AD9786 Analog Devices, AD9786 Datasheet - Page 39

• Manufacturer Part Number: AD9786
• Manufacturer: Analog Devices

Specifications of AD9786

• Resolution (bits): 16bit
• Dac Update Rate: 500MSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.5V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

Table 35. Dual Channel Complex Modulation with Hilbert
Hilbert
0
1
When complex modulation is performed, the entire spectrum is
translated by the modulation factor. If the resulting modulated
spectrum is not mirrored symmetrically about dc when the
DAC synthesizes the modulated signal, negative frequency
components fall on the positive frequency axis and can cause
destructive summation of the signals, as shown in Figure 70. For
some applications, this can distort the modulated output signal.
In Figure 71, Figure X represents a complex signal typically
found in the AD9786 signal path. Figure Y is identical to Figure X,
but it is shifted by π/2. The phase shifting in the AD9786 occurs
because the digital LO driving the digital quadrature modulator
in the Hilbert transform path is phase shifted by π/2.
X = Ae
Im
A/2
j2π(f + fm)t
Re
A/2
A/2
Figure 71. Negative Frequency Image Rejection
A/2
f
Y = Ae
Im
A/2
j2π(f + fm)t – π/2
Mode
Hilbert transform off
Hilbert transform on
Re
00
A/2
A/2
A/2
f
Im
Z = HILBERT(Y)
A/2
Re
A/2
A/2
Figure 70. Complex Modulation with Negative Frequency Aliasing
A/2
f
Im
C = X – Z
Re
00
FILTERED INTERPOLATION IMAGES
A
A
f
f
f
S
S
Rev. B | Page 39 of 56
/8 MODULATION
/4 MODULATION
The operation of the Hilbert transform (Figure Z) rotates the
negative frequency components of Figure Y by +π/2, and the
positive frequency components of Figure Y by −π/2. The result
of the Hilbert transform output is then summed with the complex
signal in the main signal path. The result is that negative frequen-
cies are cancelled and, therefore, do not fold back into the
positive side of the frequency spectrum. The Δt block in the main
signal path offsets the delay inherent in the Hilbert transform
(nine DAC clock cycle delay). When the DAC synthesizes the
modulated output, there are no negative frequency components to
fold onto the positive frequency axis out of phase; consequently,
no distortion is produced as a result of the modulation process.
–100
–150
–50
0
–0.5
ALIASED NEGATIVE FREQUENCY INTERPOLATION IMAGES
Figure 72. Negative Frequency Aliasing Distortion
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
AD9786
0.5