S-meter

An S meter (signal strength meter) is an indicator often provided on communications receivers, such as amateur radio receivers or shortwave broadcast receivers. The scale markings are derived from a system of reporting signal strength from S1 to S9 as part of the R-S-T system. The term S unit can be used to refer to the amount of signal strength required to move an S meter indication from one marking to the next.

Analogue S meters are actually sensitive microammeters, 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 AGC 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 integrated circuits for FM reception like CA3089 and CA3189 provide a DC signal to drive a 100 μA S meter.

In most consumer grade desktop tube receivers and in some military receivers the S meter was replaced with a magic eye tube.

In 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 International Amateur Radio Union (IARU) Region 1 agreed on a technical recommendation for S Meter calibration for HF and VHF/UHF transceivers in 1981.

IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms.

...
Wikipedia