Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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The vast majority of simple and cheap car radios, which include, for example, "ALAN-100+", "S-mini", do not have a built-in S-meter. When working with these stations, it is impossible to objectively assess the signal strength of the correspondent. Therefore, many owners sooner or later come to the need to install an S-meter on their radio.

A simple solution to the S-meter problem is to set the scale for the squelch resistor, as described in the article "Simple improvements to CB radios" (Radio, 1997, No. 4, p. 72,73). However, this will cause inconvenience when using, so it is still preferable to build in a pointer or LED scale S-meter. But here you will inevitably encounter the difficulty of installing the device on the front panel of the radio station. Is there another solution to this problem? In my opinion, there is. The indicators can be placed in the mains power supply housing, which is usually used when operating the station in stationary conditions, or somewhere on or near the instrument panel.

The ALAN-100+ and similar radios have an AM detector on a diode, to which you can directly connect an S-meter based on a microammeter. But I recommend not to do this, as the normal operation of the AM detector may be disturbed. It is better to include it through the buffer cascade, as described in the article mentioned above.

Still, it is best to install an additional detector, as shown in Fig. 1. To reduce its effect on the AM detector of the radio station receiver, the diode is connected in the opposite direction, i.e., a positive voltage is removed from it. Capacitor C1 is necessary to filter the voltage of the inverter, and resistor R1 is used to calibrate the scale of the device. Tests have shown that in a radio station modified in this way, a reliable indication of the input signal level up to S9 + 40 dB is possible, therefore this value was chosen as the maximum for the S-meter scale.

The author used a small-sized M4247 microammeter with a total deflection current of 100 μA and a resistance of 3 kOhm. The relationship between the readings of the microammeter and the level of the input signal in points is shown in fig. 2 (curve a). It can be seen that the scale turns out to be non-linear, and besides, it is used only by two-thirds, since the S1 level corresponds to readings of 32 μA. If, in series with the RA1 microammeter, you turn on the VD2 diode (shown in dotted line), the same as VD1, then the scale will become more convenient. The dependence for this case is shown in Fig. 2 (curve b). It is possible that when installing an additional diode, the scale will not be fully used, then you need to install another diode, the same, or germanium, for example D9.

Establishing an S-meter comes down to setting the instrument's pointer to the final mark of the scale when a signal with the maximum displayed level is applied to the radio station input. Then the scale is calibrated according to the signals of the reference RF generator. Diode VD1 and capacitor O must be soldered by surface mounting directly to the radio board from the side of the printed conductors. Trimmer resistor R1 and microammeter PA1 can be placed in the power supply housing. The connection between the radio station and the unit must be made only with a shielded wire.

In the S-meter, it is permissible to use almost any microammeter with a total deflection current of 100 ... 200 μA. For use at home and in a car, you can make two similar S-meters, while a diode VD1 (if necessary, and VD2) is placed in the radio station, a capacitor O, and a tuned resistor R1 is installed next to the microammeter. On the rear panel of the radio station, you need to install a small-sized connector, for example, from small-sized phones, where there is a ready-made hole. The microammeter installed in the car, in addition to the main function, can also perform others: measuring voltage, charging current, etc.

When installing an S-meter in a car, it should be borne in mind that not all dial gauges can withstand vibration and shaking, and it is not always convenient to read readings from them while the car is moving. In this case, the LED scale S-meter will be more reliable and convenient. The easiest way to make it is on the basis of a specialized microcircuit, such as A277D, or its complete domestic counterpart K1003PP1. The scheme of such an S-meter is shown in fig. 3.

The device provides indication of 12 input signal levels from S1 to S9 + 40 dB in the form of a continuous horizontal or vertical scale of LEDs. The number of LEDs lit is proportional to the level of the input signal.

For such an S-meter, it is necessary to install an additional detector on the VD1 diode and an R1C1 filter on the radio station board, as described for the switch version. At the same time, the time constant of the R1C1 circuit is chosen large enough to average the readings, especially when receiving AM signals.

For normal operation of the microcircuit, its conclusions 3 and 16 must be supplied with a stable reference voltage. In the case of using the radio station in a stationary version and when powered by a stabilized unit, this voltage comes directly from the power output through resistive dividers R2R5 and R3R6. When used in a car, the supply voltage from the battery will be unstable, so the right (according to the diagram) terminals of the resistors R5 and R6 must be connected to the power bus of the radio receiver (transistor 017 emitter), and the resistors R5 and R6 themselves must have a resistance of 5,1 kOhm.

