Getting an audio sign with a THD < 0.0002% made simple

When you want a really pure sine wave within the audio vary, the circuit in Determine 1 may help. It’s a easy deal: A sine wave with a THD of 1% coming from a perform generator that goes via 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 perform generator by 78 dB or extra.

Wow the engineering world together with your distinctive design: Design Concepts Submission Information

This implementation’s benefit comes from the truth that perform mills 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 perform generator, multiplies it by an element of 100, creates a 16-bit management phrase, and sends it to the oscillator via an SPI interface. The oscillator generates a square-wave sign for the filter. The nook frequency of the filter equals the elemental frequency of the enter voltage.

Determine 2 and Determine 3 exhibit the circuit operation with a 20-kHz triangular sign. Determine 2 exhibits the time area indicators from the perform generator and on the output of the filter. Determine 3’s photographs are the spectrums of those indicators. The filter passes the elemental frequency and reduces the distortion-making harmonics all the way down to the noise flooring. The distinction between the elemental and the ground is about 50 dB, which is regular for the 8-bit ADC of the oscilloscope. Extra subtle (and costlier) gear 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 perform generator and on the output of the filter.

Determine 3 The spectrum of the 20-kHz triangular sign: the elemental harmonic is 50 dB above the noise flooring. Extra subtle gear is required for the sine wave.

It’s value mentioning that the identical method 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 massive 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 perform mills) or 0.1% distortion (DDS-based perform mills); therefore the output sign might be a minimum of 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 an alternative of seconds.

Additionally, be sure to fill the interval interval with a minimum of 1000 clock pulses; the objective is to get 0.1% accuracy. This implies clock charge 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 V_PP. That is the vary the place the filter supplies minimal distortion and noise.

Jordan Dimitrov is {an electrical} engineer & PhD with 30 years of expertise. At present he teaches electrical and electronics programs at a Toronto group faculty.

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 References

1. Williams J., G. Hoover. Check 18-bit ADCs with an ultrapure sine-wave oscillator. EDN, Aug. 11, 2011, 19-23.
2. Janasek V. An extremely low-distortion oscillator with THD beneath -140 dB. http://www.janascard.cz/aj_Download.html
3. ARTA software program https://artalabs.hr/index.htm
4. TSP #234 – QuantAsylum QA403 24-bit, 0.0001% THD Audio Analyzer Assessment, Teardown & Experiments https://www.youtube.com/watch?v=BlWzpMkX5QQ
5. Horowitz P., W. Hill. The artwork of electronics. 3^rd, 2015, pg. 436.
6. Saab A. Locked-sync sine generator covers three many years with low distortion. EDN, Sep 18, 2008.
7. Elliot R. Sinewave oscillators, Part 8 – Digital era. https://sound-au.com/articles/sinewave.htm#s8

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