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ADF4360-3 leakage into AD8349 inputs

Question asked by robincoxe on Apr 8, 2011
Latest reply on Sep 6, 2011 by robincoxe

I'm observing LO leakage into the inputs (pins 1 & 2 and 15 & 16) of an AD8349 modulator in a direct conversion transmitter (tuned to 942.8 MHz).  The analog inputs to the modulator originate from an AD9763 DAC with a low-pass anti-aliasing filter with f(3db)= 3.5 MHz.   The LO signal is provided by an  ADF4360-3.  The RF outputs are configured using a broadband match as on p. 22 of the data sheet.   There is a large amount of RF leakage into the 3.3 V DC power rail thorough the 51 ohm resistors.   RF energy is also leaking into the 5V DC power rail that powers both the DAC and the modulator.   The layout guy suspects that the power net attached to pin 7 of the AD8349 may be picking up some noise from adjacent LOIP and LOIN pins 5 and 6. The net result is a large LO feedthrough signal at the output of the modulator that's subsequently amplified.  I replaced the ADF4360-3 RF output broadband match with choke inductors and tombstoned an extra inductor in series with the DC bypass capacitor and it helps a bit.   Does ADI have any other PCB layout or other advice for combating this unfortunate behavior?

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