S meters (strength meters, signal-strength meters) measure the strength of signals in communication systems such as receivers and transmitters. Typically, a portion of the rectified intermediate frequency (IF) signal from a receiver’s detector stage is applied to an ammeter. If the receiver has an automatic gain control (AGC) amplifier, this stage can be used to drive the S meter and control the gain of the receiver. S meters are calibrated in terms of S units up to a mid-scale reading of S 9. The upper half of the S meter scale is calibrated in decibels (dB) above S 9 up to 60 db. One S unit represents a 6-decibel (dB) change in received signal strength, and a reading of 60-dB indicates that the strength of the received signal is above about 50 millivolts (mV).