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ADL5385 Output Level and LO Drive

Question asked by languer on Jul 30, 2011
Latest reply on Aug 15, 2011 by THA

Hi, we're using the ADL5385 to produce a modulated waveform (1.2MHz BW) from 200MHz to 400MHz. To do this we drive the LO port with 400MHz to 800MHz from a synthesizer (ADF4112 + VCO). I have a couple of questions based on observations made so far.


1) The datasheet implies an output power at the modulator of around +5dBm. We are driving each of the the IQ inputs with 700mVpp biased at 500mV, our power is around -5dBm. Is this what you would expect? Can we increase the IQ drive to 1Vpp, how would that degrade performance?


2) When driving the LO input single-ended (using the eval board) from our synthesizer, the performance is pretty poor (the constellation wobbles and shows excessive magnitude imbalance (and phase to a lesser extent) yielding an EVM greater than 20%. If we add a low-pass filter to remove VCO harmonics the performance gets pretty good (EVM < 2%). I've read a bit on the issue and if I understand correctly what we are seeing maybe due to even-order harmonics of the VCO upsetting the divider timing inside the modulator. Two options are then offered to counter this: low-pass filter the VCO output to reduce the even-order harmonics (is 20dB sufficient? 30dB?), or use a balun (or transformer) to drive the LO differentially (do you have a recommendation?). Are the second-order VCO harmonics (or for that matter, any VCO harmonics) the root cause of what we're observing; and if so, what do you recommend to rectify this?