SDRs (Software Defined Radios) acquire and process signals differently, and determine S-readings by direct measurement of RF signal amplitude. Consequently, many SDR systems with bit depths of 14-bits or more are accurately calibrated from one end of the S scale to the other right out of the box. In cases where this is not so, a few minutes with a signal generator to set the reference level are all that is required. Low bit depth SDRs such as an 8-bit design can be somewhat accurate, but as they distinguish much coarser differences in input levels, precision at the low end of the S scale will suffer.