home Solution Amy tone generator. The simplest audio frequency generator How to make a tone crystal oscillator

Amy tone generator. The simplest audio frequency generator How to make a tone crystal oscillator

Figure 1 shows a diagram of a simple generator, designed mainly to test low-frequency equipment and determine faults in it.

The generator has one fixed frequency of 1000Hz, the value of which is set by the resistor R1. The output signal level is determined by the position of the slider resistor R13. The circuit has a system for supporting the output signal at a certain level, consisting of elements VT1, VD2, R10, R11, C6. The level of operation of the automatic output voltage maintenance system is set using the resistor R11. The harmonic coefficient of this generator is relatively large, so that it can be used to measure the non-linear distortions of low-frequency equipment. Therefore, at the output of this generator, you need to install a low-pass filter - LPF. Such a filter. Complete with a low-pass filter, this generator has a very clean tone signal with a THD level in thousandths of a percent. The generator must be powered from a stabilized source direct current with a voltage of 5 ... 12V. The schematic and PCB drawing can be downloaded here.

28.07.2018
The figure shows a diagram of a simple and very easy-to-use thermostat, DS18B20 is used as a sensor, and the controller is controlled using a ky-040 encoder. The integral temperature sensor DS18B20 has a temperature measurement range from -55 to + 125 ° C, the temperature readings are displayed on the first line of the indicator 1602 HD44780, the controller readings are displayed on the second line of the indicator ...
29.09.2014
receiver on field effect transistors receives a radio signal in the range of SV and LV. Receiver sensitivity 1…3mV/m SW and 2…5 mV/m LW. Pout=250mW, Icon=10mA(65mA max). The radio receiver can operate with a voltage drop of up to 4 V. The receiver consists of a 3-stage HF (T1-T3), detector (D1 D2) and VLF (T4 T7). Increased sensitivity and output power achieved…
20.09.2014
Twice the author had to deal with the simplest, but very unpleasant malfunction of household microwave ovens: a breakdown of a protective mica plate covering the exit of the magnetron waveguide into the oven's frying chamber. Probably, the mica plate contained metal inclusions that evaporated during the operation of the furnace magnetron, which led to a breakdown of the mica. The place of breakdown was charred, and the operation of the furnace became ...
13.10.2014
Main technical characteristics: Rated output power at load resistance: 8Ω - 48W 4Ω - 60W Frequency response with frequency response unevenness of not more than 0.5 dB and output power of 2 W - 10 ... 200000 Hz Nonlinear distortion factor at rated power in the range of 20 ... 20000 Hz - 0.05% Rated input voltage - 0.8V Output ...

It is better not to explain, but to see everything at once:

Funny toy, isn't it? But seeing is one thing, and doing it yourself is another, so let's get started!

Device diagram:

When the resistance between the PENCIL1 and PENCIL2 points changes, the synthesizer produces a melody of various tonalities. Parts marked * can be omitted. Instead of the transistor T1, KT817 is suitable; BC337, instead of Q1 - KT816; BC327. Please note that the pinout of the transistors of the original and analogues is different. Download ready printed circuit board available on the author's website.

I will assemble the circuit very compactly (which I do not advise beginners to do) on a breadboard, so I present my version of the circuit layout:

On the other hand, everything looks less neat:

I will use the button from the surge protector as the case:

In the body:

I fixed the speaker and the crown contact block on hot glue:

Complete device:

I also came across a simplified scheme:

In principle, everything is the same, only the squeak will be quieter.

Conclusions:

1) It is better to use a 2M pencil (double soft), the drawing will be more conductive.

2) The toy is interesting, but tired after 10 minutes.

3) Once the toy is tired, then you can use it for other purposes - ring the circuit, determine the approximate resistance by ear.

And finally, another interesting video:

Tone dialing (Dual-tone multi-frequency signaling, DTMF) was developed by Bell Labs in the 50s of the last century for a revolutionary at that time push-button telephone. To represent and transmit digital data in tone mode, a pair of frequencies (tones) of the speech frequency range is used. The system defines two groups of four frequencies, and the information is encoded by the simultaneous transmission of two frequencies - one from each group. This gives a total of sixteen combinations to represent sixteen different numbers, symbols, and letters. Currently, DTMF coding is used in a wide range of communications and control applications, as confirmed, for example, by Recommendation Q.23 of the International Telecommunication Union (ITU).

