Okay, but as it isn't constant over the tuning range, do I have to use the
average value of it in the formulas of the filter ?


VCO gain is basically the first derivative (slope) of the tuning voltage
vs.
frequency function. It's rare to find a VCO that tunes truly linearly, so
it will differ over the tuning range.


There are two ways I've plotted the tuning voltage vs. frequency. One
of them is the obvious way; connect a frequency counter to the output
of the VCO (taking care not to load the VCO output inappropriately;
you'll want to simulate the conditions under which the VCO will actually
be used), and then vary the tuning voltage very carefully, plotting the
voltage vs. frequency. From this you can calculate the tuning gain.

Another way, and it might be easier, is to include the VCO in a PLL.
Guess at
the parameters and you'll probably get a loop that locks up. As long as
the loop
is in lock, you can vary the frequency by programming the PLL and measure
the
tuning voltage. As long as your loop reference is reasonably accurate
you'll
get at least as good results as if you used a frequency counter and I
believe this
method is better since you're measuring the VCO gain in the system.

Enjoy!
Dana K6JQ