I and Q not 90-degrees apart (AD9854)

I am using an AD9854 Quadrature DDS and upconverting its output to between 700MHz and 1500MHz via a high quality quadrature-modulator ($1250 worth of quad-mod!). The quad-mod, with correct inputs, suppresses the lower sideband and the carrier in its output very well.

However, I was seeing the following in the quad-mod's RF o/p spectrum:

- strong baseband leakage from the DDS

- no carrier suppression

- no lower sideband suppression

I have greatly improved carrier suppression by ensuring that the I and Q DAC current-output DC-levels and signal-amplitudes are exactly balanced via the DDS's OSK function. This also somewhat improved lower-sideband suppression.

However I believe that the I and Q signals arriving at the quad-mod's inputs have some phase-skew causing the baseband leakage and the remainder of the incomplete lower-sideband suppression. Even if I and Q are precisely 90-degrees apart coming out of the DDS,  they may not stay that way as they travel through the reconstruction filter and along transmission lines etc.

I used X-Y mode on my scope to obtain a Lissajous figure displaying the phase-relationship between I and Q and I got a very elongated oval shape (a perfect circle is the result of exactly 90-degrees phase difference). I used 1MHz out of the DDS to minimise high-frequency effects on the scope-probes although I will later need to check phase at max DDS output as well (100MHz).

QUESTION:

On the AD9854 how can I trim the phase-difference between I and Q?

Many thanks,

Nick

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  • Hi Matthias,

    Thanks for your answer, it was very detailed and most helpful. I'd come to the same conclusiion that I and Q outputs are fixed at 90-deg and no trimming is possible although I was hoping I'd missed something in the DDS datasheet. It does look like I'm stuck with phase-trimming by hand. Oh well.

    I'm really surprised that there is no phase trmming though - there are similar AD products that have phase trimming; I found a good AD paper (AD1039) that shows how to correct imperfections in I/Q modulators. It shows an I/Q DAC (the AD9788) and its outputs can be individually phase trimmed.

    Regards,

    Nick

    ps

    I've attached that AD app-note for anyone who might need it.

    AN-1039CorrectingImperfectionsinIQModulatorstoImproveRFSignalFidelity.pdf
Reply
  • Hi Matthias,

    Thanks for your answer, it was very detailed and most helpful. I'd come to the same conclusiion that I and Q outputs are fixed at 90-deg and no trimming is possible although I was hoping I'd missed something in the DDS datasheet. It does look like I'm stuck with phase-trimming by hand. Oh well.

    I'm really surprised that there is no phase trmming though - there are similar AD products that have phase trimming; I found a good AD paper (AD1039) that shows how to correct imperfections in I/Q modulators. It shows an I/Q DAC (the AD9788) and its outputs can be individually phase trimmed.

    Regards,

    Nick

    ps

    I've attached that AD app-note for anyone who might need it.

    AN-1039CorrectingImperfectionsinIQModulatorstoImproveRFSignalFidelity.pdf
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