This article describes a DTMF tone generator circuit that reproduces all eight frequencies and generates the resulting two-tone output signal. The system in question was built around a Silego GreenPAK™ SLG46620V chip and Silego SLG88104V op amps. The output signal is the sum of the two frequencies defined by the row and column of the telephone keypad.

The proposed scheme uses four inputs to select the generated frequency combination. The circuit also has an enable input that triggers the generation and determines the length of time the signal is transmitted. The generator output frequency complies with the ITU standard for DTMF.

DTMF tones

The DTMF standard defines the encoding of the numbers 0-9, the letters A, B, C, and D, and the characters * and # as a combination of two frequencies. These frequencies are divided into two groups: the high frequency group and the low frequency group. Table 1 shows frequencies, groups, and corresponding symbol representations.

Table 1. DTMF Tone Coding

	Treble Group
Low frequency group

Frequencies were chosen in such a way as to avoid multiple harmonics. Also, their sum or difference does not give a different DTMF frequency. In this way, harmonics or modulation distortions are avoided.

The Q.23 standard specifies that the error of each transmitted frequency should be within ± 1.8% of the nominal value, and the total distortion (due to harmonics or modulation) should be 20 dB below the fundamental frequencies.

The resulting signal described above can be described as:

s(t) = Acos(2πfhight) + Acos(2πflowt),

where fhigh and flow are the corresponding frequencies from the high and low frequency groups.

Figure 1 shows the resulting signal for the digit "1". Figure 2 shows frequency spectrum corresponding to this signal.

Rice. 1. DTMF tone

Rice. 2. Spectrum of DTMF tone signal

The duration of DTMF signals can vary depending on the specific application that uses tone coding. For the most common applications, duration values tend to lie between manual and automatic dialing. Table 2 shows short description the typical length of time for the two dial types.

Table 2. Duration of tone dialing signals

Set type	Treble Group	Treble Group
Set type
Hand set
Automatic dialing

For greater flexibility, the DTMF generator offered in this manual, is provided with an enable input, which is used to start signal generation and determine its duration. In this case, the duration of the signal is equal to the duration of the pulse at the enable input.

The analog part of the DTMF generator circuit

ITU Recommendation Q.23 defines DTMF signals as analog signals created by two sine waves. In the proposed DTMF generator circuit, the Silego GreenPAK SLG46620V chip generates square wave signals at the desired DTMF frequencies. To get sinusoidal signals required frequency and form the resulting signal (the sum of two sine waves), you will need analog filters and an adder. For this reason, in this project, it was decided to use filters and a combiner based on SLG88104V operational amplifiers.

Figure 3 shows the structure of the proposed analog part of the device.

Rice. 3. Analog processing circuit for receiving a DTMF signal

To receive sinusoidal signals from rectangular pulses analog filters are used. After filtering, the two signals are summed and the desired output two-tone DTMF signal is generated.

Figure 4 shows the result of the Fourier transform used to obtain the spectrum of a rectangular signal.

Rice. 4. Spectrum of a rectangular signal

As you can see, the square wave contains only odd harmonics. If we represent such a signal with amplitude A as a Fourier series, then it will look like this:

Analysis of this expression allows us to conclude that if analog filters have sufficient attenuation for harmonics, then it is quite possible to obtain sinusoidal signals with a frequency equal to the frequency of the original square wave.

Taking into account the interference level tolerance defined in the Q.23 standard, it is necessary to ensure that all harmonics are attenuated by 20 dB or more. In addition, any frequency from the low frequency group must be combined with any frequency from the high frequency group. Given these requirements, two filters were developed, one for each group.

Both filters were low pass Butterworth filters. The attenuation of a Butterworth filter of order n can be calculated as:

A(f)[dB] = 10log(A(f) 2) = 10log(1+(f/fc) 2n),

where fc is the filter cutoff frequency, n is the filter order.

The difference in attenuation between the lowest frequency and the highest frequency of each group can be no more than 3 dB, so:

A(fHIGHER)[dB] - A(fLOWER)[dB] > 3 dB.

Given absolute values:

A(fHIGHER) 2 / A(fLOWER) 2 > 2.

In addition, as we said earlier, harmonic attenuation should be 20 dB or more. In this case, the worst case will be the case of the lowest frequency in the group, because its 3rd harmonic is the lowest frequency and is closest to the filter's cutoff frequency. Given that the 3rd harmonic is 3 times less than the fundamental, the filter must meet the condition (absolute values):

A(3fLOWER) 2 / A(fLOWER) 2 > 10/3.

