Function generator with electronic frequency tuning. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Function generators are used by radio amateurs to test and adjust a variety of electronic equipment. The author of the proposed article describes one of the variants of such a generator that generates sawtooth and rectangular signals.

As you know, function generators are capable of providing an output signal of triangular, sawtooth, rectangular, sinusoidal and many other shapes. True, such devices are usually used by specialists who professionally develop complex equipment. In amateur radio practice, in most cases, it is enough, for example, to be able to receive a triangular and rectangular signal. The first of them will allow you to tune the analog equipment of the 3H range and detect (of course, if you have an oscilloscope) distortions of the “step” type, signal limitations “from above” or “from below”. With the help of the second, you can check and adjust digital equipment, as well as control the dynamic characteristics of analog equipment. The generator itself, which provides such signals, is greatly simplified.

The scheme of the device is shown in fig. one.

(click to enlarge)

The generator itself is made on a DA1 chip containing two operational amplifiers. An integrator is assembled on DA1.1, and a comparator is assembled on DA1.2 (see Horowitz P., Hill W. Art of circuitry. - M .: Mir, 1998, p. 257). The frequency range of the generator from 20 Hz to 20 kHz is divided into three subranges, which are set by the switch SA1, which connects one of the capacitors C1 - C3 to the integrator. In each of the subranges, the oscillator frequency is changed by a variable resistor R2.

When charging the frequency-setting capacitor, a voltage increasing in time is formed at the output of the op-amp DA1.1 (pin 9). As soon as it reaches a certain value, the comparator changes the direction of integration. The frequency-setting capacitor begins to discharge, the voltage at the indicated output drops. The result is a triangular waveform. Through the resistor R8 and the capacitor C6, it goes to the variable resistor R9, and from its engine to the output jack XS3. The maximum voltage that can be set at the output with a variable resistor reaches 1 V.

At the output of the comparator (pin 13 of the op amp DA1.2), rectangular oscillations are formed, which are fed to the shaper, made on the DD1 chip. This chip allows you to apply voltage to the inputs,

a higher supply voltage, which allows you to connect its input 1 directly to the output of the op-amp DA1.2. The supply voltage is supplied to it through one of the zener diodes VD1 - VD4, so the output of the logic elements DD1.2 - DD1.6 will be rectangular pulses with an amplitude of 3, 5, 9, 12 V, depending on the position of the movable contact of the switch SA2.

Due to the use of a relatively powerful K561LN2 CMOS chip, its output current can reach 20...30 mA. Therefore, the device is suitable for configuring devices assembled on microcircuits of various series: K155, K176, K530, KR531, K555, K564, KR1554 and many others.

With the ratings of the elements indicated on the diagram, the frequency of the generated signal in hertz is determined by the formula: Fout \u40d (2 / C) (UR2 / Upit), where C is the capacitance of the connected frequency-setting capacitor, μF; UR2 - voltage on the engine of the variable resistor RXNUMX, V; Upit - supply voltage, V.

Since the op amp is powered by a single voltage supply, the value of UR2 will be limited from below. For the op-amp used by the author, it was 1,45 V; at a lower voltage, the generator did not work. Therefore, to obtain a tenfold overlap in frequency, a stabilized supply voltage of 15 V was chosen. True, the generator is operational even at a lower voltage, but the frequency overlap in each subrange will also be less.

Any transistor of the KT3102 series can be used in the device. Capacitors C1 - C Z-PM-2, K71, but in extreme cases, if high thermal stability is not required, - KD, KLS, K10-17; C4 - any type, C5 - C7 - K50-16, K50-35 or similar. Variable resistors - SP, SPO, SP4, fixed - MLT, S2-33. Switches - any type.

Most of the parts are mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass.

The board is installed in a case of suitable dimensions, and switches, sockets and variable resistors are attached to the case. It is desirable to provide resistor R2 with a scale and put on it the values ​​of the generated frequencies for each subband.

When setting up the device, first select a resistor R1 of such resistance that in the left (according to the diagram) position of the resistor R2 engine, stable operation of the generator is observed at the lowest frequency - 20 Hz (the movable contact of the SA1 switch is in the "20 ... 200 Hz" position). The frequencies of the subbands are set by selecting capacitors C1 - C3, and the maximum amplitude of the triangular voltage is set by selecting the resistor R8.

The operating frequency range of the generator is limited by the operating speed of the used op amp and is 40...50 kHz. If obtaining such frequencies is necessary, add another frequency-setting capacitor, use a four-position switch and set other subranges, for example, 4 ... 40 Hz, 40 ... 400 Hz, 0,4 ... 4 kHz, 4. ..40 kHz.

Author: I.Nechaev

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