Signal generators on CMOS chips. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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CMOS oscillators are popular with radio amateurs. They are used in the design of measuring instruments, audio frequency generators, probes for checking the health of radio elements and cascades of radio equipment. This article describes three variants of such generators, which can be made in the form of probes for checking and adjusting low-frequency and high-frequency components of various equipment.

Typically, when designing probes and calibrators, short pulse generators are used that produce a signal with a wide and uniform spectrum. Such a signal allows you to quickly check the cascades of radio equipment, both low-frequency (LF) and high-frequency (HF). Moreover, the shorter the pulse duration, the better - the spectrum is wider and more uniform.

As a rule, such generators consist of two main components: the rectangular pulse generator itself and the short pulse shaper. Meanwhile, you can do without a special shaper, since it is already available in the logic element of the CMOS microcircuit.

Consider, for example, the probe circuit shown in Fig. 1. This is a well-known RC generator, operating in this case at a frequency of about 1000 Hz (it depends on the ratings of parts R1, C1). A low-frequency rectangular signal comes from the output of the DD1.2 element (pin 4) through the R2C3 circuit to the variable resistor R4 - they smoothly adjust the amplitude of the signal supplied to the node under test.

The output of a high-frequency signal (short pulses) is somewhat unusual - the signal is taken from a variable resistor R3 included in the power circuit of the microcircuit. By moving the slider of this resistor, the level of the high-frequency output signal is smoothly adjusted.

Let us consider the principle of operation of such a shaper according to a simplified circuit of a logic element of the CMOS structure (Fig. 2). Its basis is two series-connected field-effect transistors with an insulated gate and different types of channel conductivity.

If a resistor R1 is connected in series with the transistors, and rectangular pulses U1 are applied to the input of the element, the following will happen (Fig. 3).

Due to the fact that the duration of the front of the pulse cannot be infinitely small, and also due to the inertia of the transistors, at the time of the action of the front, a moment will come when both transistors will be in the open state. The so-called through current will flow through them, the value of which can range from units to tens of milliamps, depending on the type of microcircuit and the voltage of the power source. The resistor will generate short voltage pulses U2. Moreover, both at the time of the action of the front, and the recession. In other words, there will be a doubling of the frequency of the initial pulses.

The resistance of the resistor should not be large in order to avoid disrupting the operation of the microcircuit elements. This means that a low-resistance load with a resistance of 50...75 Ohm can be connected to the high-frequency output.

For the considered generator, the maximum amplitude of the pulses at the high-frequency output is 100 ... 150 mV, and the current consumed from the power source does not exceed 1,6 mA. The generator is designed for use in testing 3H amplifiers, three-program loudspeakers, radio receivers on the DV and MW bands. 4.

It is built according to the principle described above, but the generator operates at a frequency of 1 MHz. Short pulses of high-frequency voltage are formed on the resistor R3 and fed through the capacitor C3 to the tested cascades. The dependence of the amplitude of harmonics on frequency is shown in fig. 5 - it decreases from 20 mV at a frequency of 1 MHz to 12 μb at a frequency of 80 MHz. which in most cases is sufficient to complete the task facing the probe. The generation frequency is precisely set by selecting capacitors C1 and C2. From the power source, the generator consumes a current of about 5 mA.

On one CMOS chip, it is not difficult to assemble a combined generator - a combination of the two described designs (Fig. 6). It also contains two outputs, and the operating modes are set by the SA1 switch.

In the lower position of the movable contact of the switch according to the diagram, only the LF generator works, therefore, there will be rectangular pulses at the LF output, and short pulses with a spectrum width of up to 1,5 MHz at the HF output. In the middle position, only a quartz oscillator works and the RF output will have a signal with a spectrum width of up to 80 MHz. At the same time, there is no signal at the LF output at all. If the movable contact of the switch is moved to the upper position, both generators will turn on, and the quartz one will be modulated by the signal of the low-frequency generator.

With a high quality factor of the quartz resonator, the HF generator can be poorly modulated by the LF generator signal. In this case, you need to disconnect pin 5 of the DD1.3 element from the switch and connect it to pin 6, and connect the wire to the switch from pin 8 (it is disconnected from pins 4 and 9 and resistor R5).

The design of all probe generators can be any, but for their stable operation, the connections between the parts must be as short as possible.

Author: I. Nechaev, Kursk

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