Conversion of Receiver Sensitivity From Microvolts to dBm by James W. Voice communication receiver sensitivity is often stated in terms of the radio-frequency voltage level at the input necessary to produce a particular signal-to-noise ratio of the desired signal at the audio-frequency output.
Contents.Technical description Analogue S meters are actually sensitive, with a full scale deflection of 50 to 100 μA. In AM receivers, the S meter can be connected to the main detector or use a separate detector at the final IF stage. This is the preferred method for CW and SSB receivers. Another approach in the days of electronic tubes (valves) was to connect the S meter to the screen grid circuit of the final IF amplifier tube. A third option is to connect the S meter to the line through a suitable level conversion circuit.In FM receivers, the S meter circuit must be connected to the IF chain before any limiter stages. Some specialized for FM reception like CA3089 and CA3189 provide a DC signal to drive a 100 μA S meter. IARU Region 1 Technical Recommendation R.1.
LCD emulation of an S meter on the Ten Tec Jupiter transceiverIn the 1930s, it was already agreed that S9 corresponds to 50 μV at the input terminal of the receiver, but this was not a measure of the power received as the input impedance of receivers was not standardized.The (IARU) Region 1 agreed on a technical recommendation for S Meter calibration for and / transceivers in 1981.IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73. This is a level of 50 at the receiver's antenna input assuming the input of the receiver is 50.For VHF bands the recommendation defines S9 to be a receiver input power of -93.
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VHF Managers Handbook, Version 5.42. P. 116.Further reading.