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參數(shù)資料
| 型號： | STEL-1376 |
| 廠商： | INTEL CORP |
| 元件分類： | 微控制器/微處理器 |
| 英文描述： | SPECIALTY MICROPROCESSOR CIRCUIT, DIP65 |
| 封裝： | 3.350 X 1.500 INCH, 0.350 INCH HEIGHT, DIP-65 |
| 文件頁數(shù)： | 4/13頁 |
| 文件大?。?/td> | 233K |
| 代理商： | STEL-1376 |
STEL-1376
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-1176 have 12 bits of amplitude resolution
and 15 BCD bits of phase resolution which results in
spurious levels which are theoretically at least 72 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 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 12.3456789 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 output
frequency range shown in this case is –59 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
Center Frequency:
15.0 MHz
Frequency Span:
30.0 MHz
Reference Level:
–10 dBm
Resolution Bandwidth: 1 KHz
Scale:
Log, 10 dB/div
Output frequency:
12.3456789 MHz
Clock frequency:
80 MHz
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