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参数资料
| 型号： | STEL-1377Q |
| 厂商： | INTEL CORP |
| 元件分类： | 微控制器/微处理器 |
| 英文描述： | SPECIALTY MICROPROCESSOR CIRCUIT, DIP63 |
| 封装： | 3.750 X 1.600 INCH, 0.40 INCH HEIGHT, DIP-63 |
| 文件页数： | 4/13页 |
| 文件大小： | 231K |
| 代理商： | STEL-1377Q |
STEL-1377Q/S
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-1177 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 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.
The
maximum non-harmonic spur level observed over the
output frequency range shown in this case is –70 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:
10.0 MHz
Frequency Span:
20.0 MHz
Reference Level:
–5 dBm
Resolution Bandwidth: 1 KHz
Scale:
Log, 10 dB/div
Output frequency:
6.789 MHz
Clock frequency:
60 MHz
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