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Shown here is a closer look at the transfer function of a linear-in-dB RF detector. The three curves that run from top left to bottom right represent the output voltage versus input power at 25ºC, -40ºC and +85ºC. At 25°C, the output voltage of the detector ranges from around 1.9V at –60 dBm input power, down to 0.6V at -10 dBm. Now, notice how the transfer function closely follows an imaginary straight line which has been laid over the trace. Now at the extremities, it is shown that the transfer function deviates from this straight line. A quick calculation suggests that this detector has a slope of approximately –25 millivolts-per-dB, that is, a 1dB change at the input power will result in a -25 mV change in output voltage. This slope is constant over the linear portion of the dynamic range. So within this range, the equation used to express the operation is: Vout = SLOPE × (PIN – INTERCEPT), where INTERCEPT is the point at which the extrapolated straight line crosses the x-axis of the plot, around +10 dBm in this case. So, the transfer function of the detector can be modeled using a very simple first-order equation. This is useful as it will allow the transfer function of the detector to be established by applying and measuring as few as two different power levels during the calibration procedure. Next, consider the behavior of this detector over temperature. It is quite difficult to see from this plot but for a particular input power level, the output voltage changes as it goes from ambient temperature to either –40°C or +85°C. If this voltage were to change by 25mV, it would be considered that there was a deviation of 1dB, remembering from earlier that the slope of the detector is -25 millivolts-per-dB. The traces that run from the bottom left to top right express the deviation at ambient, hot and cold with respect to the straight line fit. So these traces capture both the non linearity of the Vout traces along with the temperature drift.
