Scheme of a low-frequency generator.
A low-frequency generator is one of the most essential devices in an amateur radio laboratory. With it, you can set up various amplifiers, measure the frequency response, and conduct experiments. The LF generator can be a source of the LF signal necessary for the operation of other devices (measuring bridges, modulators, etc.).
The schematic diagram of the generator is shown in Figure 1. The circuit consists of a low-frequency sinusoidal generator on the operational amplifier A1 and an output divider on resistors R6, R12, R13, R14.
The scheme of the sinusoidal generator is traditional. The operational amplifier, with the help of positive feedback (C1-C3, R3, R4, R5, C4-C6) made according to the Wynn bridge scheme, is switched to the generation mode. Excessive depth of positive feedback, leading to distortion of the output sinusoidal signal, is compensated by the negative feedback R1-R2. Moreover, R1 is trimmer, so that it can be used to set the OS value such that at the output of the operational amplifier there is an undistorted sinusoidal signal of the highest amplitude.
The incandescent lamp H1 is turned on at the output of the op-amp in its feedback circuit. Together with the resistor R16, the lamp forms a voltage divider, the division factor of which depends on the current flowing through it (the H1 lamp acts as a thermistor, increasing its resistance from heating caused by the flowing current).
The frequency is set by two controls, - switch S1 selects one of the three subranges "20-200 Hz", "200-2000 Hz" and "2000-20000 Hz". In reality, the ranges are slightly wider and partially overlap each other. Smooth frequency tuning is performed by a double variable resistor R5. It is desirable that the resistor be with a linear law of change in resistance. The resistances and laws of change of the components of R5 must be strictly the same, therefore, the use of home-made dual resistors (made from two single ones) is unacceptable. The coefficient of non-linear distortion of the sinusoidal signal strongly depends on the accuracy of the equality of the resistances R5.
A knob with an arrow is fixed on the axis of the variable resistor (like a biscuit instrument switch) and a simple scale for setting the frequency. To accurately set the frequency, it is best to use a digital frequency counter.
The output voltage is smoothly regulated by a variable resistor R6. From this resistor, low-frequency voltage is supplied to the output. You can lower the set value by 10 and 100 times using an attenuator on resistors R12-R14.
The maximum output voltage of the low-frequency generator is 1.0V.
It is most convenient to control the output voltage value using a low-frequency millivoltmeter, adjusting for the attenuator value on resistors R12-R14.
The generator is turned off with a toggle switch in two directions S2, which disconnects the generator from a source of bipolar voltage ± 10V.
Most of the details are located on the printed circuit board. All resistors, switches and connectors are located on the front panel. Many details are mounted on their conclusions.
Switch S1 galet-ny on three directions and three positions. Only two directions are used. Switch S2 - toggle switch in two directions. All connectors are coaxial connectors of the "Asia" type from video equipment. Chokes L1 and L2 - from the color modules of old USCT TVs (you can use any chokes with an inductance of at least 30 μH). The H1 incandescent lamp is an indicator lamp with flexible wire leads (similar to an LED), for a voltage of 6.3V and then 20 ta. You can use another lamp for a voltage of 2.5-13.5V and a current of not more than 0.1 A.
It is desirable to adjust the generator using a frequency meter and an oscilloscope. In this case, by adjusting the resistor R1, a maximum and undistorted alternating sinusoidal voltage is achieved at the output of the generator, over the entire frequency range (this usually corresponds to an output alternating voltage of 1V). Then, by more accurate selection of R4 and R3 (these resistances must be the same), the frequency tuning ranges are set. If insufficiently accurate capacitors C1-C6 are used, it may be necessary to select them or connect “tuning” capacitors in parallel with them.
Ivanov A.
