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參數(shù)資料
| 型號： | STEL-1375A+80 |
| 廠商： | INTEL CORP |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | SPECIALTY MICROPROCESSOR CIRCUIT, DIP35 |
| 封裝： | 2.500 X 1.300 INCH, 0.350 INCH HEIGHT, DIP-35 |
| 文件頁數(shù)： | 4/13頁 |
| 文件大?。?/td> | 202K |
| 代理商： | STEL-1375A+80 |
STEL-1375A+80
12
SPECTRAL PURITY
When an NCO is used with a digital to analog converter
(DAC) to generate an analog waveform the spectral
purity of the synthesized waveform is a function of
many variables, including the phase and amplitude
quantization, the ratio of the clock frequency to output
frequency, and the dynamic characteristics of the DAC.
The sine signals generated by the STEL-1175 have 12
bits of amplitude resolution and 13 bits of phase
resolution which results in spurious levels which are
theoretically at least 75 dB down. The highest output
frequency the NCO can generate is half the clock
frequency (fc/2), and the spurious components at
frequencies greater than fc/2 can be removed by
filtering. As the output frequency fo of the NCO
approaches fc/2, the "image" spur at fc– fo (created by
the sampling process) also approaches fc/2 from above.
If the programmed output frequency is very close to
fc/2 it will be virtually impossible to remove this image
spurbyfiltering. Forthisreason,themaximumpractical
output frequency of the NCO should be limited to
about 40% of the clock frequency.
A spectral plot of the NCO output after conversion
with a DAC (AD9721) is shown below. In this case, the
clock frequency is 80 MHz and the output frequency is
programmed to 12.3456789 MHz. The maximum non-
harmonic spur level observed over the output
frequency range shown in this case is –62 dBc. The
spur levels are limited by the dynamic linearity of the
DAC. It is important to remember that when the
output frequency exceeds 25% of the clock frequency,
the second harmonic frequency will be higher than the
Nyquist frequency, 50% of the clock frequency. When
this happens, the image of the harmonic at the frequency
fc– 2fo, which is not harmonically related to the output
signal, will become intrusive since its frequency falls as
the output frequency rises, eventually crossing the
fundamental output when its frequency crosses
through fc/3. The same phenomenon occurs with the
third harmonic when the frequency crosses through
fc/4.
TYPICAL SPECTRUM
Frequency Span:
0 to 30 MHz
Reference Level:
–7 dBm
Resolution Bandwidth: 1 KHz
Scale:
Log, 10 dB/div
Output frequency:
12.3456789 MHz
Clock frequency:
80 MHz
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