The current Design Concept “Getting an audio sign with a THD < 0.0002% made simple,” discloses a low THD sine generator which led me to mud off a design that I had printed in AudioXpress journal [1] (see Determine 1).

Determine 1 The “Easy Sineman” circuit [1] relies on an easier model of the circuit having roughly -80dB THD [2].

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

Necessities for an analog oscillator

Earlier than entering into the element of how this circuit works, it’s value recalling sure necessities for an analog oscillator: a suggestions circuit which, on the oscillation frequency f_osc Hz, has a loop achieve magnitude of unity and a part shift of both 0 or a a number of of 360 levels. One technique of implementing that is to put a notch filter within the suggestions loop of an op amp. You is likely to be forgiven for pondering that f_osc is on the filter’s notch frequency f_notch Hz. However clearly, infinite attenuation just isn’t in keeping with a unity achieve loop. Not so clearly, the op amp’s inner compensation community provides a -90° part shift to its inherent -180° inverting input-to-output part shift. What’s then wanted for oscillation is a filter which, at f_osc, displays each a -90° part shift and has an attenuation of A_osc, the place A_osc is the op amp achieve magnitude at f_osc. However how can we discover a filter able to assembly such exact constraints?

The “dual-T” notch filter

The revolutionary “dual-T” notch filter in Determine 1 saves the day. It’s made up of C₁, C₂, C₃, R₁, R₂, R_3A, and R_3B. I had a necessity for a 2400-Hz oscillator and so selected the values proven. One approach to place a notch at f_notch Hz is to make use of the next course of and equations:

Select a worth C for C₁, C₂ and C₃                                                (1)

and set R₁ and R₂ equal to 1 / (2π* f_notch*C*√3)                    (2)

set R₃ = R_3A + R_3B equal to 12 / (2π* f_notch*C*√3)                   (3)

An evaluation of this filter kind reveals that there’s all the time a worth of R₃ which produces an infinite attenuation notch whatever the variations of the opposite element values as a result of tolerances. Since there may be clearly no attenuation at DC, which means any attenuation from none to infinity could be had at some frequency. The evaluation additionally reveals that there’s all the time some frequency under f_notch at which the part shift is -90°. The suitable worth of R₃ causes that part shift to coincide with the required attenuation of A_osc at f_osc. Determine 2 provides a sense for some part and achieve magnitude responses of the filter as R_3B is various. Desk 1 relates the oscillation and notch frequencies and values of R₃ for a -90° part shift at numerous attenuations A_osc.

Determine 2 Responses of the dual-T notch filter. To simulate sensible variances from the best, capacitor values had been randomly chosen to be inside 1% of 10 nF, and R₁ and R₂ to be inside 0.1% of excellent values for a 2400-Hz f_osc. A price of R₃ that produced a 130 dB notch depth was calculated, and outcomes are proven with it and with a number of barely bigger R₃ values. -90° part shifts with attenuations from 65 to 130 dB are evident for numerous R₃ values.

Desk 1. Variations within the oscillation with respect to the notch frequencies and in R3 values for a -90° part shift at numerous A_osc attenuations.

Realizing the values of f_osc and A_osc, the worth of f_notch could be calculated from Desk 1. From this, the values of the capacitors and the resistors R₁ and R₂ could be calculated from equations (1) and (2). With 0.1% resistors for R₁ and R₂ and 1% capacitors, f_notch can be saved inside a spread of the tolerance product S_tol = 1 +/- 1.01*1.001 ≈ 1.1% of the meant worth. Observe that no matter element tolerances, there may be all the time the choice of including a pot in collection with both R₁ or R₂. The combination worth of that pot plus resistor ought to have a spread of S_tol centered on the equation (2) worth. The values and tolerances of R3A and R3B ought to be chosen in order that R3 could be adjusted to inside S_tol – NO_tol (see Desk 1) of the equation (3) worth.

It’s value noting that with the better-known twin-T notch filter [3], I used to be unable to satisfy the part and attenuation necessities concurrently by various solely a single resistor worth. Even when this had been attainable, the capacitors within the twin T are conveniently an identical, whereas the twin-T’s requirement of a worth ratio of two limits capacitor decisions. That is additionally a very good time to say that polystyrene capacitors supply the bottom harmonic distortion [4], with non-metalized polypropylene being a secondary selection.

