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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Low harmonic signal generator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Nonlinear distortion of AF signals, which characterize the quality of sound recording and reproducing equipment, is usually estimated by the harmonic coefficient, which for high-quality devices should not exceed an approximate threshold value of 0,1%. To measure distortions of this level, a signal generator with a harmonic coefficient several times lower is required, therefore, when developing the proposed device, the main attention was paid to reducing the nonlinear distortion of the signal. Main technical characteristics:
The range of generated frequencies of the device is divided into four sub-ranges, in each of which the frequency is changed by a double variable resistor. The output voltage can be adjusted smoothly and discretely in steps of 20 dB. The functional diagram of the generator is shown in fig. 1. Its basis is a wideband amplifier A1, the positive feedback circuit (POS) of which is formed by a band-pass filter R1C1R2C2 (Win's bridge), and the negative feedback (NFB) is formed by nodes and elements for stabilizing the output voltage amplitude R3, R4, U1, A2-A7.
The bandpass RC filter is similar to a parallel oscillatory circuit and at the quasi-resonance frequency fp=1/2piRC (at R1=R2=R and C1=C2=C) provides the maximum transfer coefficient equal to 1/3, the highest quality factor and the best selective properties. The oscillation frequency can be tuned by a consistent change in the resistance of resistors R1 and R2 or the capacitance of capacitors C1 and C2. It is obvious that for the self-excitation of the generator, the transfer coefficient of the amplifier A1, set by the OOS circuit, must be equal to three. With such a low gain, it is easy to achieve a wide frequency range and a very low (less than 0,01%) level of distortion of the amplifier itself using deep feedback. To obtain a low harmonic coefficient of the generator, the amplitude of the output voltage must be stabilized at a certain level. To do this, the amplifier is covered with a non-linear OOS circuit, in which a thermistor or a field-effect transistor is often included as a controlled attenuator. However, in the first case it is difficult to achieve in a simple way the harmonic coefficient of the generator at medium frequencies is less than 0,05%, in the second - less than 0,1%, therefore, special attention was paid to reducing distortion in the controlled attenuator. The FOS voltage supplied to amplifier A1 can be represented as the sum of two components: a constant, the amplitude of which is always equal to 1/3 of the output voltage, and a variable, the nature of the envelope of which is determined by the properties of the FOS circuit, and the amplitude depends on destabilizing factors: temperature and time drift parameters of the elements, changes in the filter gain in the frequency range, etc. (the amplitude of the second component is several orders of magnitude smaller than the first). This prompted the idea to use a two-channel OOS circuit to reduce non-linear distortion, by applying a constant component to the inverting input of amplifier A1 through a channel containing only linear elements (divider R3R4 and adder A7), and a variable through the amplitude stabilization channel (U1, A2-A6) , which generates a corrective signal, which is added in the adder A7 with a constant component. The second channel works as follows. The output signal of amplifier A1 is rectified by the rectifier U1, and the voltage taken from it is compared in the integrator A2 with the exemplary one that sets the level of output oscillations. The integrated differential voltage controls the attenuator A4 directly, and the attenuator A5 through the inverting follower A3. In the stationary (steady-state) mode of operation of the generator, with the transfer coefficients of the divider R3R4 and the filter equal to 1/3, the difference between the input voltages, as well as the output voltages of the integrator A2 and the follower A3 are close to zero. Therefore, the amplitudes of the signals at the outputs of the attenuators A4 and A5 are the same and the output voltage of the differential amplifier A6 is also close to zero. In non-stationary mode, a change in the amplitude of the output signal of amplifier A1 causes a deviation of the rectified voltage in one direction or another relative to the reference and, consequently, the output voltages of the integrator A2 and the follower A3. Under the action of these control signals, the transmission coefficients of the attenuators A4 and A5 change in opposite directions, and a sinusoidal voltage appears at the output of the amplifier A6, leading the generator to a stationary mode. With an increase in the amplitude of the output oscillations relative to the stationary value, a signal appears at the output of the amplifier A6, which is in phase with the output, and when it decreases, it is out of phase. The use of controlled attenuators operating at a small signal and partial compensation of non-linear distortion products made it possible to significantly reduce the level of generator harmonics.