Literature:
1. Ovechkin M. Low-frequency measuring complex, Zh. Radio No. 4, 1980.
Radioconstructor 08-2016
A deep vacuum is created in the balloon, which is necessary for the unhindered passage of electrons. The electronic searchlight of the tube consists of a cathode, a control electrode and two anodes and is located in a narrow elongated part of the cylinder. Cathode TO It is made in the form of a small nickel cylinder, on the end part of which an oxide layer is applied, which emits electrons when heated. The cathode is enclosed in a control electrode (modulator) M also cylindrical. At the end of the control electrode there is a small hole (diaphragm) through which the electron beam passes. Several tens of volts of negative voltage relative to the cathode are applied to the control electrode, with the help of which the brightness of the glow of the spot on the tube screen is regulated. The control electrode acts like the control grid of a vacuum tube. At a certain value of this voltage, the tube is blocked, and the luminous spot disappears. The specified adjustment is placed on the front panel of the oscilloscope and is labeled "Brightness".
Preliminary focusing of the electron beam is performed in the space between the modulator and the first anode. The electric field between these electrodes presses the electrons to the axis of the tube and they converge to a point ABOUT at some distance from the control electrode (Fig. 33.2). Further focusing of the beam is performed by a system of two anodes A 1 And A 2
The first and second anodes are made in the form of open metal cylinders of various lengths and diameters, inside which diaphragms with small holes are located at some distance from each other.
A positive accelerating voltage is applied to the anodes (at the first
300-1000 V, for the second 1000-5000 V and more). Since the potential of the second anode A 2 above the potential of the first anode A 1 , then the electric field between them will be directed from the second anode to the first. Electrons that have fallen into such an electric field will be deflected by it in the direction towards the axis of the tube and receive acceleration in the direction of movement towards the screen . Thus, the action of the anode system is equivalent to the action of an optical system of collecting and diverging lenses. Therefore, the focusing anode system of a cathode ray tube is sometimes called electronic static lens. Accurate focusing of the beam is performed by changing the voltage at the first anode. This adjustment is placed on the front panel of the oscilloscope and labeled "Focus".
The formed electron beam after the second anode enters the space between two pairs of mutually perpendicular deflecting plates X 1 X 2 And Y 1 Y 2, called electrostatic deflection system. First pair of plates X 1 X 2, placed vertically causes the beam to deviate in the horizontal direction. Plates of the second pair Y 1 Y 2, placed horizontally cause the beam to deviate in the vertical direction. When a constant voltage is applied to a pair of plates, the electron beam is deflected towards the plate that is under a positive potential, which leads to a corresponding movement of the luminous spot on the screen.
When an alternating voltage is applied to the plates, the movement of the luminous spot across the screen forms luminous lines.
Screen E A cathode ray tube is a glass surface coated on the inside with a thin layer of a special substance (phosphor) that can glow when bombarded with electrons.
To obtain an image on the screen of the tube, the investigated signal voltage is applied to the vertical deflection plates Y 1 Y 2, a pa plate X 1 X 2- sawtooth voltage called sweep voltage (Fig. 33.3).
Location on AB the sweep voltage is linearly dependent on time, and under the action of this voltage, the light spot moves along the tube screen along the horizontal axis in proportion to time. Location on Sun the sweep voltage drops sharply, and the light spot returns to its original position.
If simultaneously with the sweep voltage to the plates Y 1 Y 2 bring the investigated sinusoidal voltage, then on the screen of the tube you will get one period of the sinusoid (Fig. 33.4).
The positions 0, 1, 2, ... of the light spot on the screen of the tube at the corresponding moments of time are determined by the instantaneous values of the investigated and developing voltages.
If the sweep period Tr is selected as a multiple of the period of the voltage under study, then the oscillograms obtained in subsequent periods are superimposed on each other and a stable and clear image of the process under study is observed on the screen
Sawtooth voltage generator for varicaps.
When working with a high-frequency generator tunable by a varicap, it was necessary to make a sawtooth voltage control generator for it. There are a great many circuits of "saw" generators, but none of those found fit, because. to control the varicap, an output voltage swing of 0 - 40V was required when powered by 5V. As a result of reflection, the following scheme turned out.
