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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 RST code. 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.
==Technical description==
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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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