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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Logarithmic quasi-peak indicator on the K1003PP1 chip. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment Our magazine has repeatedly addressed the topic of developing audio signal level indicators. This time, the author of the article offers an interesting version of a logarithmic indicator on a microcircuit designed to build linear scales. The device uses an original input signal rectifier, which provides a clear fixation of the peak level. The importance of using quasi-peak indicators in sound recording and broadcasting was discussed in detail in [1]. In the same article, a diagram of a variant of such a device was proposed, in which imported microcircuits form a logarithmic scale. However, the domestic dual-mode microcircuit K1003PP1 [2] allows you to build a logarithmic indicator that is no worse. The scheme of the proposed device is shown in fig. 1. The input full-wave rectifier, as in [1], is built on the K157DA1 chip.
When a short pulse of 3H oscillations appears at the input of the device, the capacitor C3 is charged to a higher voltage than C2, the transistor VT1 closes. Capacitor C2, charged almost to the peak voltage of the input signal, slowly discharges with a time constant τ1 = C2R5 = 2 s (curve 1 in Fig. 2). Capacitor C3 discharges much faster - with a constant τ2 = C3R3 = 0,2 s (curve 2). When the voltage at C3 becomes 0,6 V less than at C2 (shift by 0,6 V in Fig. 2), without choosing the ratio of the amplitudes of the output signal of the channels of the DA1 microcircuit, you can change the ratio of the peak level indication time and the decay time (see curve 3 in Fig. 2. It can also be adjusted by changing the time constants τ1 and τ2. We also note that the resistor R5 can be excluded altogether (R5 = ∞), in this case, during the indication time interval, the voltage across the capacitor C2 will be practically unchanged . Such a construction of a quasi-peak detector circuit is useful in that the indication and decay time do not depend on the signal level. At the same time, when the rectifier capacitor is discharged by direct current [1], the indication time (which is rather arbitrary, since the signal on the capacitor begins to fall off immediately after the end of the input pulse) is the shorter, the smaller the amplitude of the input signal peak. The generated output voltage of the rectifier amplifies approximately three times the op-amp DA2, after which it goes to the indicator on the DA3 chip and the LEDs HL1 - HL12. To provide a logarithmic indication mode, the input voltage through a divider formed by resistors R8 - R10 is fed to the UB input of the DA3 chip, which determines the upper level of the input signal indication. Therefore, as the input signal increases, the voltage at the UB input increases, which stretches the scale and makes it close to logarithmic. The calculation of the parameters of the elements is simple. Let the voltage at the output of the op-amp DA2, equal to 6 V, should correspond to the glow of the HL12 LED (+4 dB), the voltage, 3 times less, U2 = 2 V (by 10 dB) - HL7 (-6 dB), and another 4 times smaller U1 = 0,5 V (by 12 dB) - HL1 (-18 dB). From the description of the operation of the K1003PP1 chip given in [2], it follows that the number of the next turning on LED can be calculated by the formula NCB = 13(UBX - UH)/(UB - UH). where IV, UH, UB are the voltages at the inputs of the microcircuit UBx, UH, UB, respectively. Substituting the above points into this formula and taking into account that UB = UB0 + k UBX (UB0 is the input voltage UB at UBX = 0), you can get a system of three equations with three unknowns: k, UH, UВ0. The result of its solution are the following values: k = 0,765, UH = 0,353 V, UBO = 1,88V. On fig. Figure 3 shows graphs illustrating the correspondence of the number of the glowing LED to the level of the input signal in decibels at various values of k. It can be seen that for the calculated value of k = 0,765, the dependence is close to linear, and the "scale division" is about 2 dB within the entire displayed range. If, on the other hand, greater reading accuracy is required in the upper part of the scale, by reducing the value of k to 0,25, you can get a "scale division" in the upper part of 1 dB, and in the lower part - 5 dB, while maintaining the indication range of about 22 dB. Practically in the device according to the scheme in Fig. 1 coefficient k determines the ratio of the resistance of resistors R8 - R10 (moreover, R9 \u10d R12), and the voltage UH can be set with a tuning resistor R0. The voltage UB8 will then be set automatically. With the selected value of k, the resistor R8 can be calculated using the formula R0,5 = 9R1 ( 1 / k - XNUMX). Shown in fig. 1 connection of LEDs ensures the formation of a luminous line of variable length. If it is desirable to get a scale with one luminous dot, it is enough to connect the cathodes of the LEDs to the corresponding outputs of DA1, and the anodes to the +12 V circuit [2]. Each channel of the stereo amplifier indicator is assembled on a 100x65 mm printed circuit board made of one-sided foil fiberglass (Fig. 4).
The board is designed for the use of MLT resistors, tuning resistors - SPZ-19a, K73-17 capacitors for an operating voltage of 400 V (C2 and C3), KM-5 and KM-6 (the rest). It is also possible to use LEDs of the AL307BM and AL307NM series, but before installation, their body with a diameter of just over 5 mm must be sawn to a size of 5 mm. If you use LEDs with a luminous surface size of 2,5x5 mm (for example, the KIPM01 series), and capacitors C2 and C3 for a voltage of 63 V, you can significantly reduce the height of the board. For mounting the DA1 chip, it is better to use a socket, since its parameters deteriorate from the slightest overheating [1]. Before installing the LEDs, their leads were bent at right angles so that their axes were parallel to the PCB. The LEDs on the left channel board are installed on the side of the microcircuits, on the right channel board - on the side of the printed conductors. The boards are placed perpendicular to the front panel of the amplifier. Setting up the indicator is easy. First, a sinusoidal signal with a frequency of about 1000 Hz and a voltage corresponding to the level of +4 dB should be applied to its input, with a trimming resistor R1 to achieve the glow of HL12 "in the glow floor", and then reduce the input voltage by 12 times (by 22 dB) and resistor R12 set the same brightness HL1. Since the adjustments are dependent, repeat the indicated operations one or two more times, after which, with the resistor R1, refine the calibration at an input signal level of 0 dB. The sensitivity of the indicator at the level of +4 dB is 80 ... 100 mV. If it is necessary to obtain a significantly lower sensitivity, a resistor should be installed in series with the capacitor C1, which forms the necessary divider with R1. Literature
Author: S. Biryukov
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