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Baseband phase error problem about AD8345

Question asked by BG2BHC on Apr 8, 2013
Latest reply on Apr 9, 2013 by BG2BHC



I am using a custom AD8345 board to generate SSB signals.


The IQ frequency I want is about a few kHz to a few tens of kHz. I use 15 kHz as the test frequency.


If I drive the IQ inputs in quadrature, the output sideband rejection is rather poor (-16 dB). If I shift the Q channel phase for about 17 degrees, I got the optimum sideband rejection. That large amount of phase error was not I expected.


Is that a normal performance for AD8345? If not, what could be the problem?


The following schematic diagram is what I am using. Two AD8476s are used to transform single-ended signals to differential signals. The Lissajous figure measures at the inputs of AD8345 shows a tipsy ellipse when I shift the phase, but a circle when the driving signals are in quadrature. So I don't think that part has much problem.


Many thanks!