If these equations apply to both groups, then the filters used must be second-order filters. This means that they will have two resistors and two capacitors if implemented with op amps. With third order filters, the sensitivity to component tolerances would be lower. The selected filter cutoff frequencies are 977 Hz for the low band and 1695 Hz for the high band. With these values, the differences in signal levels in the frequency groups are consistent with the above requirements, and the sensitivity to changes in the cutoff frequency due to component tolerances is minimal.

Schematic diagrams of the filters implemented using the SLG88104V are shown in Figure 5. The ratings of the first R-C pairs are chosen in such a way as to limit the output current of the SLG46620V chip. The second filter element determines the gain, which is 0.2. The amplitude of the square wave sets the op-amp's operating point at 2.5 V. Unwanted voltages are blocked by the output filter capacitors.

Rice. 5. Schematic diagrams of output filters

At the output, the filter signals are summed, and the resulting signal is the sum of the harmonics selected from the group of low and high frequencies. To compensate for the attenuation of the filter, the amplitude of the output signal can be adjusted using two resistors R9 and R10. Figure 6 shows the circuit of the adder. Figure 7 shows the entire analog part of the circuit.

Rice. 6. circuit diagram adder

Rice. 7. Analog part of the circuit

Digital part of the DTMF tone generator circuit

The digital part of the DTMF tone generator circuit includes a whole set of square wave generators - one for each DTMF frequency. Since eight counters are required to create these generators, the GreenPAK SLG46620V chip was chosen for their implementation. At the exits digital circuit two square wave signals are generated, one for each frequency group.

Square waveforms are generated by counters and D-flip-flops and have a duty cycle of 50%. For this reason, the counter switching frequency is twice the required DTMF frequency, and the DFF flip-flop divides the output signal by two.

The clock source for the counters is the built-in 2 MHz RC oscillator, the frequency of which is additionally divided by 4 or 12. The divider is selected taking into account the bit depth and the maximum value of each counter required to obtain a specific frequency.

Fewer samples are required to generate high frequencies, so 8-bit counters are used for their formation, clocked from an internal RC generator, whose signal is divided by 4. For the same reason, lower frequencies are implemented using 14-bit counters.

The SLG46620V chip has only three standard 14-bit counters, so one of the lower frequencies was implemented using an 8-bit CNT8 counter. In order for the number of samples to fit in the range of 0 ... 255, to clock this CNT8, it was necessary to use the signal of the RC generator divided by 12. For this circuit, a frequency with largest number counts, that is, the most low frequency. This made it possible to minimize the error.

Table 3 shows the parameters of each square wave.

Table 3 Parameters of square wave generators

	Clocking	Frequency error [%]
Low frequency group



Treble Group

As can be seen from the table, all frequencies have an error of less than 1.8%, so they comply with the DTMF standard. These design characteristics, based on the ideal value of the RC oscillator frequency, can be adjusted to take into account the measurement of the RC oscillator output frequency.

Although in the proposed scheme all generators operate in parallel, the signal of only one generator from each group will be fed to the output of the microcircuit. The choice of specific signals is determined by the user. This uses four GPIO inputs (two bits for each group) with the truth table shown in Table 4.

Table 4 Frequency selection table from the low frequency group


Low frequency group

Table 5 Frequency selection table from the high frequency group


Treble Group

Figure 8 shows the logic diagram of the 852 Hz square wave generator. This pattern is repeated for each frequency with appropriate counter settings and LUT configuration.

Rice. 8. Rectangular pulse generator

The counter generates an output frequency determined by its settings. This frequency is equal to twice the frequency of the corresponding DTMF tone. The meter configuration parameters are shown in Figure 9.

Rice. 9. Example of setting the counter of the rectangular pulse generator

The output of the counter is connected to the clock input of the D-Flip Flop trigger. Since the DFF output is configured as inverted, if you connect the DFF output to its input, the D-flip-flop will be converted to a T-flip-flop. The DFF configuration options can be seen in Figure 10.

Rice. 10. An example of setting up a trigger of a rectangular pulse generator

The signal from the output of DFF is fed to the input of the truth table LUT. LUT truth tables are used to select one signal for each specific combination of R1-R0. An example LUT configuration is shown in Figure 11. In this example, if a "1" is applied to R1 and a "0" is applied to R0, the input signal is passed to the output. In other cases, the output is "0".

