In the context of wideband receiver design, what are the advantages and disadvantages of Analog IQ sampling vs creating IQ digitally (with Hilbert transform)?

Block Diagram For IQ conversion

Why do we use different ADCs to create independent IQ channels when we can use Hilbert Transform to create the Q channel and use only one ADC?

  • $\begingroup$ Could you add a block diagram of each method? It's not completely clear from your question how each system works. $\endgroup$
    – MBaz
    Commented Dec 4, 2020 at 14:50
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    $\begingroup$ Also, is this question homework? $\endgroup$
    – MBaz
    Commented Dec 4, 2020 at 14:51
  • $\begingroup$ I also removed the tags to which you didn't relate in the question. Be careful to only tag the topics that you actually touch upon in the question's text! $\endgroup$ Commented Dec 5, 2020 at 12:46
  • $\begingroup$ @MBaz I have added a generic block diagram for IQ conversion hope its more clear now and no this is not homework, I was studying about hilbert transform and wondered why we use additional hardware to create IQ when it can be done digitally. $\endgroup$
    – malik12
    Commented Dec 5, 2020 at 14:27
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    $\begingroup$ It depends on what you're doing -- so if you have a scheme in mind you should ask another question about that. But if you're downconverting to baseband and you downconvert with a single mixer and then Hilbert transform, at the point where you downconvert you lose information -- doing a Hilbert transformation on the result rearranges things, but doesn't make information reappear. $\endgroup$
    – TimWescott
    Commented Dec 6, 2020 at 16:51

1 Answer 1


You're right. Whenever you can do things digitally you should because of all the advantages that you've listed. But a long time ago it was expensive to do those low pass filters digitally. If the LPF required is narrow (and requires a steep roll off), it would require many taps (multipliers) and they might require many bits so as to not lose precision to numerical accuracy issues. You also have to have the ADC twice as fast in the all digital case.

The disadvantages of doing the things in the analog realm are many though. You would have to carefully tune LPFs to have the same group delay. There could be many calibration issues.

Here's a paper describing a Hilbert Transform design


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