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).
Selecting S Meters
Selecting S meters requires an analysis of performance specifications such as frequency response, sensitivity, gain, detectable power range, power dissipation, display type, and operating temperature. Frequency response is a measure of the effectiveness at which applied frequencies are transmitted. It is the range of frequencies for which s meters maintain a constant sensitivity within defined boundaries. Variations from this linear or flat sensitivity are given in decibels (dB). S meter sensitivity is a measure of a s meter responsiveness. Gain is the factor by which the input signal is multiplied. Gains are frequently greater than unity, but may be fractional when a reduction (attenuation) of signal amplitude is desired. Power dissipation is the amount of power that is consumed and converted to heat. Display types for s meters include analog devices such as meters with needles, and digital products with alphanumeric displays.
Applications and Industries
S meters are used in a variety of applications and industries. Some products are used in antenna development, experiments, and adjustments. Others are used to measure the loss of television coaxial (coax) cables in buildings, or to detect crosstalk from electronic instruments due to radio frequency interference (RFI). S meters are also used to determine RF levels in radio rooms and enclosed areas, and detect 60-cycle noise from motors. They can be used as power meters in very low power applications, and as a way to define the difference between balanced and unbalanced transmissions.