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    Resistor terminating in anti-aliasing filter

    Hi guys, I'm using LTC5586 IQ modulator and in the datasheet, it suggest an anti-alias low pass filter with inserted-resistors, which is hard to understanding for me. I would like to ask why they do so. in this filter, if all the resistors were removed, it is normal 5th order low pass filter- which could be already designed for 50ohm terminators.
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    the example is low pass filter, what if I use band pass filter ?
    If you can, please tell me where can I learn this technique.

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    Re: Resistor terminating in anti-aliasing filter

    An anti-alias filter is used if you are sampling the signal, such as for A/D conversion, and the sample-rate is close to 1/2 the maximum frequency of interest.
    A filter is needed to remove any noise or undesired signals about 1/2 the sample rate since they will be sampled and appear as a lower frequency in the signal band (aliased).
    The closer the highest signal frequency is to 1/2 the sample rate, the higher the order of filter needed to suppress the undesired signal/noise.
    If the sample frequency is much higher than the desired signal frequency, the undesired signals below 1/2 the sample frequency can be removed by digital filtering, so less analog filtering is needed.
    Here's more info.

    Normally a low-pass filter is sufficient to perform the anti-alias function.
    Why would you want to use a band-pass, which just adds complexity?
    Zapper
    Curmudgeon Elektroniker


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    Re: Resistor terminating in anti-aliasing filter

    Tks crutschow,
    my application use low IF architecture, the IF is 170MHz. sampling frequency Fs=245.76 -> Fs/2=122.88Mhz, the image should be 75.76M. so I need bandpass filter to obtain my If signal. Did I misunderstand st ?



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    Re: Resistor terminating in anti-aliasing filter

    As for the question implied in the thread title, the termination resistors are part of the filter design. Omitting it changes the filter characteristic. A passive filter can be be designed with arbitrary termination on both ends, a resistive component is required at least at one end.

    If your application is IF rather than baseband sampling, you don't need to observe the Nyquist criterion. A bandpass filter may be used, but it isn't necessarily required if the input signal is already band limited. But even if you don't have signal components at image frequencies there may be still noise power that can be suppressed by a band pass.



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