Establishing oscillation amplitude

After all, the elephant within the room is what I haven’t but talked about—the requirement for establishing an oscillation amplitude. A method of doing that is to parallel the R₃ resistor plus pot with a non-linear resistor whose worth varies inversely with sign stage. Sadly, any such non-linearity will increase harmonic distortion. So it is sensible to decide on a non-linear element designed particularly for low harmonic distortion audio functions. The NE570 (an improved model of the SA571 seen in Determine 1) is a low harmonic distortion compressor/expandor IC meant for audio functions [5]. A block diagram of the half seems in Determine 3.

Determine 3 A block diagram of the perform of the SA571 and NE570 compandor IC, curtesy of On Semiconductor.

As can been seen, the half has a “delta G” cell whose present achieve is managed by the capacitively filtered output of the rectifier. The capacitively-coupled inputs to each features are linked in Determine 1 via resistors I’ve added to scale back the features’ working ranges. These are pushed by the output of the LME49720 op amp U_2A. (The op amp supplied with the SA571/NE570 is of the 741 kind and shouldn’t be utilized in extraordinarily low THD functions. Its output and one finish of the 20K resistor R3 could be left unconnected. Its inverting enter is linked to that of U_2A.) Observe the 1.8-V reference which is the unavoidable DC working voltage of the delta G cell and each inputs of U_2A.

The SA571/NE570 are twin components, and use is product of the secondary unit. Its rectifier capacitor pin is grounded to disable its delta G cell, whose enter is floating. The uncommitted aspect of its R₃ is linked to its op amp output to supply a secure 3 VDC supply. This supply drives the Determine 1 R₁₀ pot to produce a present to the THD trim pin. R₁₀ is adjusted to null out the small quantity of two^nd harmonic distortion produced by the delta G cell (and probably by U_2A). I powered the circuit from batteries for portability and added the LEDs to maintain contemporary 9-V batteries from exceeding the +/- 18 V most energy provide scores of the op amp. The SA571’s 30k resistor connecting the op amp inverting inputs to floor is unavoidable. With Determine 1’s R₃, it biases that op amp’s output to roughly 4.5 V( (≈45k/30k + 1)*1.8 V ). This stage could be decreased by connecting a resistor from the 3-V supply to U_2A‘s inverting enter (not supplied within the Determine 1 circuit). With or with out this extra resistor, bear in mind to maintain a correct DC bias throughout output electrolytic capacitor C₅.

The added passive elements on the NE570 inputs are chosen to permit R₃ to be adjusted for a 3 Vrms output from U_2A, the extent at which its datasheet signifies that that op amp displays the bottom THD.

Measuring distortion

To measure distortion, I attenuated the oscillator output’s elementary by working the sign via a second twin T filter with a pot in collection with every resistor. By laboriously tweaking every pot in flip, I used to be in a position to attenuate the basic by 70 dB. The filtered output was utilized to an SR770 spectrum analyzer which might precisely measure alerts inside an 80 dB dynamic vary. Tweaking the THD pot to reduce the two^nd harmonic stage, I measured the degrees of the oscillator harmonics and utilized corrections for the filter attenuations at every frequency (see Desk 2.) I then took the rms of the degrees corrected for the attenuations of the second twin T filter and arrived at a THD greater than 130 dB under the oscillator elementary.

Desk 2 Attenuation of upper harmonics by a dual-T filter tuned as described within the textual content to maximise attenuation of the oscillator elementary.

The NE570 and LME49720 datasheets and components can be found on-line and thru DigiKey. Small portions of the NE570 for experimenters could be had from quite a few eBay distributors.

I consider that it’s powerful to beat the mixture of simplicity and efficiency afforded by this design and welcome feedback from anybody who builds and assessments it.

Christopher Paul has labored in numerous engineering positions within the communications business for over 40 years.

Associated Content material

• Getting an audio sign with a THD < 0.0002% made simple
• Measure an amplifier’s THD with out exterior filters
• Extremely-low distortion oscillator, half 2: the actual deal
• A easy circuit with an optocoupler creates a “tube” sound
• Learn how to management your impulses—half 2

References

1. Paul, C, The Easy Sineman, audioxpress, November 2013, p. 52
2. Jung, “Acquire Management 1C for Audio Sign Processing,” Ham Radio, 1977, not out there.
3. https://learningaboutelectronics.com/Articles/Notch-filter-calculator.php#answer1
4. https://www.tedss.com/LearnMore/Polystyrene-Movie-Capacitors gives a big selection of polystyrene capacitors
5. ON Semiconductor, NE570 datasheet, https://www.onsemi.com/pdf/datasheet/ne570-d.pdf

The publish A easy, purely analog -130dB THD sine wave generator appeared first on EDN.

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