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HomeElectronicsExtremely-low distortion oscillator, half 1: how to not do it.

Extremely-low distortion oscillator, half 1: how to not do it.

Editor’s Notice: This DI is a two-part collection.

Half 1 discusses audio oscillators, particularly the Wien bridge and biquad, state-variable, or two-integrator-loop configuration.

Half 2 will add distortion-free suggestions to the biquad to provide a pure sine wave.

Over time, the Design Concepts (DI) column has featured many attention-grabbing oscillators, however none that I can recall was particularly designed to provide a very clear sine wave. Placing that omission along with the necessity to rebuild my outdated sub-0.01% sine generator gave me the right excuse to do some exploring, ending up with this DI, which is in two components. First, we’ll take a look at methods to not do it, then half 2 will present learn how to get Audio Precision® distortion ranges for RadioShack costs.

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The options

We gained’t even contemplate squashing a triangle wave to present one thing moderately sinusoidal, as 0.3% THD is the perfect that this method can provide with out complicated, multi-breakpoint “squashing” networks. Equally, phase-shift oscillators are out; their q-factor is low, and three-gang pots usually are not catalog objects. And if I had been designing one thing for contemporary manufacturing, the starting-point could be a 24-bit DAC fed by a small processor containing a big look-up desk, however that’s not one thing that may be knocked up from obtainable components in a day.

So, what’s fallacious with a great outdated Wien bridge circuit? The related Wikipedia web page incorporates a lot historic, sensible, and mathematical element, and it studies that distortion ranges right down to 0.0003% (3  ppm) might be achieved, so we’ve one benchmark, although that’s probably for a spot frequency relatively than for a multi-range, fully-tunable gadget. A sensible goal is 96 dB or 0.0015%, which is absolutely the restrict for CD-type 16-bit linear PCM audio, whereas a extra arbitrary objective is 120 dB, or 1 ppm. At these ranges, THD could also be dominated by circuit noise, which we’ll conveniently ignore for now.

Wien bridge oscillator

To verify issues out, I breadboarded a primary circuit utilizing an LM4562 op-amp, carefully-matched resistors and polystyrene capacitors, and amplitude-stabilised utilizing a photoconductive opto-isolator (basically an LED and an LDR) pushed by some heavy filtering. (A thermistor solely works for increased output ranges and may be very bouncy.) Determine 1 exhibits the schematic and Determine 2 the output spectrum at near 1 kHz for a stage of -20 dBV (about -22 dBu, or 0.283 Vpk-pk).

Determine 1 A easy Wien bridge oscillator, utilizing a photoconductive opto-isolator to assist stabilise the amplitude.

Determine 2 The spectrum of the oscillator working at ~1 kHz, its output being at -20 dBV.

The spectrum implies a THD of about -76 dB or 0.02%: solely so-so. Nevertheless, I’ve learnt to be cautious of FFTs when the dynamic vary of the sign to be examined exceeds about 90 dB, and like to notch out a lot of the elemental, permitting a clearer view of the harmonics. Determine 3 exhibits the results of this: a lot better, with a THD of maybe -88 dB, or 0.004%.

Determine 3 The spectrum of the identical sign, however with the elemental principally notched out to indicate the harmonics extra precisely.

Higher, and never unhealthy for a lash-up, however nonetheless off-target. (Notice that the dimensions now exhibits the relative stage of the harmonics, in dBc, because the oscillator’s output is at 20 dBV and the notch filter has a voltage achieve of 10 dB or 20 dB.) With a bit of extra thought and much more fiddling—or vice versa—we might most likely enhance its efficiency to the benchmark stage, however a distinct starting-point seems extra promising. The largest drawback is the amplitude management loop as a result of eradicating all of the ripple results the damping badly, rising the loop settling time. The low Q-factor of a Wien bridge, 1/3, does us no favours in any respect.

Bi-quad loop filter

My favorite circuit for complete filters and oscillators has at all times been the bi-quad(ratic), state-variable, or two-integrator-loop configuration, one topology of which is proven in Determine 4.

Determine 4 A traditional bi-quad or two-integrator-loop filter, with its high-pass, bandpass, and low-pass outputs.

It’s possible you’ll nicely recognise one thing like this from 100 and extra NatSemi/TI datasheets and app notes. Its fundamentals return to the Fifties, I believe, when “operational amplifiers” often meant racks of glowing bottles, and it’s versatile, designable, and controllable. This model is minimize for a Q-factor of ~16 and a achieve of ~10. We’ll now bundle it with a dashed line and deal with it as a module. Assume ±5 V to ±15 V provides, with loads of decoupling caps.

To make it oscillate, we take the bandpass (BP) output and feed it again to the enter at an appropriate stage. That is usually finished by limiting the BP sign with a pair of back-to-back diodes, a lot as proven in Determine 5.

Determine 5 The filter with added diode-limited suggestions turns into an oscillator, however with loads of harmonics, giving a THD of round 0.0%.

With the values proven, the diodes compress the sign to ~2/3 of the output stage. Lower than this, and we lose stability; extra, and the harmonics turn out to be extreme. The suggestions community proven retains the impedances across the diodes low to permit clear operation as much as 100 kHz and past, whereas the added thermistor improves the amplitude stability with temperature. Match the diodes for ahead voltage to minimise even-harmonic distortion. The third harmonic produced by the diodes is decreased by about 22 dB by the point it reaches the LP output, increased harmonics being attenuated much more.

The spectrum for the uncooked (LP) output exhibits a THD of ~0.08%, which is about the perfect that this method can provide. (The “notched” spectrum—not proven—confirmed fewer and decrease peaks, however the third harmonic—the limiting issue—was nonetheless on the identical stage.) As a result of there isn’t any management loop as such, there might be no loop stability points, although the settling-time is considerable at low frequencies. It’s nonetheless a great foundation for a multi-range general-purpose AF oscillator.

With correspondingly bigger capacitors and resistors, it additionally works nicely at very low frequencies, although FET-input op-amps are wanted to keep away from leakage. With tuning elements of 5µ7 (= 4µ7 + 1µ0; PET dielectrics) and 3M3 resistors, and with TL072s fitted instead of the LM4562s, the waveform at ~8 mHz, or a calculated 118.2 s/cycle, seems like Determine 6.

Determine 6 Utilizing µF and MΩ for tuning, a diode-stabilised bi-quad will simply work down within the mHz area—about 8 mHz, on this case.

Why anybody would need to use a purely analog method to producing such low-frequency alerts escapes me, however making an attempt it was irresistible, even when it took an hour or so to cool down correctly. (I lacked the persistence to strive even bigger values of timing elements. And don’t even consider asking for the spectrum.)

To be continued…

Partially 2, we’ll take the bi-quad filter and add distortion-free suggestions, just like that used with the Wien bridge however on steroids, to provide a significantly pure sine wave.

Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio gear, many datacomm merchandise, and technical safety package. He has ultimately retired. Principally. Kind of.

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