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Nyquist Frequency

en.wikipedia.org

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Re: Nyquist Frequency

#71

Post by kimburgess »

coolandsmartrr wrote: post #63

I saw the Nyquist Frequency mentioned in the American Cinematographer Magazine. The article illustrate how detailed patterns, like sweaters, can produce a fuzzy jagged artifact called moire. This is because there is too much information for camera's sensor to interpret and summarize the details into pixels (ie. surpassing the Nyquist Frequency). Their suggested solutions were to 1) get a wide-angle lens to reduce det…

  When a grid’s misaligned
  with another behind
  That’s a moiré…

  When the spacing is tight
  And the difference is slight
  That’s a moiré

Re: Nyquist Frequency

#73

Post by kimburgess »

regularfry wrote: post #64
coolandsmartrr wrote: post #63

I saw the Nyquist Frequency mentioned in the American Cinematographer Magazine. The article illustrate how detailed patterns, like sweaters, can produce a fuzzy jagged artifact called moire. This is because there is too much information for camera's sensor to interpret and summarize the details into pixels (ie. surpassing the Nyquist Frequency). Their suggested solutions were to 1) get a wide-angle lens to reduce det…

Yep. Strictly speaking what's happening is that the pattern has a higher spatial frequency than the sensor, and the light detection acts as a non-linear interaction which aliases the higher frequencies down into the bandwidth of the sensor. A wide-angle lens would change the effective bandwidth of the system, as would a larger sensor: all either would do is change the apparent size of the moire pattern (possibly so i…

You can buy them: https://rawlite.com/olpf-for-blackmagic-pocket-cinema-camera..., https://www.grassvalley.com/products/optical_low_pass_filter

Re: Nyquist Frequency

#74

Post by gct »

kayson wrote: post #18
EarthIsHome wrote: post #3

One misconception that many make regarding the Nyquist frequency is thinking that the sampling rate needs to be twice the highest frequency. Your sampling should really really be twice the bandwidth . e.g. your bandwidth is 100 MHz centered at 1 GHz (it needs to actually be bandlimited to 100 MHz**). You do not need to sample at 2.2 GHz. You sample at 200 MSPS (really, you should sample a little more than that, say 2…

I know what you're getting at, but your statement, as others have pointed out, is incorrect. Your sampling rate always always has to be twice the highest frequency of the signal you are sampling. If you are sampling an RF-modulated signal with a center frequency of 1GHz and 100MHz of baseband bandwidth, then yes, you do need to sample at 2.2GHz+. And some applications do exactly that. If you're taking the RF signal,…

I'm afraid you're mistaken (source: worked as DSP engineer for 15 years). Often you apply your filter around the RF frequency you want and then sample at a lower rate. You're right that the signal will get aliased doing that, but the information is always preserved.

If you sample s.t. your folding frequencies are in an appropriate place, you can fold your desired region into the first nyquist region without needing to mix it down. This is especially desirable if you can avoid having to build an IQ mixer because they're hard to keep balanced.

The worst case doing this is that your signal spectrum is reversed in frequency, but you can correct that easily digitally.

Re: Nyquist Frequency

#75

Post by kayson »

squeaky-clean wrote: post #58
kayson wrote: post #55

Earlier quoted context omitted.

Yes, but this only works if, as the page points out, the signal is bandpass filtered, which GP did not mention. It's not true in the general sense, nor is it practical for many (most?) RF systems, especially those with multiple channels.

> your bandwidth is 100 MHz centered at 1 GHz Implies a bandlimited signal centered around 1ghz.

I can see why you might think that but consider that in RF systems, while the wanted signal is bandlimited, you also have a lot of unwanted "blockers" all over the spectrum that need to be dealt with before sampling.

Re: Nyquist Frequency

#76

Post by [deleted] »

gct wrote: post #74
kayson wrote: post #18

Earlier quoted context omitted.

I know what you're getting at, but your statement, as others have pointed out, is incorrect. Your sampling rate always always has to be twice the highest frequency of the signal you are sampling. If you are sampling an RF-modulated signal with a center frequency of 1GHz and 100MHz of baseband bandwidth, then yes, you do need to sample at 2.2GHz+. And some applications do exactly that. If you're taking the RF signal,…

I'm afraid you're mistaken (source: worked as DSP engineer for 15 years). Often you apply your filter around the RF frequency you want and then sample at a lower rate. You're right that the signal will get aliased doing that, but the information is always preserved. If you sample s.t. your folding frequencies are in an appropriate place, you can fold your desired region into the first nyquist region without needing t…

[deleted]

Re: Nyquist Frequency

#77

Post by kayson »

gct wrote: post #74
kayson wrote: post #18

Earlier quoted context omitted.

