Audio Range Function Generator

In the development and research of radio-electronic equipment in laboratory conditions, one of the most important components of the measuring complex is a universal source of test signals. A function generator operating in the audio frequency range was developed on the basis of a microcontroller. The generator circuit described in Radiohobby, 2000, No. 5, p. 79.

As a result of the analysis of this scheme, a significant drawback was identified in it. Due to the fact that the counter is made on two functional elements, the logic levels on the address pins are set non-simultaneously, which will lead to the appearance of high-frequency surges at the generator output. The developed device is devoid of these shortcomings. Generated output waveforms (saw, triangle, sinusoid, meander) are programmed in flash memory. The generator uses the firmware provided in Radiohobby, 2000, No. 5, p. 79. Schematic diagram of the function generator is shown in fig. one.

Main technical parameters

(click to enlarge)

A master oscillator is assembled on the MS 4046 (DD1) chip, with the help of which the frequency is tuned in the range of 0 ... 4 MHz. Variable multi-thread resistor R1 is designed to adjust the frequency. On the 556IE10 (DD2) chip, an address counter is assembled, at the outputs of which signals are generated ("address is being dialed") that arrive at the address inputs of the Winbond W27C512 (DD4) EPROM. This flash memory chip is programmed with output waveforms. From the PROM outputs, the signals go to the DAC0800 (DA4) DAC, from the output of which the signal goes to the high-speed operational amplifier K574UD1 (DA5). Button SB1 is designed to select the form of the output signal. On the elements of the 155LAZ chip (DD3.2, DD3.3), an anti-bounce button assembly of the SB1 button is assembled. From pins 5 and 9, which also receive signals from the PROM outputs, the signal is fed to the decoder chip SN74LS156N (DD7). Data about the selected signal is taken from its outputs, which are displayed by one of the LEDs. To indicate the selected form of the output signal, a node was assembled on the D-trigger of the 155TM2 (DD6) microcircuit, the input of which receives pulses that occur when the button is switched.

The device is powered by an unstabilized + 12 V voltage source, which is fed to the integral timer DA1 and the positive voltage regulator DA2. A multivibrator is implemented on the DA1 timer, from the output of which rectangular pulses are taken. They are rectified and fed to the input of the negative voltage regulator DA3. The voltage taken from its outputs is used to power the generator. When setting up the device, first, by selecting C1, set the frequency to about 4 MHz at pin 4 of the DD1 microcircuit, then with the resistor R11 in the sinusoid generation mode, controlling the output signal with an oscilloscope, set the signal symmetry relative to the common wire. The maximum oscillator output frequency is determined mainly by the speed of the PROM, which for the Winbond W27C512 chip is approximately 4 MHz, so the maximum output frequency is: 4 MHz/128 steps = 30 kHz. The measured harmonic distortion of the sinusoidal signal is 0,0076% at a frequency of 1 kHz.

This oscillator was designed to work with a digital oscilloscope that displays input frequencies, so there is no additional indication of the output signal frequency. Devices made on microcontrollers are quite promising. For example, the proposed oscillator can be increased in frequency by using a master oscillator with a higher frequency. Also, by increasing the bit depth of flash memory, you can increase the sampling rate. The generator can be connected to a personal computer with the ability to programmatically change the shape of the output signal by changing the flash-memory firmware. Resistor R1 - SP5-44-1 or other multi-turn, R9, R11 - SPZZA or other small ones. The software for this design can be found on the magazine's website at dian.ru.

Authors: Andrey Kolokoltsev, Andrey Dakhnovich, Andrey Babaev, Tambov; Publication: cxem.net

See other articles Section Knots of amateur radio equipment. Generators, heterodynes.

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