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ML6420CS-1 参数 Datasheet PDF下载

ML6420CS-1图片预览
型号: ML6420CS-1
PDF下载: 下载PDF文件 查看货源
内容描述: 三/双相位均衡,低通滤波器的视频 [Triple/Dual Phase-Equalized, Low-Pass Video Filter]
分类和应用: 有源滤波器光电二极管信息通信管理LTE
文件页数/大小: 20 页 / 273 K
品牌: MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
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ML6420  
ALIASING: THE PROBLEM  
Aliasing is a signal distorting process that occurs when an  
analog signal is sampled. If the analog signal contains  
frequencies greater than half of the sampling rate, those  
frequencies will be altered and “folded back” in the  
frequency domain. These frequencies represent a distortion  
of the original signal as represented in the sampled domain,  
and cannot be corrected after sampling.  
Since they have flat amplitude and linear phase, they are  
low distortion. And since the aliasing is removed at the  
analog input to the ADC, the clock rates are minimized,  
an expensive DSP half band filter is eliminated, and  
significant power is conserved.  
Oversampling vs Nyquist sampling  
THE RESULT OF ALIASING IN A TV PICTURE  
Clearly the purely analog monolithic solution versus the  
analog/digital solution using DSP filtering are different  
ways of solving the same problem. Other than costs  
(purely analog is many times less expensive) there are no  
real differences in performance for applications that  
require flatness specs of ±0.5db to 4.5MHz for consumer  
and pro-sumer video applications. The ML6420/ML6422  
are also phase corrected for flat group delay, a feature not  
found in typical low cost analog filters, and a  
Aliasing causes several disturbing distortions to a picture.  
Since the folded spectrum adds to the original spectrum, it  
will sometimes be in phase, and sometimes out of phase  
causing ripples in response that depend on the position of  
the picture element relative to the clock. The net effect is  
that picture elements, edges, highlights, and details will  
“wink” in amplitude as they move across a picture if they  
have high frequency content above the Nyquist frequency  
of the sampler.  
characteristic often associated with digital filters alone.  
The following section highlights the importance of linear  
phase response in video applications.  
ANTI-ALIASING  
TIME DOMAIN RESPONSE:  
TRANSIENTS AND RINGING  
Anti-aliasing reduces the bandwidth of the signal to a  
value appropriate for the sample processing system. Some  
detail information is lost, but only the information that  
cannot be unambiguously displayed is removed. Assuming  
that the passband contains the “real” picture information,  
the only distortion that occurs is due to amplitude and  
phase variations of the anti-aliasing filter in the passband.  
The following section shows approaches using digital and  
analog filters in an oversampled system, and a monolithic  
analog filter as a lower cost alternative.  
The phase response of filters is often ignored in  
applications where time domain waveforms are not  
relevant. But in video applications the time domain  
waveform is the signal that is finally presented on the  
screen to the viewer, and so time domain characteristics  
such as pulse response symmetry, pre-shoot, over-shoot  
and ringing are very important. Video applications are  
very demanding in that they require both sharp cutoff  
characteristics and linear phase. The application of DSP to  
the problem is based on the linear phase characteristic of  
a particular class of digital filters known as symmetrical  
FIR filters. Use of these filters guarantees the best possible  
time domain characteristics for a given amplitude  
characteristic. In the analog domain phase linearity is not  
automatic (except for special phase linear filters such as  
Bessel or Thomson filters, both of which have inadequate  
amplitude characteristics for most video anti-alias  
applications) and it is often assumed that linear phase is  
unachievable. This is not true. Similarly, in the digital  
domain it is often assumed that sharp cutoff amplitude  
characteristics can be achieved without overshoot and  
ringing. This is also not true. Phase linear filters whether  
digital or analog have symmetrical response to  
OVERSAMPLING  
Aliasing cannot be removed once it occurs, it must be  
prevented at the signal sampler. Many current systems are  
choosing to prevent aliasing by increasing the clock rate of  
the sampler. This is known as “oversampling”. Doubling  
the clock rate greatly reduces the burden on the analog  
anti-alias filter, but the increased data rate greatly  
increases the size, complexity and cost of the Digital  
Signal Processing (DSP) circuitry. Since the higher clock  
rate generates more samples than are necessary to  
represent the desired passband content, a digital filter may  
be used to decimate the signal back to a lower sample  
rate, saving size, complexity and power in the  
downstream circuitry. Since this digital filter itself is a  
complex digital block, this method cannot be considered  
the lowest cost approach to solving the anti-alias problem.  
symmetrical inputs. High roll-off rate uncompensated  
filters (whether analog or digital) have ringing and  
overshoot. In the example below, the traditional 2T test  
pulse is applied to a traditional, non-phase linear analog  
filter, the ML6420 pure analog anti-alias filter (5.5MHz)  
and the combined analog/digital filters (9.3MHz analog  
filter and half-band digital filter.)  
NYQUIST SAMPLING  
In traditional systems, before the advent of higher speed  
ADCs, anti-aliasing filters were designed in the analog  
domain. The movement toward higher sampling rates was  
an attempt to circumvent the difficult challenge of  
designing a sharp roll-off, linear phase, non-distorting  
analog filter. The ML6420 series of filters solve this  
problem where it is best solved, in the analog domain.  
Since they are monolithic, their application is simple.  
As seen in Figure 19c, the ML6420 filters provide a time  
domain response that is comparable to more complex and  
expensive filters.  
Typical Passive Filter  
9