EVAL-AD9837SDZ Analog Devices Inc, EVAL-AD9837SDZ Datasheet - Page 11

• Manufacturer Part Number: EVAL-AD9837SDZ
• Description: EVALUATION BOARD
• Manufacturer: Analog Devices Inc
• Series: -r

Specifications of EVAL-AD9837SDZ

• Main Purpose: Timing, Direct Digital Synthesis (DDS)
• Embedded: No
• Utilized Ic / Part: AD9837
• Primary Attributes: -
• Secondary Attributes: Graphical User Interface
• Kit Application Type: Clock & Timing
• Application Sub Type: Clock Generator
• Kit Contents: Software CD, USB Cable
• Silicon Manufacturer: Analog Devices
• Silicon Core Number: AD9837
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

THEORY OF OPERATION
Sine waves are typically thought of in terms of their magnitude
form: a(t) = sin(ωt). However, sine waves are nonlinear and not
easy to generate except through piecewise construction. On the
other hand, the angular information is linear in nature; that is,
the phase angle rotates through a fixed angle for each unit of
time. The angular rate depends on the frequency of the signal
by the traditional rate of ω = 2π f .
Knowing that the phase of a sine wave is linear and given a
reference interval (clock period), the phase rotation for that
period can be determined as follows:
Solving for ω,
ΔPhase = ω Δ t
ω = Δ Phase/ Δ t = 2 πf
2
–1
0
0
28
Figure 18. Sine Wave
2π
0Hz
2π
MAGNITUDE
PHASE
f
OUT
4π
4π
sin(x)/x ENVELOPE
IMAGE
FIRST
x = π (
f
C
SYSTEM CLOCK
–
f
OUT
f
/
f
C
f
C
)
6π
6π
f
SECOND
C
IMAGE
Figure 19. DAC Output Spectrum
+
f
OUT
Rev. 0 | Page 11 of 28
(1)
(2)
FREQUENCY (Hz)
2
f
IMAGE
THIRD
C
–
f
OUT
Solving for f and substituting the reference clock frequency for
the reference period (1/f
The AD9837 builds the output based on this simple equation. A
simple DDS chip can implement this equation with three major
subcircuits: numerically controlled oscillator (NCO) plus phase
modulator, SIN ROM, and digital-to-analog converter (DAC).
Each subcircuit is described in the Circuit Description section.
The AD9837 provides a sampled signal with its output following
the Nyquist sampling theorem. Specifically, its output spectrum
contains the fundamental plus aliased signals (images) that occur
at multiples of the reference clock frequency and the selected
output frequency. A graphical representation of the sampled
spectrum with aliased images is shown in Figure 19.
The prominence of the aliased images depends on the ratio of
f
prominent and of a relatively high energy level as determined by
the sin(x)/x roll-off of the quantized DAC output. In fact, depend-
ing on the f
be on the order of −3 dB below the fundamental.
External filtering is required if the aliased image is within the
output band of interest.
2
OUT
f
C
2
to MCLK. If the ratio is small, the aliased images are very
FOURTH
f
f = Δ Phase × f
C
IMAGE
+
f
OUT
OUT
/reference clock ratio, the first aliased image can
3
f
C
IMAGE
FIFTH
MCLK
–
f
OUT
∕2 π
MCLK
3
f
C
3
= Δt),
f
IMAGE
C
SIXTH
+
f
OUT
AD9837
(3)