The sawtooth voltage is formed on the capacitor C1, the charging current of which is determined by the resistors R1-R2 and (to a much lesser extent) the parameters of the transistors of the current mirror VT1-VT2. A rather large internal resistance of the charging current source makes it possible to obtain a high linearity of the output voltage (photo below; vertical scale 10V / div). Basic technical problem in such circuits is the discharge circuit of the capacitor C1. Usually, unijunction transistors, tunnel diodes, etc. are used for this purpose. In the above circuit, the discharge is produced by ... a microcontroller. This achieves ease of setting up the device and changing the logic of its operation, because. the selection of circuit elements is replaced by the adaptation of the microcontroller program.
The voltage across C1 is monitored by a comparator built into the microcontroller DD1. The inverting input of the comparator is connected to C1, and the non-inverting input to the reference voltage source on R6-VD1. When the voltage on C1 reaches the reference value (approximately 3.8V), the voltage at the output of the comparator jumps from 5V to 0. This moment is monitored by software and leads to reconfiguring the GP1 port of the microcontroller from input to output and applying a logic 0 level to it. As a result, capacitor C1 turns out to be shorted to ground through the open transistor of the port and discharges quickly enough. At the end of the C1 discharge at the beginning of the next cycle, the GP1 output is again configured to the input and a short rectangular sync pulse is generated at the GP2 output with an amplitude of 5V. The duration of the discharge and synchronizing pulses is set by software and can vary over a wide range, because The microcontroller is clocked by an internal oscillator at a frequency of 4 MHz. When varying the resistance R1 + R2 within 1K - 1M, the frequency of the output pulses at the specified capacitance C1 changes from about 1 kHz to 1 Hz.
The sawtooth voltage at C1 is amplified by the op-amp DA1 up to the level of its supply voltage. The desired output voltage amplitude is set by resistor R5. The choice of the type of op-amp is due to the possibility of its operation from a 44V source. The 40V voltage for powering the op-amp is obtained from 5V using a pulse converter on the DA2 chip, included according to the standard scheme from its datasheet. The operating frequency of the converter is 1.3 MHz.
The generator is assembled on a board measuring 32x36 mm. All resistors and most capacitors are size 0603. The exceptions are C4 (0805), C3 (1206), and C5 (tantalum, frame A). Resistors R2, R5 and connector J1 are installed on the reverse side of the board. When assembling, you should first install the microcontroller DD1. Then, the wires from the programmer connector are temporarily soldered to the board conductors and the attached program is loaded. The program was debugged in the MPLAB environment, the ICD2 programmer was used for loading.
Although the described device has solved the problem and is still successfully operating as part of a sweep generator, to expand its capabilities, the above scheme can be considered rather as an idea. The upper frequency limit in this circuit is limited by the discharge time C1, which in turn is determined by the internal resistance of the output transistors of the port. To speed up the discharge process, it is desirable to discharge C1 through a separate low resistance MOSFET. In this case, it is possible to significantly reduce the time of the software delay for the discharge, which is necessary to ensure full discharge capacitor and, accordingly, the saw output voltage drop to almost 0V (which was one of the requirements for the device). To thermally stabilize the operation of the generator, it is desirable to use an assembly of two PNP transistors in one package as VT1-VT2. At a low frequency of the generated pulses (less than 1 Hz), the final resistance of the current generator begins to affect, which leads to a deterioration in the linearity of the sawtooth voltage. The situation can be improved by installing resistors in the emitters VT1 and VT2.
Subject: Ramp generators andcurrent.
General information about sawtooth pulse generators (GPI).
Linear voltage generators.
Linearly changing current generators.
Literature:
Bramer Yu.A., Pashchuk I.N. impulse technology. - M.: Higher school, 1985. (220-237).
Bystrov Yu.A., Mironenko I.G. Electronic circuits and devices. - M.: graduate School, 1989. - S. 249-261,267-271.
General information about sawtooth pulse generators (GPI).
Tension sawtooth called such a voltage, which for some time changes according to a linear law (increases or decreases), and then returns to its original level.
Distinguish:
linearly increasing voltage;
linearly falling voltage.
Sawtooth pulse generator - a device that generates a sequence of sawtooth pulses.
Appointment of sawtooth pulse generators.
Designed to obtain voltage and current that varies in time according to a linear law.
