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I would like to know in general where the advantages and disadvantages are:

  1. Downsampling of a high sampled continuous analog signal in FPGA or µC and
  2. Direct use of a lower sampling rate, i.e. using an analog to digital converter with a lower sampling rate ? Are there any differences or does it have the same effect ? Because almost every DSP-System i see uses a way Oversampling followed by Decimation (Downsampling + AAF-Filter) instead of adequate Sampling Rate without Decimation.

Greetings

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  • $\begingroup$ uhm, i still dunno what exactly is the question. is it about the relative merits of two different ways of doing sample-rate-conversion? perhaps asynchronous sample-rate-conversion (ASRC)? one is the analog way with an D/A, analog LPF, and A/D at the new rate. the other is purely digital. is this what the question is about? $\endgroup$ – robert bristow-johnson Sep 28 '20 at 4:44
  • $\begingroup$ Hi, the question is about the advantages and disadvantages between sampling a signal either at the desired sampling rate (using an ADC with exactly this sampling rate), or first a significant oversampling and then reaching the desired sampling rate by downsampling. - Because almost every DSP-System i see uses a way Oversampling followed by Decimation (Downsampling + AAF-Filter) instead of adequate Sampling Rate without Decimation. $\endgroup$ – Rabobsel Sep 28 '20 at 9:00
  • $\begingroup$ so is the question about what is called "Sigma-Delta" conversion? $\endgroup$ – robert bristow-johnson Sep 28 '20 at 17:11
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With downsampling you have complete control over the process and it comes down to what compromise of processing complexity, delay, aliasing and loss of passband you can accept.

With a lower rate A/D you are pretty much at the mercy of someone elses spectral trade-offs and in addition you get the quantization/noise of one analog pass.

-k

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  • $\begingroup$ I disagree with your second paragraph -- it assumes that you don't get any say in the analog circuitry before the lower-rate ADC. In theory you could do any amount of filtering in analog-land before ADC-ing. It's just that in many practical problems, it's easier to sample fast and do filtering in digital-land, then decimate. $\endgroup$ – TimWescott Sep 27 '20 at 18:35
  • $\begingroup$ True. But commercial AD and DA (at least for audio) tends to employ multi rate topologies, and might offer a choice between a couple preset filter characteristics. While you could, in principle, do additional purely analog filtering in between them, I dont think that is practical for the kind of filter orders one typically are interested in here. My point about noise/quantization still stands. $\endgroup$ – Knut Inge Sep 27 '20 at 18:40
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And additional consideration not mentioned that comes up in radio design is in the decision to use quadrature sampling of a baseband signal (as in "Zero-IF receivers") over a "Digital-IF" receiver that is achievable when the signal can be sampled at a much higher rate as a real signal. The Digital IF signal avoids the quadrature imbalance errors that would be introduced in the analog at both the local oscillators and notably in the I and Q signal paths after the down-conversion from analog RF or IF takes place. This latter effect is much more challenging to compensate for given its variation over frequency.

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There are many advantages, but the most obvious to me

Advantage 1 :

Oversampling followed by decimation allows you use to simpler and smaller anti-aliasing filters. These filters cost less and take up less space on a PCboard.

Advantage 2 :

In multi-channels application, the tolerance and variation of the analog components of your anti-aliasing filters can cause skew between the channels because of the analog bandwidth tolerance. The cut-off frequency is determined by the value of the resistors and capacitors. While it's easy to get resistor with a ± 0.1% tolerance, it is much harder to get capacitors with a ± 0.1% tolerance. A ballpark estimate yields a ± 1% bandwidth tolerance for an order-1 filter, 2% for an order-2 filter, etc.

Oversampling followed by decimation will make the overall bandwidth more dependant on the digital anti-aliasing filter (in the decimation process) and less on the analog anti-aliasing filter. If your application requires a 0.1 % bandwidth tolerance between multiple channels, oversampling is the way to go.

Disadvantage :

You need faster ADCs and faster digital electronics (CPU, or FPGA). It costs more and usually draws more power.

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