Problem with designing a PIN diode attenuator

Hallo
I try to design PIN diode attenuator and the problem is that in every version I've tryied the attenuation was nonlinear dependent to control voltage or current. Dependence of attenuation vs conrol voltage (current) MUST be linear in this design, and that is my problem. Maybe some one of you can help me?

Buy a linearizer from these guys:
http://www.impellimax.com/

try this link may it help
http://www.radio-electronics.com/inf...attenuator.php

visit the www.semiconductor.agilent.com to get the HSMP3814 attenuator example!
there about 16dB linear dynamic region! if you have more wide linear dynamic region, you can use a table to calibrate it,good luck!