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I and Q signals

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  • Christopher Jullum Kise
    Hi Guys Sorry for the loon intro.. After building my softrock, I found that I would like to make sure that I got the correct adjustments for
    Message 1 of 2 , Sep 4 2:25 AM
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      Hi Guys
      Sorry for the loon intro..

      After building my softrock, I found that I would like to make sure
      that I got the correct adjustments for Phase,amplitude,Bandwidth,
      which I guess is the main possible differences in the soundcards
      channels + Cabling. Beeing that the main problems is the differences
      in between the channels..

      So I found several guides.. for adjusting these factors, but ended up
      with the feeling that these factors would change with temperature and
      moment of cables, potentiometers.

      So, i thought i needed to read up a bit on the I/Q signals..
      So i found this guide: www.nonstopsystems.com/radio/article-sdr-is-qs.pdf

      After reading that I think understood the I and Q signals to be the
      identical, but the Q signal is 90 degrees phase shifted ?

      If so why do we do this transfer of both of these signals?, when
      channels on ok soundcards like the emu 0204 can have as much as 5db
      differences in channels?

      Could we not create the Q signal from the I signal in software? so
      that this could be a perfect clone of the I signal and just adjust the
      time(phase) shift

      I feel i must have misunderstood something ... I guess this had been
      done if it was possible, so i hope you could help me into the correct
      way of thinking..

      Regards
      LA6ZSA
      Christoper
    • gordon@gjcp.net
      ... Not so much as you d notice. Without paying any particular attention to phase and amplitude, you should get at least -40dB separation, which is good
      Message 2 of 2 , Sep 4 2:33 AM
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        On Tue, Sep 04, 2012 at 11:25:56AM +0200, Christopher Jullum Kise wrote:
        >
        > So I found several guides.. for adjusting these factors, but ended up
        > with the feeling that these factors would change with temperature and
        > moment of cables, potentiometers.

        Not so much as you'd notice. Without paying any particular attention to phase and amplitude, you should get at least -40dB separation, which is "good enough".o

        It's worth improving, but don't worry too much about it.

        > After reading that I think understood the I and Q signals to be the
        > identical, but the Q signal is 90 degrees phase shifted ?

        Not quite. The carrier is shifted by 90 degrees; the I and Q signals are effectively the incoming signal multiplied by the two phase-shifted carriers.

        > Could we not create the Q signal from the I signal in software? so
        > that this could be a perfect clone of the I signal and just adjust the
        > time(phase) shift

        You can, but you need to sample at four times the frequency. This is how direct-downconversion SDR works.

        --
        Gordon JC Pearce MM0YEQ
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