Rice. 11. An example of setting up the truth table of a rectangular pulse generator

As mentioned above, the proposed circuit has an enable input. If there is a logical unit "1" at the enable input, then the generated rectangular signals are fed to a pair of microcircuit outputs. The transmission duration is equal to the pulse duration at the enable input. To implement this function, several more LUT truth table blocks were required.

For the high band, one 4-bit LUT and one 2-bit LUT are used, as shown in Figure 12.

Rice. 12. Treble group output circuit

4-bit LUT1 configured as logical element OR, so it outputs a logical one "1" if any of its inputs have a "1". The C1/C0 truth tables allow only one of the generators to be selected, so the 4-bit LUT1 determines which signal is output. The output of this LUT is connected to a 2-bit LUT4, which only transmits a signal if the enable input is a logical "1". Figures 13 and 14 show the 4-bit LUT1 and 2-bit LUT4 configurations.

Rice. 13. 4-bit LUT1 configuration

Rice. 14. 2-bit LUT4 configuration

Since 4-bit LUTs were no longer available, two 3-bit LUTs were used for the low pass group.

Rice. 15. Bass group output circuit

The complete internal circuit of the GreenPAK SLG46620V is shown in Figure 16. Figure 17 shows the final circuit diagram of the DTMF generator.

Rice. 16. Block diagram of the DTMF tone generator

Rice. 17. Schematic diagram of the DTMF tone generator

Testing the DTMF Generator Circuit

At the first stage of testing the proposed DTMF generator, it was decided to check the frequencies of all generated rectangular signals using an oscilloscope. As an example, Figures 18 and 19 show square wave outputs for 852 Hz and 1477 Hz.

Rice. 18. 852Hz square wave

Rice. 19. 1477Hz square wave

Once the frequencies of all the square wave signals were checked, testing of the analog part of the circuit began. The output signals for all combinations from the group of low and high frequencies were investigated. As an example, Figure 20 shows the sum of 770 Hz and 1209 Hz signals, and Figure 21 shows the sum of 941 Hz and 1633 Hz signals.

Rice. 20. DTMF tone 770Hz and 1209Hz

Rice. 21. DTMF tone 941Hz and 1633Hz

Conclusion

In this article, a DTMF tone generator circuit based on the Silego GreenPAK SLG46620V chip and Silego SLG88104V operational amplifiers was proposed. The generator gives the user the ability to select desired frequency combinations from four inputs and control the enable input, which determines how long the outputs will generate.

Characteristics of the SLG46620V chip:

Type: programmable mixed signal IC;
Analog blocks: 8-bit ADC, two DACs, six comparators, two filters, ION, four integrated oscillators;
Digital blocks: up to 18 I/O ports, connection matrix and combinatorial logic, programmable delay circuits, programmable function generator, six 8-bit counters, three 14-bit counters, three PWM generators/comparators;
Communication interface: SPI;
Supply voltage range: 1.8…5 V;
Operating temperature range: -40…85 °C;
Box version: 2 x 3 x 0.55 mm 20-pin STQFN.

E. KUZNETSOV, Moscow
Radio, 2002, No. 5

Tone pulses can be used to check the dynamic performance of meters and levelers, as well as noise suppression devices. A stand with a tone pulse generator will also be useful in the study of amplifying and acoustic equipment.

Linearity frequency response and the accuracy of the readings of the level meters is easy to check using a conventional generator sound signals, but to check their dynamic parameters, a tone pulse generator (GTI) is needed. Such generators offered by radio amateurs often do not comply with the standards, where the frequency of the sinusoidal signal in pulses is taken to be 5 kHz for testing level meters (DUT), and the beginning and end of the pulses coincide with the signal transitions through "zero".

Similar problems arise when adjusting audio signal level autoregulators. The release time of 0.3...2 s is easy to see on the oscilloscope screen, but the response time of the limiter (limiter) or compressor can be less than 1 ms. To measure and observe transients in audio equipment, it is convenient to use the GTI. In this case, it is desirable to change the pulse filling frequency using an external tunable generator. For example, at a duty cycle of 10 kHz, the duration of one period is 0.1 ms, and when observing the operation process, determining the operation time is not difficult. Sound pulses from the GTI output should have a level difference of 10 dB.

