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参数资料
| 型号： | STEL-1175+125/MC |
| 厂商： | INTEL CORP |
| 元件分类： | 数字信号处理外设 |
| 英文描述： | 8-BIT, DSP-NUM CONTROLLED OSCILLATOR, CQCC68 |
| 封装： | CERAMIC, LDCC-68 |
| 文件页数： | 4/14页 |
| 文件大小： | 253K |
| 代理商： | STEL-1175+125/MC |
STEL-1175+125
12
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 spur by filtering. For this
reason, the maximum practical 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 (Sony CX20202A-1) is shown below. In this case,
the clock frequency is 60 MHz and the output frequency is
programmed to 6.789 MHz. This 10-bit DAC gives better
performance than any of the currently available 12-bit
DACs at clock frequencies higher than 10 or 20 MHz.
The maximum non-harmonic spur level observed over the
entire useful output frequency range in this case is 74
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. It would
be necessary to select a DAC with better dynamic linearity
to improve the harmonic spur levels. (The dynamic
linearity of a DAC is a function of both its static linearity
and its dynamic characteristics, such as settling time and
slew rates.) At higher output frequencies the waveform
produced by the DAC will have large output changes from
sample to sample. For this reason, the settling time of the
DAC should be short in comparison to the clock period.
As a general rule, the DAC used should have the lowest
possible glitch energy as well as the shortest possible
settling time.
TYPICAL SPECTRUM
Center Frequency:
6.7 MHz
Frequency Span:
10.0 MHz
Reference Level:
5 dBm
Resolution Bandwidth: 1 KHz
Video Bandwidth:
3 kHz
Scale:
Log, 10 dB/div
Output frequency:
6.789 MHz
Clock frequency:
60 MHz
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