Woohoo ! I finally found, I had used 92 ohms resistors instead of 92 Kohms
resistors... how inattentive I am sometimes ;-) But it isn't yet perfect,
I've took a picture of the oscilloscope, showing the output signal that is
fed into the VCO (http://www.mcarsweb.com/_divers/oscillo.jpg the vertical
scale is enlarged). It is slightly varying, at a frequency of about 187 KHz.
I don't know if I should change the filter values, or add a additional small
filter, with a resistor and a capacitor between the output and the ground.
What do you think about that ?

Damien