I know what you're getting at, but your statement, as others have pointed out, is incorrect. Your sampling rate always always has to be twice the highest frequency of the signal you are sampling. If you are sampling an RF-modulated signal with a center frequency of 1GHz and 100MHz of baseband bandwidth, then yes, you do need to sample at 2.2GHz+. And some applications do exactly that. If you're taking the RF signal,…

I'm afraid you're mistaken (source: worked as DSP engineer for 15 years). Often you apply your filter around the RF frequency you want and then sample at a lower rate. You're right that the signal will get aliased doing that, but the information is always preserved. If you sample s.t. your folding frequencies are in an appropriate place, you can fold your desired region into the first nyquist region without needing t…

I'm afraid I'm not mistaken (source: I design integrated RF transceivers) ;)

Yes, you can subsample if you have a suitably bandpass-limited signal. But that's not the general case, nor is it what the nyquist-shannon theorem proves, which is where "nyquist frequency" comes from.

Nyquist frequency by the original definition is 2X highest frequency, though some papers textbooks evidently have started using it to mean 2X bandwidth, enough so that wikipedia[1] actually mentions it.

In integrated circuits, IQ mixing isn't problematic as we can fairly easily do gain and phase calibration to correct for the mismatch.

[1] https://en.m.wikipedia.org/wiki/Nyquist_frequency#Other_mean...

Re: Nyquist Frequency

#78

Post by detaro »

kayson wrote: post #77
gct wrote: post #74

Earlier quoted context omitted.

I'm afraid you're mistaken (source: worked as DSP engineer for 15 years). Often you apply your filter around the RF frequency you want and then sample at a lower rate. You're right that the signal will get aliased doing that, but the information is always preserved. If you sample s.t. your folding frequencies are in an appropriate place, you can fold your desired region into the first nyquist region without needing t…

I'm afraid I'm not mistaken (source: I design integrated RF transceivers) ;) Yes, you can subsample if you have a suitably bandpass -limited signal. But that's not the general case, nor is it what the nyquist-shannon theorem proves, which is where "nyquist frequency" comes from. Nyquist frequency by the original definition is 2X highest frequency, though some papers textbooks evidently have started using it to mean 2…

(use \* to escape an * and prevent it being parsed as an italics marker.)

Re: Nyquist Frequency

#79

Post by kayson »

kayson wrote: post #56
abstrakraft wrote: post #20

Earlier quoted context omitted.

From an information theoretic perspective (which is the perspective Nyquist was originally coming from, though it didn't yet have that name), you don't need to mix the signal down. Assuming it is truly band-limited, you can sample the signal directly at RF, and reproduce it from those samples. Additionally, you will need to modulate the reproduced signal into the original band, which means you need to know where that…

To clarify: "band-limited" usually means X(w) = 0 for abs(w) > B for some B, where X is the frequency spectrum. And that's the definition Shannon used in the original proof, which is where the idea of Nyquist Frequency comes from. If you add the additional constraint of the signal being "bandpass-limited" where, X(w) = 0 for A > abs(w) > B for some A, B, then yes, you can under sample. And that's where the informatio…

Looks like I bundled that second inequality. Band pass is X(w) = 0 for abs(w) or abs(w) > B

Re: Nyquist Frequency

#80

Post by OscarCunningham »

regularfry wrote: post #64
coolandsmartrr wrote: post #63

I saw the Nyquist Frequency mentioned in the American Cinematographer Magazine. The article illustrate how detailed patterns, like sweaters, can produce a fuzzy jagged artifact called moire. This is because there is too much information for camera's sensor to interpret and summarize the details into pixels (ie. surpassing the Nyquist Frequency). Their suggested solutions were to 1) get a wide-angle lens to reduce det…

Yep. Strictly speaking what's happening is that the pattern has a higher spatial frequency than the sensor, and the light detection acts as a non-linear interaction which aliases the higher frequencies down into the bandwidth of the sensor. A wide-angle lens would change the effective bandwidth of the system, as would a larger sensor: all either would do is change the apparent size of the moire pattern (possibly so i…

Or you can vibrate the camera: https://en.wikipedia.org/wiki/Pentax_K-3.
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