Classification of sawtooth pulse generators:
By element base:
on transistors;
on lamps;
on integrated circuits (in particular, on op-amps);
By appointment:
sawtooth voltage generators (GPN) (another name - linearly varying voltage generators - CLAY);
sawtooth current generators (GPT) (another name - linearly varying current generators - GLIT);
By the method of switching on the switching element:
sequential circuit;
parallel circuit;
According to the method of increasing the linearity of the generated voltage:
with a current-stabilizing element;
compensation type.
Sawtooth pulse generator device:
The construction is based on an electronic key that switches the capacitor from charge to discharge.
The principle of operation of sawtooth pulse generators.
Thus, the principle of obtaining an increasing or falling voltage is explained by the process of charging and discharging a capacitor (integrating circuit). But, because the arrival of pulses on the integrating circuit must be switched, it is used transistor key.
The simplest schemes of sawtooth pulse generators and their functioning.
Schematically, the functioning of the GUI is as follows:
Parallel circuit:
When the electronic key is opened, the capacitor slowly, through the resistance R, is charged to the value E, thus forming a sawtooth pulse. When the electronic key is closed, the capacitor quickly discharges through it.
The output pulse has the following form:
When the polarity of the power supply E is reversed, the output waveform will be symmetrical with respect to the time axis.
Serial scheme:
When the electronic key is closed, the capacitor is quickly charged to the value of the power source E, and when it is opened, it is discharged through the resistance R, thus forming a linearly falling sawtooth voltage, which has the form:
When the polarity of the power supply is reversed, the shape of the output voltage U out (t) will change to a linearly increasing voltage.
Thus, it can be seen (it can be noted as one of the main drawbacks) that the greater the amplitude of the voltage across the capacitor, the greater the nonlinearity of the pulse. Those. it is necessary to form an output pulse at the initial section of the exponential charge or discharge curve of the capacitor.
SAWTOOL VOLTAGE GENERATOR- linearly changing generator (current), electronic device, forming a periodic voltage (current) sawtooth. Main The purpose of H. p. n. is to control the time sweep of the beam in devices using cathode ray tubes. G. p. n. also used in devices for comparing voltages, time delay and pulse expansion. To obtain a sawtooth voltage, the process (discharge) of a capacitor in a circuit with a large time constant is used. The simplest G. p. (Fig. 1, a) consists of integrating circuit RC and a transistor that performs the functions of a key controlled periodically. impulses. In the absence of pulses, the transistor is saturated (open) and has a low resistance of the collector-emitter section, capacitor WITH discharged (Fig. 1, b). When a switching pulse is applied, the transistor turns off and the capacitor is charged from a power source with a voltage of - E to- direct (working) course. Output voltage G. p. n. taken from the capacitor WITH, changes according to the law. At the end of the switching pulse, the transistor opens and the capacitor WITH quickly discharges (reverse) through a low resistance emitter - collector. Main characteristics G. p. n.: sawtooth voltage amplitude, coefficient. nonlinearity and coefficient. using the power supply voltage. When in this scheme
Forward run time T p and the frequency of the sawtooth voltage are determined by the duration and frequency of the switching pulses.
The disadvantage of the simplest G. p. is small k E at small. The required values of e lie in the range of 0.0140.1, with the smallest values related to the comparison and delay devices. The non-linearity of the sawtooth voltage during forward running occurs due to the decrease in charging current due to the decrease in voltage difference. An approximate constancy of the charging current is achieved by including a non-linear current-stabilizing two-terminal device (containing a transistor or a vacuum tube) in the charge circuit. In such G. p. 
And 
. In G. p. with positive voltage feedback, the output sawtooth voltage is fed into the charging circuit as a compensating emf. In this case, the charging current is almost constant, which provides the values \u200b\u200b1 and \u003d 0.0140.02. G. p. n. used for scanning in cathode ray tubes with e-magn. beam deflection. To obtain a linear deviation, a linear change in current in the deflection coils is necessary. For a simplified equivalent coil circuit (Fig. 2, a), the current linearity condition is satisfied when a trapezoidal voltage is applied to the coil terminals. Such a trapezoidal stress (Fig. 2, b) can be obtained in G. p. when included in the charging circuit will add. resistance R e (shown in Fig. 1, A dotted line). The deflecting coils consume high currents, so the trapezoidal voltage generator is supplemented with a power amplifier.
