The S-meter is essentially a milliammeter designed to indicate signal level. Usually the dial is calibrated from 0 to 9, with each suc
Signal Strength Meter
ceeding point indicating a doubling of signal level.
Notice the two common methods of S-meter placement. At A above, the meter is placed in the plate circuit of a variable-mu tube. A movement of 0-200 microamperes is used, and a variable shunt resistor makes possible zero adjustment for no signal response. At B, the S-meter is located between cathode and ground. It uses a movement of 0-1 milli-
ampere in series with a variable resistor. It is in parallel with the cathode bias resistor.
In both cases, the tube operates class A. Therefore, the only time there is a change in plate current is when AVC is applied to the grid. With a large signal, the AVC voltage becomes greater and increases the bias on the tube. This reduces the plate current and causes the proper reading on the S-meter. Consequently, the meter must be arranged to show
DETECTOR—AVC—AMPLIFIER I s-meter I
DETECTOR—AVC—AMPLIFIER I s-meter I
S-Meter With Separate Amplifier 57
minimum signal strength when current is maximum (no signal and no AVC), and maximum signal when current is minimum (maximum signal and maximum AVC). The meter movement is the reverse of that in the usual milliammeter. To read left to right on increasing signal strength, the meter deflects from right to left when current is increased. Such a meter is called a right-zero meter.
Sometimes the S-meter is used with a separate amplifier circuit. Notice in the diagram at the bottom of page 57 that a multielement tube is used. The AVC voltage is applied to the control grid of the pentode section of the tube. Since the AVC voltage is negative, an increase in signal strength causes a decrease in plate current.
Only a small part of the current, probably about one tenth of the total tube current, flows through the meter. The meter response is not perfectly linear, but the meter dial is calibrated according to the characteristics of the tube used in the amplifier circuit.
The S-meter can also be located in the AVC rectifier, as shown below. A portion of the diode current flows through the meter. Here, signal strength is in proportion to current flow, and the meter is calibrated for left-to-right deflection.
Though not as common as S-meters, electron ray tubes are used in some communications equipment. The electron ray tube is not intended as an accurate signal strength indicator. It is a tuning indicator that shows when a signal is tuned to maximum strength. As shown in the circuit diagram below, the indicator responds to the AVC voltage applied to the grid of the triode section.
The triode amplifies the AVC signal. But the plate of the triode section serves also as control grid of the indicator section. (Notice the connection between the plate of the triode section and the grid of the indicator section.) The indication on the fluorescent target is controlled by the difference in potential between the deflecting probe (the control grid of the indicator section) and the fluorescent target (the plate of the indicator section).
If the probe and target are at the same potential, the electrons flowing from the cathode to the target are not deflected. Consequently, the electron shadow of the deflecting probe appears only as a dark line on the target . This is the condition that exists when a large AVC voltage prevents the triode section from conducting.
With little or no AVC (little or no signal if input
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