I don't have any ground plane. I only have a veroboard, which I took the
unused tracks apart. I also don't have an "anti-slewing filter"; I first
tried to do it but the output signal was very "unclean", so I removed the
two capacitors. I'm now having two resistors in serial between the
phase-detector and the op-amp, and it is much better.
I would be interested if you could mail me the schematic of your loop filter
;-) (mail it to )

Damien

I had used that chip for several years, with a 2.5kHz reference frequency,
and never ran into those kinds of problems. My first iteration of that
synthesizer did have some high level reference sidebands (-40dBc), but
after
I redid the board with a top layer ground plane, all of the problems
cleared
up. Are you using a ground plane on your board? Are you also using that
anti-slewing filter at the op-amp input, whereby you split one of the Rs,
and use a shunt C between the two?
If you would like a schematic of my loop filter, give me a shout.

Pete