When you want a really pure sine wave within the audio vary, the circuit in Determine 1 can assist. It’s a easy deal: A sine wave with a THD of 1% coming from a operate generator that goes by means of a trackable low-pass filter, which attenuates the distortion-causing harmonics by an element of 7900 (-78 dB) or extra. The result’s a sine wave with lower than 0.0002% (2 ppm) distortion.
Determine 1 A frequency multiplier and a monitoring low-pass filter cut back the THD of the sine sign coming from a operate generator by 78 dB or extra.
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This implementation’s benefit comes from the truth that operate turbines present two indicators, a sine wave and a sq. wave, with the identical frequency. The sine wave goes to the low-pass filter. A voltage divider reduces amplitude in order that it doesn’t exceed the enter vary of the filter. The switched-capacitor filter wants a clock sign with frequency that’s 100 instances bigger than the frequency of the sign being filtered. The microcontroller–oscillator pair generates the clock sign. The microcontroller (µC) measures the frequency of the sign coming from the operate generator, multiplies it by an element of 100, creates a 16-bit management phrase, and sends it to the oscillator by means of an SPI interface. The oscillator generates a square-wave sign for the filter. The nook frequency of the filter equals the basic frequency of the enter voltage.
Determine 2 and Determine 3 reveal the circuit operation with a 20-kHz triangular sign. Determine 2 reveals the time area indicators from the operate generator and on the output of the filter. Determine 3’s pictures are the spectrums of those indicators. The filter passes the basic frequency and reduces the distortion-making harmonics right down to the noise flooring. The distinction between the basic and the ground is about 50 dB, which is regular for the 8-bit ADC of the oscilloscope. Extra subtle (and costlier) tools is required to catch the distinction of about 80 dB anticipated for the sine wave. Curious readers can get a way of this enterprise in references [1-4].
Determine 2 The time area indicators from the operate generator and on the output of the filter.
Determine 3 The spectrum of the 20-kHz triangular sign: the basic harmonic is 50 dB above the noise flooring. Extra subtle tools is required for the sine wave.
It’s price mentioning that the identical strategy has been used to filter a sq. wave sign or a digitally generated sine wave with a really small variety of stairs, see references [5-7]. Regardless of the big attenuation of the filter, the output sign just isn’t a pure sine wave because of the excessive degree of harmonics within the enter sign: 43% for the sq. wave and 11-12% for a “sine” wave made of 5 stairs. The proposed circuit makes use of an enter sign with 1% distortion (analog operate turbines) or 0.1% distortion (DDS-based operate turbines); therefore the output sign might be not less than 10 or 100 instances cleaner than with the earlier circuits.
When you resolve to make the circuit, measure the interval, not the frequency of the sign coming from the sign generator. The longest measurement time might be 50 ms as a substitute of seconds.
Additionally, be sure you fill the interval interval with not less than 1000 clock pulses; the objective is to get 0.1% accuracy. This implies clock fee of 20 MHz or extra for the shortest interval of fifty µs. Decrease clock charges needs to be used for longer durations to keep away from getting too giant numbers for the captured clock pulses.
Lastly, maintain the sine sign amplitude from 3 to 9 VPP. That is the vary the place the filter offers minimal distortion and noise.
Jordan Dimitrov is {an electrical} engineer & PhD with 30 years of expertise. At the moment he teaches electrical and electronics programs at a Toronto neighborhood faculty.
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References
- Williams J., G. Hoover. Check 18-bit ADCs with an ultrapure sine-wave oscillator. EDN, Aug. 11, 2011, 19-23.
- Janasek V. An extremely low-distortion oscillator with THD beneath -140 dB. http://www.janascard.cz/aj_Download.html
- ARTA software program https://artalabs.hr/index.htm
- TSP #234 – QuantAsylum QA403 24-bit, 0.0001% THD Audio Analyzer Overview, Teardown & Experiments https://www.youtube.com/watch?v=BlWzpMkX5QQ
- Horowitz P., W. Hill. The artwork of electronics. 3rd, 2015, pg. 436.
- Saab A. Locked-sync sine generator covers three a long time with low distortion. EDN, Sep 18, 2008.
- Elliot R. Sinewave oscillators, Part 8 – Digital era. https://sound-au.com/articles/sinewave.htm#s8
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