Two sinusoidal logic generators. Scheme, description

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Two sinusoidal logic generators. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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Usually, low-frequency sinusoidal signal generators are built on operational amplifiers. But logic elements can also work in analog mode - as amplifiers. In the literature, this topic has been touched upon repeatedly, but mainly they were analog signal amplifier circuits (low-frequency amplifiers on CMOS microcircuits, direct amplification receivers, etc.). But any amplifier, even made of logic elements, can be turned into a generator - it's all about feedback ...

Figure 1 shows a diagram of a fixed-frequency sinusoidal LF generator implemented on two logical inverters of the K561LN2 microcircuit.

Two sinusoidal logic generators
Fig. 1

The inverters are switched to analog mode using the OOS on resistors R1 and R3. each of which is connected between the input and output of the inverter. The amplifiers obtained in this way are connected in series (as two stages) through the resistor R4. Moreover, the transfer coefficient of the first stage depends on the ratio of the resistances R1 and R2. Since these resistors are the same, the transfer coefficient of the first stage is equal to one. The transfer coefficient of the second stage is determined by the ratio of the resistances R4 and R3, and it can be adjusted by the resistor R4.

Resistors R1-R2, together with capacitances C1 and C2, form a Wien bridge, tuned to a certain frequency, which is determined by the well-known formula:

F=1/(RC), where R=R1=R2, C=C1=C2.

To get an unlimited and undistorted sinusoid, you need to adjust the gain of the amplifier under the built-in resistor R4 accordingly. In this circuit, when powered by a 9V source, the best shape of the sinusoid is obtained at its effective value of about 1V.

This oscillator, although based on logic gates, is purely analog and its output product does not contain any pulse components or voltage steps that need to be filtered.

Figure 2 shows a digital sine wave oscillator circuit that generates a sine wave voltage of 976,5625 Hz (at a 500 kHz crystal frequency). Here, a sinusoidal voltage is formed from rectangular pulses using a DAC on the elements of the D2 chip and resistors. The period consists of 32 steps. Finally, the output signal is formed by the operational amplifier A1, and the RC circuit turned on at its output. which smoothes the steps that form a sinusoid.

Two sinusoidal logic generators
Fig. 2

The frequency of the output sinusoid will be 512 times lower than the frequency of the quartz resonator or input pulses, which, when operating from an external source of pulses, can be applied to pin 11 D1. In this case, the details R1, R2, Q1, C1, C2 are excluded

The scheme is attractive in that it allows you to get a sinusoidal low-frequency signal of quartz frequency stability.

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