In foreign literature, it is usually proposed to measure the response time with an abrupt increase in the signal level by 6 dB above the normalized value, but real signals have a significantly larger level difference. The use of such a technique often explains the "clicking" of imported automatic level controls. In addition, in almost any sound generator, you can jump the level by 10 dB, using such a level difference is convenient for observation. Therefore, in domestic practice, it is customary to measure the dynamic parameters of autoregulators when the levels change by 10 dB.

Unfortunately, the signal level switches of many generators at the moment of switching give a short-term voltage surge, and it is not possible to use them to measure the response time, since the autoregulator "shuts up". In this case, the GTI can be very useful.

Most radio amateurs rarely have to make such measurements, and it is advisable to include such a device in a measuring stand with more features. On its front panel there are switching elements, very convenient for connection. measuring instruments and custom hardware. On fig. 1 shows the approximate location of connectors (terminals or sockets) and switches. The bench diagram (Fig. 2) shows these switching circuits.

Device diagram

Click on the image to enlarge (opens in a new window)

Input sockets Х1 ("ВХ.1") and Х2 ("ВХ.2") are intended for connection of inputs of adjustable equipment. Toggle switches SA1 and SA2 allow you to connect the inputs to connectors X2 and X3 or close them to a common wire when measuring the level of integrated noise. Compared to buttons, toggle switches provide a more visual representation of how inputs are connected. A generator is connected to the central sockets X2 and XZ audio frequency and a voltmeter to control the input voltage. Connectors X5 and X8 are designed to connect outputs of adjustable equipment. One of the outputs can be connected with the SA3 toggle switch to connectors X6 and X7 for measuring instruments. When setting up audio equipment, it is convenient to use a non-linear distortion meter and an oscilloscope.

For switching circuits, no power sources are needed, therefore, with such switching it is very convenient to check various equipment.

If the dual toggle switch SA4 (Fig. 1) is in the "POST" position, the signal with a constant level applied to X2, X3, comes, depending on the position of the toggle switches SA1 or SA2, to the connectors X1, X4 to the inputs of the equipment under test. If you move SA4 to the upper position, then the signal from the generator will go to inputs 1 and 2 through the GTI circuits. In this case, the stand must be connected to the network alternating current 220 V.

The power switch SA5 is located on the rear panel, and only the LEDs HL1, HL2 (indication "+" and "-") are displayed on the front panel, signaling the presence of a bipolar supply voltage of ╠15 V.

An electronic switch DA4 is used to form tone pulses. At pins 16 and 4, the signal voltage value changes from the normalized value to zero, and at pins 6, 9, the level difference is set during adjustment variable resistor R15. The mode is selected using the SA9 toggle switch.

The pulse filling tone signal comes from the generator to the electronic switch through the buffer op-amp DA1.1. The second op-amp DA1.2 is used as a comparator, issuing a synchronization signal for the beginning of the pulse when the filling signal passes through "zero". The pulses from the comparator are fed to the clock input of the D-flip-flop DD2. Input D (pin 9) receives a pulse from a single vibrator assembled on the second trigger DD2.

The pulse duration is changed using the SA8.2 switch, which changes the resistance in the C15 charging circuit connected to the R input (pin 4) of the one-shot. To set the pulse duration, a conventional oscilloscope is sufficient. The single vibrator is triggered by signals coming from a rectangular pulse generator on inverters DD1.1 ≈ DD1.3, or in manual mode button SA6 "START". If the SA7 toggle switch is set to the "AUTO" position, the duty cycle (period) of the pulses is set using the variable resistor R11 "SLE".

It is very difficult to observe transient processes on the oscilloscope screen with a tone pulse duration of 3 ms and a large duty cycle. The task is simplified for oscilloscopes with external trigger during a waiting sweep. For their synchronization on the rear panel of the stand, the socket X9 "SYNCHR." is displayed. The trigger pulse is applied to electronic key with some delay relative to the synchronizing one, determined by the choice of parameters R13, C13.

The high level at which the DA4 electronic switch passes the tone signal appears with a positive voltage drop from the comparator after the appearance of a pulse from the one-shot and ends after the end of this pulse (with the next signal drop from the comparator). Thus, the beginning of the tone pulse coincides with the transition of the filling signal through "zero" and the requirement for generating an integer number of periods is satisfied. When the switch position SA8 "U Out" the voltage at the control input DA4 is zero and can be set output voltage generator corresponding to the nominal input level. In the switch position SA8 "STROKE." the DA4 chip is controlled by voltage coming directly from the clock generator. Its switching frequency is set by a variable resistor R11.

