I am evaluating the ADF4350 using the EVAL-ADF4350EB2Z system. The loop filter was re-done for 20kHz, the reference was kept at 10MHz, output frequency tunes from 400MHz to 800MHz, output frequency step at 200kHz (integer-N design).
A few initial observations on the evaluation follow:
1) The wideband operation required replacing the RF output bias inductors with 50-ohm resistors (i.e. L2 and L3). Otherwise the power variation over the operating band was extereme (over 15dB), and some unexplained high-level spurious outputs were present at the higher operating frequencies. No issue here, just a friendly reminder for some future user.
2) Using the x2 option on the reference input causes some huge spurs at 400kHz from the carrier (x2 the channel step?). These "reference" spurs showed-up at -30dBc (way too high).
3) Using the x1 reference option, the "reference" spurs show up at 200kHz and 100kHz. Levels are at -72dBc for 200kHz, and -75dBc for 100kHz.
1) Using the x1 reference (Ref->10MHz, R->25, VCO->divide-by-4 for 600MHz); (a) why do I get spurs at 100kHz? (b) why are the levels for the "reference" spurs (whether they are 200kHz or 100kHz) near -70dBc?
2) What is the charge pump leakage current? Originally I thought somewhere around 1-5nA; but with the reference spurs I am experiencing; this may be much higher?