Good day dear radio amateurs! I welcome you to the site ""
We assemble a signal generator - a functional generator. Part 1.
In this lesson Beginner radio schools we will continue to fill our radio laboratory with the necessary measuring instruments. Today we will start collecting function generator. This device is necessary in the practice of a radio amateur to set up various amateur radio circuits- amplifiers, digital devices, various filters and many other devices. For example, after we assemble this generator, we will take a short break during which we will make a simple light and music device. So, in order to properly adjust the frequency filters of the circuit, this device is just very useful to us.
Why is this device called a function generator, and not just a generator (low frequency generator, generator high frequency). The device that we will make generates three different signals at its outputs at once: sinusoidal, rectangular and sawtooth. As a basis for the design, we will take the scheme of S. Andreev, which is published on the website in the section: Circuits - Generators.
To begin with, we need to carefully study the circuit, understand the principle of its operation and collect necessary details. Thanks to the use of a specialized microcircuit in the circuit ICL8038 which is just designed to build a function generator, the design is quite simple.
Of course, the price of a product depends on the manufacturer, on the capabilities of the store, and on many other factors, but in this case we are pursuing one goal: to find the necessary radio component that would be acceptable quality and most importantly, affordable. You probably noticed that the price of a microcircuit is highly dependent on its marking (AC, BC and SS). The cheaper the chip, the worse its characteristics. I would recommend opting for the “BC” chip. Her characteristics are not very different from the “AC”, but much better than that of the “SS”. But in principle, of course, this microcircuit will also work.
We assemble a simple function generator for the laboratory of a beginner radio amateur
Good day to you dear radio amateurs! Today we will continue to collect our function generator. So that you do not jump through the pages of the site, I post it again circuit diagram function generator, the assembly of which we are engaged in:
I also post the datasheet technical description) chips ICL8038 and KR140UD806:
(151.5 KiB, 6,245 hits)
(130.7 KiB, 3,611 hits)
I have already collected the necessary parts to assemble the generator (I had some of them - constant resistances and polar capacitors, the rest were bought at a radio parts store):
The most expensive parts were the ICL8038 chip - 145 rubles and switches for 5 and 3 positions - 150 rubles. In total, this scheme will have to spend about 500 rubles. As you can see in the photo, the five-position switch is two-section (there was no one-section), but this is not scary, more is better than less, especially since the second section may come in handy for us. By the way, these switches are exactly the same, and the number of positions is determined by a special stopper, which can be set to the required number of positions yourself. In the photo I have two output connectors, although in theory there should be three: common, 1:1 and 1:10. But you can put a small switch (one output, two inputs) and switch the desired output to one connector. In addition, I want to pay attention to the constant resistor R6. There is no rating of 7.72 MΩ in the line of megaohm resistances, the nearest rating is 7.5 MΩ. In order to get the desired value, you will have to use a second 220 kOhm resistor, connecting them in series.
I also want to draw your attention to the fact that we will not finish the assembly and adjustment of this circuit to assemble the functional generator. For comfortable work with the generator, we must know what frequency is generated in this moment work, or we may need to set a certain frequency. In order not to use additional devices for these purposes, we will equip our generator with a simple frequency meter.
In the second part of the lesson, we will study another method of manufacturing printed circuit boards - the LUT method (laser ironing). We will create the board itself in the popular amateur radio program for creating printed circuit boards – SPRINT LAYOUT.
How to work with this program, I will not explain to you yet. In the next lesson, in a video file, I will show you how to create our printed circuit board in this program, as well as the entire process of manufacturing the board using the LUT method.