The S-meter works as follows. When the signal level at the receiver input is one point, the HL1 LED lights up. As the signal increases to the level of S9+40 dB, all other LEDs light up sequentially, i.e. the entire column is highlighted. Such a scale can be much more convenient for quick readings, especially if you use LEDs of different glow colors.

All parts of the S-meter, except for VD1, R1 and C1, are placed on a printed circuit board, a sketch of which is shown in fig. 4.

The microcircuit and resistors are installed on the side of the printed conductors, and the LEDs are installed on the opposite side. In the device, it is better to use rectangular LEDs in a plastic case, for example, the KIPMO1 and KIPM02 series with letter indices A, B (red), and C, D, D (green). Imported LEDs similar in design are also applicable, it is only necessary that their operating voltage does not exceed 2 ... will be worse. Trimmer resistors R2,5 and R307 - SDR - 341, constant - mlt.

If the radiating area of ​​the LEDs is small, then the digital designations are applied on the front panel next to the LEDs, if this area is at least 5X5 mm, then the digital designations are applied directly to them. such as black paint. As noted earlier, it is convenient to use LEDs of different glow colors, for example, up to S8 inclusive - green, and from S9 and above - alternately red and green. There are many such options and therefore the radio amateur can choose them at his discretion. But first you need to calibrate the scale.

Calibration is carried out as follows. In parallel with the capacitor C1, a DC voltmeter is connected, preferably with an input resistance of at least several hundred kilo-ohms, and by applying signals from the S1 level to the S9 + 40 dB level to the input, the DC voltage is measured. This should be done in the middle of the frequency range (18 - 20 channels). Then, resistor R2 sets the voltage at pin 16 of the DD1 chip, which is approximately equal to the minimum measured value, and resistor R3 at pin 3 sets the maximum measured value. Then, the signal level S1 is applied to the input and the resistor R2 causes the HL1 LED to light up, and by applying the level S9 + 40 with the resistor R3, the HL12 lights up. The last step of the setup should be repeated 2 - 3 times and then remove the relationship between the number of lit LEDs N and the input signal level. After that, you can at your discretion determine the color of a particular LED.

The obtained dependence is shown in fig. 5 (curve a). In principle, it could already be successfully used, but still, according to the author, it is not very convenient due to some unevenness. Therefore, an attempt was made to make the scale more uniform and to simplify the device. It should be noted that for a different instance or type of radio station, the dependence may turn out to be different, so do not rush and immediately do the option described below.

In this variant, a constant voltage from the output of the detector of the threshold noise reduction system, namely, from the collector of transistor Q7, was chosen as the signal at the input of the microcircuit. Measurements showed that when the signal level changes from SI to S9 + 40 dB, this voltage changes from 3,4 to 1,6 V, i.e., as the input signal increases, the voltage decreases. Since the standard inclusion of the microcircuit allows only an increasing positive voltage to be indicated, it was necessary to develop a non-standard circuit in which the measured voltage is supplied to the inputs intended for supplying the reference voltage, and the reference voltage is supplied to the input for supplying the measured voltage. This made it possible to make the microcircuit work "in reverse" - with a decrease in the input positive voltage, the number of burning LEDs increases. A fragment of the modified circuit diagram is shown in fig. 6. It can be seen that the device has been simplified, since there is no need to install an additional diode detector on the radio station board.

The scale is calibrated in a similar way, that is, when the input signal level changes from S1 to S9 + 40 dB, the constant voltage at the collector of transistor Q7 is measured. Resistor R1 set at pin 17 DD1 voltage equal to the minimum measured. Then, the level S1 is applied to the input of the radio station and the resistor R3 causes the first LED to light up, and by applying the level S9 + 40 dB, the resistor R1 causes the last LED to light up.

All calibration work must be carried out carefully and repeated several times, after which the relationship between the input signal level and the number of burning LEDs is already removed. The author obtained the dependence shown in Fig. 5 (curve b). In conclusion, you can choose the color of the glow of specific LEDs.

A feature of the latter option is that in the transmission mode ("TX") all the LEDs of the scale will light up. If this turns out to be unnecessary or the radio station will be operated in a car, then the output of the resistor R1, right according to the diagram, must be connected to the power output of the radio station receiver, as mentioned earlier, using a 5,1 kΩ resistor.

The connection of the LED S-meter to the radio station must be made with a shielded wire. The device draws about 9 mA when the LEDs are off and 60 mA when they are all on.

The microcircuit allows for smooth adjustment of the brightness of the glow of all LEDs at the same time. To do this, a variable or trimming resistor with a resistance of 22 ... 47 kOhm must be installed between the power output and the common wire, and the upper output of the resistor R4 according to the diagram should be connected to its engine.

Author: I. Nechaev, Kursk

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