A pitch-linear VCO, half 2: taking it additional

Editor’s Be aware: This DI is a two-part sequence.

Half 1 exhibits how one can make an oscillator with a pitch that’s proportional to a management voltage.

Half 2 exhibits how one can modify the circuit to be used with greater provide voltages, implement it utilizing discrete components, and modify it to carefully approximate a sine wave.

In Half 1, we noticed how one can make an oscillator whose pitch, versus frequency, may be made proportional to a management voltage. On this second half, we’ll have a look at some other ways of arranging issues for different attainable functions.

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

To start out with, Determine 1 exhibits a revised model of the essential circuit, constructed with B-series CMOS to permit rail voltages of as much as 18 or 20 V relatively than the nominal 5 V of the unique.

Determine 1 A variant on Half 1’s Determine 2, permitting operation with a provide of as much as 20 V.

Aside from U2’s change from a 74HC74 to a CD/HEF4013B, the principle distinction is in U1. With a 12 V rail, TL062/072/082s and even LM358s and MC1458s all labored effectively, as did an LM393 comparator with an output pull-up resistor. The management voltage’s span will increase with provide voltage, however stays at ~±20% of Vs. Be aware that as a result of we’re solely sensing inside that central portion, the restricted enter ranges of these gadgets was not an issue.

One thing that was an issue, even with the unique 5-V MCP6002, was a frequent incapability to start oscillating. In contrast to the 74HC74, a 4013 has active-high R and S inputs, so U1a’s polarity have to be flipped. It tends to start out up with its output excessive, which successfully locks U2a into an all-1s situation, forcing Q1 completely on. That explains the necessity for R5/C5/Q2. If (when!) the sticky situation happens, Q2 will activate, shorting C2 in order that Q1 can flip off and oscillation begin. A reverse diode throughout R5 proved pointless on the low frequencies concerned.

This is also constructed utilizing the additional constant-current sink, proven in Half 1’s Determine 4, however then U1 would want to have rail-to-rail inputs.

A model that lacks any logic

That is an extension of the primary model that I attempted, which was constructed with out logic ICs. It’s neat and works, however U1a might solely output pulses, which wanted stretching to be helpful. (Utilizing a flip-flop assured the obligation cycle, whereas the spare part, used as a monostable, generated a lot better-defined reset pulses.) The circuit proven in Determine 2 works round this and may be constructed for just about any rail voltage you select, so long as U1 and the MOSFETS are chosen appropriately.

Determine 2 This all-discrete model (other than the op-amps) makes use of a second part to provide an output having an obligation cycle near 50%.

U1b’s circuitry is a reproduction of U1a’s however with half the time-constant. It’s reset in the identical approach and its management voltage is identical, so its output pulses have half the width of a full cycle, giving a sq. wave (or practically so). Ideally, Q1 and Q3 must be matched, with C3 precisely half of C1 relatively than the sensible 47n proven. R7 is barely vital if the rail voltage exceeds the gate-source limits for Q1/3. (ZVP3306As are rated at 20 V max.)

Purity comes from overclocking a twisted ring

The ultimate variation—see Determine 3—goes again to utilizing logic and has a fairly sinusoidal output, do you have to want that.

Determine 3 Right here the oscillator runs 16 occasions quicker than the output frequency. Dividing the heart beat charge down utilizing a twisted-ring counter with resistors on its 8 outputs offers a stepped approximation to a sine wave.

The oscillator itself runs at 16 occasions the output frequency. The heartbeat-generating monostable multivibrator (MSMV) now makes use of a pair of cross-coupled gates, and never solely feeds Q1 but additionally clocks an 8-bit shift register (carried out right here as two 4-bit ones), whose closing output is inverted and fed again to its D enter. That’s generally known as a twisted-ring or Johnson counter and is a kind of digital Möbius band. Because the sign is shifted previous every Q output, it has 8 excessive bits adopted by 8 low ones, repeated indefinitely. U2c not solely performs the inversion but additionally delivers a quick, strong excessive to U3a’s D enter at start-up to initialize the register.

U2 and U3 are proven as high-voltage CMOS components to permit for operation at far more than 5 V. Once more, U1 would then want altering, maybe to a rail-to-rail enter (RRI) half if the additional present supply is added. 74HC132s and 74HC4015s (or ’HC164s) work tremendous at ~5 V.

The Q outputs feed a standard level by resistors chosen to provide an output which, although stepped, is near a sine wave, as Determine 4 ought to clarify. R4 units the output stage and C4 supplies some filtering. (Totally different units of resistors can provide completely different tone colours. For instance, if they’re all equal, the output (if stepped) might be an excellent triangle wave.)

Determine 4 Waveforms illustrating the operation of Determine 3’s circuit when it’s delivering ~500 Hz.

The steps correspond to the fifteenth and seventeenth harmonics, which, although considerably filtered by C4/R4, are nonetheless at ~-45 dB. To cut back them, add a easy two-pole Sallen–Key filter, like that in Determine 5, which additionally exhibits the filtered spectrum for an output of round 500 Hz.

Determine 5 An acceptable output filter for including to Determine 3, and the ensuing spectrum.

The 2nd and third harmonics are nonetheless at round -60 dB, however the others at the moment are effectively beneath -70 dB, so we are able to declare round -57 dB or 0.16% THD, which might be worse at 250 Hz and higher at 2 kHz. This method received’t work too effectively in order for you the complete 4–5-octave span (additional present sink) until the filter is made tunable: maybe a few resistive opto-isolators mixed with R14/15, pushed by one other voltage-controlled present supply?

All that’s fascinating, however relatively pointless. In spite of everything, the principle function of this design thought was to make helpful audible tones, not precision sine waves, which sound boring anyway. However a secondary function must be to push issues so far as attainable, whereas having enjoyable experimenting!

A musical coda

Given a pitch-linear tone supply, it appeared foolish to not strive make some type of musical thingy utilizing a tappable linear resistance. A few toes, or about 10kΩ’s-worth, of Teledeltos chart paper (which I at all times knew would turn out to be useful, because the saying goes) wrapped spherical a size of plastic pipe with a clean, shiny croc clip for the faucet or slider (plus a 330k pull-down) labored fairly effectively, permitting tunes to be picked out as on a Stylophone or an air guitar. Electro-punk lives! Although it’s not a lot “Eat your coronary heart out, Jimi Hendrix” as “Get these earplugs in”.

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

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