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0 pointskayson3y ago0 comments

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...

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gct3y ago

You have to have a band limited signal to sample anyways, where it's at in the spectrum doesn't matter. The first thing you'll do before feeding anything to an ADC is running it through a filter to make _sure_ it's band limited. Whether that filter's at DC or some Rf doesn't matter.

Here's the result from his original paper where he specifically says that it doesn't have to be at DC:

https://imgur.com/uSywML7

kaysonOP3y ago

My point is that practically speaking, it does matter where the signal is, depending on how you filter it. If you lowpass filter an RF- (or, more realistically, IF-) centered signal, you can't just sample it at 2X bandwidth because you'll get aliases from the unwanted content between DC and the bottom frequency edge of the signal.

It may not be a common scenario anymore, but it was very common in the early GSM days when the signal wasn't mixed to DC but near-DC.

gct3y ago

Ah yes you're right that you have to be careful, it'll fold at multiples of the nyquist frequency and you want to make sure your SOI is entirely contained in one of those zones.

detaro3y ago

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

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gct3y ago

Here's the result from his original paper where he specifically says that it doesn't have to be at DC:

https://imgur.com/uSywML7

kaysonOP3y ago

It may not be a common scenario anymore, but it was very common in the early GSM days when the signal wasn't mixed to DC but near-DC.

gct3y ago

Ah yes you're right that you have to be careful, it'll fold at multiples of the nyquist frequency and you want to make sure your SOI is entirely contained in one of those zones.

detaro3y ago

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

j / k navigate · click thread line to collapse