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| 型号： | EVAL-AD9837SDZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 3/28页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD9837 |
| 产品培训模块： | Direct Digital Synthesis Tutorial Series (1 of 7): Introduction Direct Digital Synthesizer Tutorial Series (7 of 7): DDS in Action Direct Digital Synthesis Tutorial Series (3 of 7): Angle to Amplitude Converter Direct Digital Synthesis Tutorial Series (6 of 7): SINC Envelope Correction Direct Digital Synthesis Tutorial Series (4 of 7): Digital-to-Analog Converter Direct Digital Synthesis Tutorial Series (2 of 7): The Accumulator |
| 标准包装： | 1 |
| 主要目的： | 计时，直接数字合成（DDS） |
| 嵌入式： | 否 |
| 已用 IC / 零件： | AD9837 |
| 次要属性： | 图形用户界面 |
| 已供物品： | 板，CD，文档 |
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Data Sheet
AD9837
Rev. A | Page 11 of 28
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 = 2πf.
MAGNITUDE
PHASE
+1
0
–1
228
0
2π
4π
6π
2π
4π
6π
0
90
70
-02
3
Figure 18. Sine Wave
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:
ΔPhase = ωΔt
(1)
Solving for ω,
ω = ΔPhase/Δt = 2πf
(2)
Solving for f and substituting the reference clock frequency for
the reference period (1/fMCLK = Δt),
f = ΔPhase × fMCLK∕2π
(3)
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
fOUT to MCLK. If the ratio is small, the aliased images are very
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 fOUT/reference clock ratio, the first aliased image can
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.
09
07
0-
0
40
SYSTEM CLOCK
fOUT
fC – fOUT
fC + fOUT
2
fC – fOUT
2
fC + fOUT
3
fC – fOUT
3
fC + fOUT
fC
0Hz
FIRST
IMAGE
SECOND
IMAGE
THIRD
IMAGE
FOURTH
IMAGE
FIFTH
IMAGE
SIXTH
IMAGE
2
fC
3
fC
FREQUENCY (Hz)
S
IG
N
A
L
AM
P
L
IT
UD
E
sin(x)/x ENVELOPE
x = π (
f/fC)
Figure 19. DAC Output Spectrum
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