Schematic diagram of the device. Its main amplifier contains two differential input stages (VT1, VT2 and VT5, VT6) connected in parallel for the amplified signal. Due to this, the amplifier is symmetrical for both half-waves of the AC voltage, which significantly reduces the level of even harmonics, especially the second, largest component of the signal spectrum in most high-quality RC oscillators. Another feature of the amplifier is the low current flowing through the resistors R39, R32.2 and R40 connected to the bases of the differential stage transistors. It is equal to the difference in base currents, therefore, by selecting transistors with similar current transfer coefficients h21e, it can be significantly reduced. As a result, it turned out to be possible to reduce the requirements for the consistency of the sections of the dual variable resistor R32 and connect its first section (R32.1) directly to the bases of the transistors VT1, VT5 (without an isolation capacitor). In order to reduce the inherent noise of the amplifier, the quiescent current of the differential stages is chosen to be relatively small (about 100 μA). The signals from the collectors of transistors VT1 and VT5 are fed to a symmetrical voltage amplifier made on transistors VT7, VT9 and VT8, VT10. To reduce non-linearity, it is covered by local OOS (resistors R13 and R15), which reduces its transmission coefficient to 8...12. Resistors R19, R20 create conditions close to the voltage source mode for the output stage on composite transistors VT12VT14 and VT13VT15, which also improves the linearity of the amplifier. The quiescent current of this stage is set by a trimming resistor R16. For stable operation with a large depth of feedback and a wide bandwidth, the amplifier provides frequency correction with circuits R1C1 and R11C2 connected in parallel with the load resistors (R2 and R10) of the differential stages. The cutoff frequency of the frequency response of the amplifier with open-loop feedback, set by these circuits, is in the range of 20 ... 25 kHz. As a result of pairing the frequency response of the uncorrected amplifier and correction circuits, the section of the characteristic with a steepness of 6 dB per octave has become more extended. The cutoff frequency of the voltage amplifier is in the region of several megahertz. In addition, to increase the stability margin of the entire amplifier, a boosting link C19R69 is included in the OOS circuit. The output signal of the amplifier passes through a repeater on a VT16 transistor, is rectified by a VD6 diode and fed to an integrator made on an op-amp DA1. The exemplary voltage is supplied from the trimmer resistor R35. From the output of the op-amp, a voltage equal to the result of integrating the difference of the indicated signals acts on the gate of the transistor VT17.1, and through the inverting follower on the op-amp DA2 - on the gate of the transistor VT17.2. Together with resistors R52-R55, these transistors form controlled attenuators. The non-linearity of the characteristics of transistors is reduced by the OOS circuits, consisting of resistors R49, R50 and R56, R57. It has been experimentally established that in order to obtain the best results, the constant voltage at the gates of field-effect transistors should be within 20 ... 50% of the cut-off voltage, and the resistance of resistors in the CNF circuits should be much greater than the resistance of their channels. This is taken into account in the described attenuators, and the voltage at the inverting input of the op-amp DA2 can be adjusted with a trimmer resistor R33 in order to set the best ratio of voltages that control the attenuators in stationary mode. The difference in the output signals of the attenuators is amplified by a differential amplifier on the op-amp DA4 and through the CFO voltage adder, made on resistors R66-R68, R70-R72, R40, acts on the inverting input of the main amplifier. The transmission coefficient of the OOS circuit, close to 1/3, is set by trimming resistors R68, R70-R72 in each subband separately. Frequency control, switching of subranges, as well as destabilizing factors cause changes in the output voltage, which is accompanied by processes that restore its previous level. For example, with an increase in the output signal, the voltage at the output of the rectifier (VD6) increases and, consequently, the control voltage at the gate of the transistor VT17.1 decreases, and at the gate of the transistor VT17.2 it increases. For this reason, the gains of the attenuators change in opposite directions, and the amplitude of the common-mode output signal of the amplifier at the op-amp DA4 increases, while the gain of the main amplifier decreases. As a result, the amplitude of the output signal of the generator and the rectified voltage at the inverting input of the op-amp DA1 return to the previous, stationary value. The output voltage of the generator is measured with an AC voltmeter at the op-amp DA3. The microammeter RA1 is included in the diagonal of the rectifier bridge VD7--VD10 in the OOS circuit, covering the OS. The output voltage of the generator is set by a variable resistor R26 and a stepped attenuator consisting of a resistive divider R27-R30 and switch SA2. The generator is powered by a bipolar stabilized source. The current consumed from it is less than 100 mA. Details and design. The device mainly uses MLT resistors with a permissible resistance deviation from the nominal value of ±5 and ±10%. Resistors R31, R39, as well as R27-R30 are selected with an accuracy of ±0,5 ... 