Supersized log-scale audio meter – EDN


On the finish of the DI for a Easy log-scale audio meter, I promised to indicate improve it to work higher. With these fixes, it now has near-digital efficiency, with sooner response and smoother operation. Even this supersized model is available in two flavours, one comparatively easy, the opposite, maxed out. It may now out-perform the usual peak program meter (PPM) specs (for which this an excellent reference), and has a span of over 60 dB with straightforward setting of the specified minimal and most ranges.

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

Whereas the aim for the unique model was to supply one thing easy and useful, the purpose of this DI is to see how intently we are able to match the efficiency of some traces of DSP code, irrespective of how a lot {hardware} it might take. The unique used only one twin op-amp; this strategy inflates that to 2 quad packs. Excessive: in fact. Instructive enjoyable: undoubtedly, at the very least for us analogeeks.

The underlying precept is similar as earlier than—drive present by means of a diode, measure the ensuing voltage, which is proportional to the logarithm of the enter, and seize the height worth—however the implementation is completely different. Determine 1 reveals the fundamental circuit.

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Determine 1 We take the log of the enter sign; its peak degree is captured on C2, which is discharged slowly and linearly; and temperature- and level-corrections are utilized within the present supply which drives the meter.

The audio enter to be measured is now utilized by means of R1, a 10k mounted resistor moderately than a thermistor. The thermistor gave compensation for the diodes’ tempco by scaling the (linear) enter; with the mounted resistor, we’ll apply an offset to the (logged) sign later within the circuit to attain the identical consequence. A1’s output is a logarithmically-squashed model of the enter. For now, we solely want its constructive peaks.

A2 and Q1 kind a easy peak detector. At any time when A1.OUT is larger than the voltage on C2, A2/Q1 dumps present into C2 till the voltages match. Utilizing a transistor moderately than a diode enormously improves the velocity; as drawn, with R2 = 22R, it can seize a single half-cycle at 20 kHz, as proven in Determine 2 which is manner sooner than the PPM spec requires. (For a slower, extra lifelike response, enhance R2. 1k5 provides a ~5 ms response time to inside 1 dB of the ultimate studying.) This will appear to be a number of op-amps in need of a “correct” peak detector, nevertheless it does the job in hand: it’s been Muntzed. (Muntz? Who he? This will clarify.) Taking A2.IN- straight from C2, which could appear extra traditional, results in overshoot or slows the response, relying on the worth of the collection resistor.

Determine 2 The assault or integration time could be very quick; the decay or return time, a lot slower, and linear.

Now that now we have charged C2 quick, we have to discharge it slowly. A3 buffers its voltage, with D3/R4 bootstrapping R3 to present a linear fall in voltage equal to twenty dB in 1.7 s, which, extra by blissful accident than by design, is strictly what we wish.

Now we go the sign by means of D4, whose tempco of about -2 mV/°C compensates for that of D1/2. It additionally drops the extent by its VF of about 600 mV, which wants restoring. D5 is proven as a generic 1.25 V shunt stabiliser, and its actual sort or worth is just not crucial. (I used an LM385 which was handy; with a clear, secure detrimental provide rail, it may be designed out.) It offers an correct supply for offsetting not solely D4’s VF, but additionally the sign as a complete, to set the meter needle’s zero level. R8 permits adjustment of this from about -62 dBu (R8 = 10k) to +1 dBu (R8 = zero).

A4 drives the meter motion, buffering the voltage from D4, the offset-voltage compensation being utilized by means of R9. A4 drives present by means of the meter into R11, the ensuing voltage throughout that being fed again by means of R10 to shut the suggestions loop. The meter has D6 in collection with it to forestall underswings, and D5 catches detrimental swings on A4. (Disgrace we are able to’t do the identical for A2.)

Calibration is easy. Apply the minimal enter degree on the enter, or apply a DC voltage similar to the minimal detrimental peak worth to the sign finish of R1, and regulate R8 for zero indication on the meter. Now apply the utmost degree—I selected +10 dBu—and set R11 for full-scale deflection. R8 should be set first, then R11.

Temperature stability is nice. In line with LTspice, the tempco is zero at round +1 dBu enter and cheap at different ranges, giving a studying appropriate inside 1 dB down at -50 dB or so for 15 to 35°C. Frustratingly, I might solely get higher compensation by including further resistors and a thermistor in a community round R10, the values differing in response to the specified span: too many interactions. An additional stage might have mounted this, however . . . Determine 3 reveals the response of the meter, each simulated and dwell.

Determine 3 Simulated and measured responses when arrange for a 50 dB span with a +10 dBu most studying, displaying the results of temperature and op-amp offset.

We now have a high-performance meter, with near-digital accuracy and even precision. However it’s nonetheless solely half-wave sensing, and has a few residual bugs. For full-wave operation, we are able to add inverter A5, and so forth., to the output of A1, together with a second peak-detection stage, A6 and Q2, successfully paralleled with A2 and Q1, so as to add within the contribution from positive-going inputs: see Determine 4. If A1 and A5 have zero offset voltage or if a couple of trimmer-derived millivolts are utilized to A2.IN+ and A5.IN+, C3 will be omitted. The enter offsets inherent in real-world (and low-cost) op-amps restrict the span, as they result in inaccuracies at low ranges, the place the sign to be measured is comparable with them.

One other manner of including bipolar detection would have been to make use of a full-wave rectifier on the enter, however the further op-amp offsets made this strategy too inaccurate with out messy trimming.

Determine 4 Further elements will be added for full-wave detection.

This circuit responds sooner than a meter motion can comply with. C2 could also be charged nearly instantaneously by a transient, however its voltage will decay by an indicated 11.8 dB/second (or 20 dB in 1.7 s). Thus, if the meter takes 85 ms to reply, it can under-read that transient by 1 dB. Determine 5 reveals treatment this.

Determine 5 Last additions: a “power-on reset”, and a monostable to present ~100 ms maintain time after a peak to permit the meter motion to catch up.

A7 and A8 kind a monostable, which is triggered by a pointy enhance in C2’s voltage and generates a constructive pulse at A7.OUT. Connecting this to R4, which not goes to Vs-, by way of a diode cures the issue: whereas A7.OUT is low, C2 will discharge within the regular manner, however whereas it’s excessive, C2’s discharge path is successfully open-circuited. As proven, and with +/-6 V rails, this maintain time is ~100 ms. Alter C5 or R16 to range this. The consequence will be seen in Determine 2.

A ultimate contact is a power-on reset, additionally proven in Determine 5. (Digital circuits often have them, so why ought to we be not noted?) A pointy rise of the constructive rail activates Q3—which can be nearly any n-MOSFET—for a couple of hundred milliseconds, clamping C2 to floor whereas the circuitry stabilises. With out this, C2 could cost to a excessive degree at power-on, taking many seconds to get better.

Though a 100 µA meter motion is proven, A4 will comfortably drive a number of milliamps. Choose or regulate R11 to swimsuit.

When you could not need to construct a whole meter like this, the strategies and concepts used right here could properly turn out to be useful for different tasks. However if you happen to do, you’ll want to use an ebonite-cased motion, full with polished brass inlays, and with a pointer based mostly on a Victorian town-hall clock’s minute hand. Electro-punk lives!

—Nick Cornford constructed his first crystal set at 10, and since then has designed skilled audio tools, many datacomm merchandise, and technical safety package. He has eventually retired. Largely. Form of.

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