Using the ADF4360-9 to produce output frequencies in the range of 70MHz to 90MHz. Using both outputs, but using them singled ended (kind of a dual output), but both outputs are driving 50 ohm impedance. Using a 150nH inductor to bias up the RFout, instead of the 51 ohms resistor.
I'm seeing higher harmonic content than the data sheet would indicate, though the datasheet doesn't give any conditions on the harmonic measurement. The second order harmonic is down only -9dBc (compared with -16dBc typical) and the third is down only -12dBc (compared with -21dBc typical).
Are these measurements fairly normal? Are there performance differences over different frequency ranges? What can one do to reduce the harmonic contect coming out of the VCO?
Ideally you should load the output with 50 ohms worth of inductance followed by a 50 ohm low-pass filter.
I suspect the output is clipping with the value of inductance that is being used, which is probably too big -- it's exactly the same as putting too large a collector resistor into an emitter-coupled amplfier -- the load line clips. (Undortunately, the data sheets are sketchy on this topic... 50 ohms, for example, provides a broadband resistive match but the right matched load (an inductor, etc.) can provide a bit more output power.