RF detectors have two distinct transfer functions. The blue trace on the left has a so-called linear-in-dB transfer function. So, as the input power changes dB for dB, the output voltage, scaled on the left axis, changes by a constant amount. As a result, devices like this are generally talked about having a constant with a unit of millivolts-per-dB. In contrast to this, the black trace on the right does not have a constant millivolts-per-dB slope. Instead it appears to increase exponentially on this plot. However, if this transfer function were to be plotted on a volts out versus volts in plot, it would show a straight line. This has given rise to the term linear in volts-per-volt detector. This linear in volts-per-volt transfer function, by the way, is the transfer function that would be expected from a discrete diode-based RF Detector. RF detectors with linear in volts-per-volt transfer functions tend to have lower dynamic range due to the fact that their transfer functions flatten out at lower input power levels.