After the electronic switch, through the DA1.3 repeater and the SA1 and SA2 toggle switches, the tonal pulses are fed to the inputs of the tunable equipment. The device also has an inverter DA1.4 and a switch SA10, which can be used to change the phase of the signal on one of the inputs in relation to the other. Such an inverter is needed, for example, when checking the common-mode signals in stereo systems, in speakers, but it may be more useful instead to assemble a built-in tone generator on this op-amp according to the circuit shown in Fig. 3 . In such a generator, it is easy to obtain Kg less than 0.2%, and for many tests one can dispense with the use of an external generator for the stand.

To test the level meters, you need to connect the inputs of two channels (for stereo meters) to the corresponding input connectors. Then, in the "U Vyx" position of switch SA8, set the normalized value of the signal level with F = 5 kHz at the generator output and check the readings of both channels of the meter. For example, in a level meter, the LEDs corresponding to the value "0 dB" should light up simultaneously, and the scale error here should not exceed 0.3 dB. The SA9 toggle switch is set to "-80 dB". Then the switch SA8 is switched in turn to the positions "10 ms", "5 ms" and "3 ms" and check the compliance with the readings of the DUT. The "200 ms" setting of SA8 is used to test average level meters, which unfortunately predominate in household equipment.

In order to accurately control the return time value, the variable resistor R11 ("RMS") sets the frequency of the rectangular pulse generator signals, at which, immediately after the LED extinguishing, corresponding to the value of -20 dB on the DUT scale, the next pulse would follow. Then it is not difficult to determine the period of the signals using an oscilloscope. The extinction of the LEDs in both channels must occur synchronously.

When checking the dynamic parameters of the autoregulators of the signal level, the position "-10 dB" of the SA9 switch is used. The inputs and outputs are connected to the appropriate connectors. The outputs of the channels are monitored in turn, although with a two-channel oscilloscope, nothing prevents both outputs from being monitored simultaneously. At the output of the audio frequency generator, when the SA8 switch is in the "U Out" position, a signal is set with a level 10 dB higher than the normalized value. Then switch SA8 to pulses of any duration, and switch SA7 ≈ to the "MANUAL" position. The key remains off and allows you to control the voltage at connectors X1 and X2, which must correspond to the normalized value. Then switch SA7 transfer the GTI to auto mode work and, having selected the desired pulse duration and duty cycle, observe transient processes at the output of the autoregulator. If the oscilloscope is running in clock-triggered sleep mode, it is easy to determine the trip time and the presence of trip noise or overshoot.

The GTI uses four chips and the current consumption is very low. This allows instead of integrated stabilizers to use simple parametric voltage regulators on zener diodes. On the other hand, by installing more powerful integrated stabilizers DA2, DA3 of the DA7815 and DA7915 series, they can be used to power custom device breadboards by placing an additional connector on the rear panel (not shown in the diagram). The microcircuits provide protection against short circuits, which are not uncommon during experiments.

The front panel of the stand has dimensions of 195x65 mm. The body of the stand is made of steel.

To connect the equipment under test, socket-terminals of the ZMP type are convenient. In addition to them, depending on the equipment being tested, it is possible to install connectors of the appropriate design on the test bench panel, for example, tulip, jack, ONTS-VG or other sockets.

Double toggle switch SA4 ≈ PT8-7, P2T-1-1 or similar. Switch SA2 ≈ biscuit PG2-8-6P2NTK. Button SA6 "START" can be of any type without fixing, for example, KM1-1.

Chip DA2 K590KN7 can be replaced with a similar functional purpose. As DA1, you can use a chip with four op-amps of types LF444, TL084, TL074 or K1401UD4.

Mounting the device board ≈ printed or hinged on a breadboard.

The stand with GTI can be used for testing compander noise reduction systems, dynamic filters and other sound equipment.

LITERATURE
1. E. Kuznetsov. Audio level meters. - Radio, 2001, No. 2, p. 16, 17.
2. Chips for household radio equipment. Directory. - M.: Radio and communication, 1989.
3. Turuta J. Operational amplifiers. Directory. - M.: Patriot, 1996.