 + B/4J
A/2 + B/2J
Q
C
C
C
C
REAL CHANNEL (OUT)
In fact, image rejection will still occur 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
Figure 31 represents the two-stage upconversion from complex
baseband to carrier.
*
– F
C
–F
–F
–F
+j t
Q
Q
Q
A/4 – B/4J
A/4 – B/4J
*
–F
bit (Register 01h, Bit 1). In purely complex terms,
Q
REJECTED IMAGES
+ F
C
B/2J
A/2J
–F
A/2
B/2
F
F
F
C
C
C
C
F
A/4 + B/4J
A/4 + B/4J
Q
– F
TO QUADRATURE
MODULATOR
C
F
F
F
Q
Q
Q
–A/4 + B/4J
A/4 – B/4J
A/2 – B/2J
F
Q
+ F
C
AD9775