[Julesomega]

When you say "linearity" do you mean flat frequency response, or amplitude linearity? Frequency response is really no problem, but a linear amplitude response will be difficult to achieve with a diode detector at signal levels below about .5V P-P. If you want accurate voltage measurements at lower levels you should look out for an AC millivoltmeter, which all the major manufacturers made. Mark Hennessy listed some buget DVMs in another thread recently, where he lists some currently available that measure accurately to below 10mV RMS. I am rather fond of the "Advac" range, and Farnell and Philips models.