SAWTOOL VOLTAGE GENERATOR- a generator of a linearly changing voltage (current), an electronic device that forms a periodic. fluctuations in voltage (current) of a sawtooth shape. Main The purpose of H. p. n. is to control the time sweep of the beam in devices using cathode ray tubes. G. p. n. also used in devices for comparing voltages, time delay and pulse expansion. To obtain a sawtooth voltage, the process of charging (discharging) a capacitor in a circuit with a large time constant is used. The simplest G. p. (Fig. 1, a) consists of integrating circuit RC and a transistor that performs the functions of a key controlled periodically. impulses. In the absence of pulses, the transistor is saturated (open) and has a low resistance of the collector-emitter section, capacitor WITH discharged (Fig. 1, b). When a switching pulse is applied, the transistor turns off and the capacitor is charged from a power source with a voltage of - E to- direct (working) course. Output voltage G. p. n. taken from the capacitor WITH, changes according to the law. At the end of the switching pulse, the transistor opens and the capacitor WITH quickly discharges (reverse) through a low resistance emitter - collector. Main characteristics G. p. n.: sawtooth voltage amplitude, coefficient. nonlinearity and coefficient. using the power supply voltage. When in this scheme
Forward run time T p and the frequency of the sawtooth voltage are determined by the duration and frequency of the switching pulses.
The disadvantage of the simplest G. p. is small k E at small. The required values of e lie in the range of 0.0140.1, with the smallest values related to the comparison and delay devices. The non-linearity of the sawtooth voltage during the forward stroke occurs due to the decrease in the charging current due to the decrease in the voltage difference . An approximate constancy of the charging current is achieved by including a non-linear current-stabilizing two-terminal device (containing a transistor or a vacuum tube) in the charge circuit. In such G. p. 
And 
. In G. p. with positive voltage feedback, the output sawtooth voltage is fed into the charging circuit as a compensating emf. In this case, the charging current is almost constant, which provides the values \u200b\u200b1 and \u003d 0.0140.02. G. p. n. used for scanning in cathode ray tubes with e-magn. beam deflection. To obtain a linear deviation, a linear change in current in the deflection coils is necessary. For a simplified equivalent coil circuit (Fig. 2, a), the current linearity condition is satisfied when a trapezoidal voltage is applied to the coil terminals. Such a trapezoidal stress (Fig. 2, b) can be obtained in G. p. when included in the charging circuit will add. resistance R e (shown in Fig. 1, A dotted line). The deflecting coils consume high currents, so the trapezoidal voltage generator is supplemented with a power amplifier.
A sawtooth voltage is a voltage that increases in proportion to time and decreases abruptly. On fig. 46, A shows an ideal sawtooth voltage having a rise time t out and fall time t sp, equal to zero. It is obvious that the period of such tension T equal to the rise time. Real sawtooth voltage generators have a voltage that is not quite linearly increasing and its decay time is not equal to zero (Fig. 46, b).
Sawtooth voltage is used to scan the electron beam in cathode ray devices.
Rice. 46. Curves of changes in the ideal (a) and real (b) sawtooth voltage
Consider the operation of a controlled transistor sawtooth voltage generator with capacitive feedback (Fig. 47).
Rice. 47. Sawtooth voltage generator circuit
The generator is controlled by pulses of negative polarity through a diode VDI. In the initial state, the transistor VT1 locked by a positive voltage supplied from the emf source. E bae through a resistor R2,diode VDI and resistor R1.Capacitor WITH charged via R K , R 1,VDI And R2 up to voltage Ye ke.When a control pulse is applied, the diode VD1 is locked. Transistor VTI opens, since the voltage to its base is now supplied through a resistor R. The discharge of the capacitor through the open transistor begins. The potentials of the base and collector at the moment of unlocking the transistor abruptly decrease. capacitive Feedback between the collector and the base keeps the capacitor discharge current almost unchanged.
At the end of the control pulse, the diode is unlocked, the transistor is closed by the emf source voltage. E bae and the capacitor begins to charge WITH.
To ensure the full discharge of the capacitor and obtain the maximum amplitude of the sawtooth voltage, the duration of the control pulses is selected based on the ratio
τ = (1,1 – 1,2)t res
Where t res- capacitor discharge time.
The frequency of the sawtooth voltage is determined by the parameters of the discharge circuit and is limited by the frequency properties of the transistor.