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I and Q not 90-degrees apart (AD9854)

Question asked by DrPhibes on Sep 15, 2011
Latest reply on Oct 6, 2011 by schwarz

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).



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


Many thanks,