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6th December 2017, 12:23 #1
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I Q signals lower sampling rate
According to this article https://www.dsprelated.com/showarticle/192.php by using IQ signals we have at the end the benifit that Each A/D converter operates at half the sampling rate of standard realsignal sampling. Can somebody explain why? I mean the signal has still the same BW.

6th December 2017, 12:23

6th December 2017, 12:38 #2
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Re: I Q signals lower sampling rate
The argument is referring to sampling rate, not signal bandwidth.
If I understand right, it says that a signal with carrier fc can be either sampled with a single ADC at fs >= 2fc (Nyquist sampling) or quadrature sampled with two ADCs at fs = fc.

6th December 2017, 12:38

6th December 2017, 13:47 #3
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Re: I Q signals lower sampling rate
I don't think this is what he means. After all you can still sample in lower sampling rate than the Nyquist rate if you use undersampling techniques https://en.wikipedia.org/wiki/Undersampling.
It has to do with spliting the signal to two ADCs and in that way the sampling rate is not 2B anymore but B. But still dont understand why.

6th December 2017, 13:47

6th December 2017, 16:04 #4
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Re: I Q signals lower sampling rate
With IQ signals, positive and negative frequencies are not the same. This means that the represented range is fs/2 to +fs/2, so the bandwidth is equal to fs.
It is the baseband signal that is represented by the IQ samples. The carrier is not visible in the IQ "domain".
A fixed value of the IQ baseband signal (= zero frequency) corresponds to the carrier frequency in the RF domain.

6th December 2017, 16:26 #5
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Re: I Q signals lower sampling rate
Still don't get it. Supposed our signal has a BW of B as in Fig18 what is the sampling rate of each ADC and waht is the benifit over notcomplex down convertion?

6th December 2017, 16:26

6th December 2017, 17:09 #6
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Re: I Q signals lower sampling rate
The sample rate of each A/D is fs, as usual. With "notcomplex" sampling the frequency goes from 0 to fs/2, which means that BW = fs/2.
With complex IQ signals, the frequency goes from fs/2 to fs/2, which means that BW = fs.
Figure 18 assumes complex sampling. With noncomplex sampling, both the negative and positive frequencies will map to the range 0  fs/2, so they can't be separated.
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