1%. Trimmer resistors - SP3-44, SP3-27 or SP3-16. For frequency tuning, a double wire variable resistor PTP was used, but this does not exclude the use of other types of resistors with a resistance of 2 ... 50 kOhm (with a corresponding change in the capacitance of capacitors C8-C15). To facilitate the establishment of the generator and obtain the harmonic coefficient indicated at the beginning of the article, the unbalance of the sections of the resistor R32 should not exceed 2..3%. Capacitors C1, C2, C4, C5, C7, C19 - KM4 or KM5; C3, C6 - K50-6; C16-C18 - K50-3; C8-C15 - K73, K76, MBM. To reduce the frequency setting error in the subbands, the capacitance of the latter must be selected with an accuracy of no worse than 1 ... 2%. The capacitance values indicated in the diagram are obtained by connecting two capacitors in parallel (for example, C8, C12 are made up of capacitors with a capacity of 3,3 and 0,68 μF). Diodes KD521A can be replaced with KD522A, KD522B, KD509A, KD510A, zener diode KS162A - with KS156A. Static current transfer coefficients h21e of transistors VT1, VT2, VT5, VT6 should not differ by more than 20%, and transistors VT7-VT10 - by 30%. For transistors VT1-VT6, these coefficients should be within 150 ... 250, VT7-VT10 - within 100 ... 200, VT12-VT15 - 80 ... 200. Instead of those indicated in the diagram, it is possible to use transistors of the KT315 (VT1-VT3, VT10-VT12, VT14) and KT361 (VT4-VT7, VT9, VT13, VT15) series, instead of assembling KPS104V - KPS104E, as well as transistors KP303V - KP303E with cutoff voltages, differing by no more than 30%. OU K140UD7 can be replaced by K140UD8A, K140UD8B, K140UD6. The device has an M261M microammeter with a total deflection current of 100 μA and a loop resistance of about 800 ohms. Switches SA1, SA2 - PG3, socket XS1 - СР50-73. Most of the generator elements are placed on a printed circuit board made of fiberglass with a thickness of 2 mm. Resistor R25 is soldered to the outputs of the level regulator R26, divider resistors R27-R30 - to the outputs of switch SA1. Capacitors C8-C15, C19 and resistors R31, R39, R67-R72, R40 are mounted on an additional printed circuit board installed next to the dual variable resistor R32 (since the dimensions and pattern of printed conductors of the board depend on the dimensions of the capacitors, its drawing is not given). Resistor R60 and capacitor C17 are mounted on the terminals of the RA1 microammeter.
The adjustment of the device begins with measuring the voltages at the outputs of a stabilized power source, which should be within ± 14,5 ... 16 V. After that, one of the terminals of the resistor R66 is temporarily soldered and the DC operating mode of the amplifier is checked. The voltage drop across the resistors R2, R10 should be within 2,3 ... 2,7 V, on the resistors R12, R14 - 1,7 ... 2,1 V, and on R13, R15 - 1,1 .. .1,5 V. The trimming resistor R16 sets the quiescent current of the output stage 1,5 ... 2,5 mA. The DC voltage at the output of the amplifier must be no more than ±10 mV. If necessary, this is achieved by shunting the resistor R5 or R6 with an additional high resistance resistor (15 ... 150 kOhm). Then make sure that there is no parasitic self-excitation of the amplifier. If it is, increase the capacitance of the correction capacitors C1, C2 and select the elements of the boost circuit R69C19. After that, the op-amp DA1, DA2, DA4 is balanced, the output of the resistor R66 is soldered and the sliders of the resistors R32, R33, R35, R37 are set to the middle position, and the SA1 switch is set to the "x10" position (100 ... 1000 Hz). Trimmer resistors R70 and R35 achieve generation in this subrange, resistor R35 sets the maximum output voltage of 5 V. Next, the synchronization input of the oscilloscope is connected to the output of the generator and the waveform at the output of the op-amp DA4 is checked. Trimmer resistors R70 and R33 achieve the smallest possible amplitude of this signal and close control voltages at the gates of transistors VT17 (they are measured with a voltmeter with a high-resistance input), which should be in the range of -0,4 ... -1,6 V. Stable generation and the smallest the amplitude of the undistorted signal at the output of the op-amp DA4 in the remaining subranges is achieved by trimming resistors R68, R71, R72. With insufficient stability of the signal amplitude in frequency, the resistance of the resistor R44 is increased. Low-frequency (0,1 ... 1 Hz) oscillations that occur in order to stabilize the amplitude are eliminated by connecting a resistor with a resistance of several kilo-ohms in series with the capacitor C16. Graduate the scale and check the multiplicity of frequency change when switching subranges using a digital frequency meter. Establishing a voltmeter on the op-amp DA3 comes down to setting the required sensitivity by selecting the resistor R59. The unevenness of the transfer coefficient of the voltmeter in the frequency band 10 ... 105 Hz should not exceed 1%. Author: N. Shiyanov
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