Hello,
About the quartz, I haven't used a 10.24MHz one as in the given schematic,
but a 5.12 MHz one. The R ratio is 1024, so the reference frequency is 5
KHz. The N ratio used is from 196 to 284 (with the 64/65 prescaler; VCO goes
from 63 to 91 MHz). I don't think the loop is unstable, because the VCO
output is quite clean (only a bit blur, see
http://www.mcarsweb.com/_divers/vco.jpg).
The loop filter values have been calculated with the fomulas given in the
motorola application note 980.

So, if you think I should use a additional RC network, could you tell me how
to calculate the values :-$ ?
Thank you ;-)

Damien