Arrow S-meter for CB radio station. Scheme, description

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Arrow S-meter for CB radio station. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The S-meter built into the radio usually has a coarse scale, and inexpensive models simply do not have it. The article describes an easy-to-make S-meter with a dial indicator, which a radio amateur can independently enter into almost any CB radio station.

The scheme of the S-meter is shown in fig. 1. The intermediate frequency amplifier is made on the transistor VT1. A dynamic load is introduced into the collector circuit of the transistor: a resistor R3 and a source-drain channel of the field-effect transistor VT5 connected in parallel (through R2). In the absence of a signal, the channel resistance is high, since VT2 is closed by the bias voltage at the source, which sets the divider on resistors R5R6. A constant voltage from the rectifier on diodes VD1VD2 is supplied to the gate of the field-effect transistor VT3. An indicator is included in the drain circuit of this transistor - a pointer microammeter RA1.

Pointer S-meter for CB radio station

The same voltage is supplied to the gate of the transistor VT2. With an increase in the signal level of the received radio station, the transistor VT2 opens, reducing the total load resistance in the collector circuit VT1. Accordingly, the gain of the cascade on the transistor VT1 also decreases. This expands the range of the S-meter in terms of signal level.

The device uses resistors of the MLT 0,125 type. Capacitors C1 - KD, the rest - KM6. Indicator RA1 - M4248 microammeter with a total deflection current of 100 μA.

The dimensions and configuration of the board on which the S-meter is mounted depend, of course, on the available space in the radio itself. It can be, for example, as shown in Fig. 2. The PA1 S-meter indicator will most likely need to be taken out: there is usually no free space for it on the front panel of the station.

Pointer S-meter for CB radio station

Resistor R10, shunting microammeter PA1, is selected so that the total deviation of the arrow PA1 corresponds to the maximum possible signal at the input of the radio station. In order for the arrow of the device to be at zero in the absence of a signal, the transistor VT3 is closed by selecting the voltage at its source with resistors R8 and R9.

By selecting some resistors, you can modify the characteristic of the S-meter, the view of which is shown in Fig. 3. Resistor R3 affects the slope of the characteristic in the initial part (i.e., the sensitivity of the S-meter to weak signals), R5 - the slope after the inflection (i.e., the sensitivity of the S-meter to strong signals), R6 affects the beginning of the inflection.

Pointer S-meter for CB radio station

The author tested the operation of the S-meter together with the radio station "TAIS-RM41" [1]. The connection point is terminal 1 of the K174UR7 microcircuit.

With the ratings indicated on the diagram, the dynamic range in terms of the voltage level of the inverter at the input of the S-meter is 10 ... 1200 mV. If necessary, this interval can be shifted to the region of larger or smaller values by selecting capacitor C1. So, for example, at С1=39 pF this range will be 20...2400 mV.

The S-meter installed in the radio station and working in conjunction with its AGC, which in its own way "compresses" the signal in the RF and IF path, of course, will need to be calibrated. This is done by applying high-frequency signals with levels corresponding to the S-scale to the antenna input of the station (see, for example, [2]).

Literature

Vakhrushev S. Radio station "TAISRM41". - Radio, 1998, No. 6, p. 87.
Vinogradov Yu. S-meter calibrator. - Radio, 1998, No. 7, p. 74, 75.

Author: Yuri Vinogradov, Moscow

See other articles Section Civil radio communications.

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