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Old 25th February 2012, 09:48 AM   #1
Elvee is offline Elvee  Belgium
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Default A simplistic distortion-meter

Here is a very simple and easy THD meter project for beginners.

Of course, it cannot compare with commercial instruments, or even projects like Cordell's, but the investment in time, money and effort is on a completely different scale.

It only requires standard, consumer-grade parts (5% carbon resistors and 10% capacitors, common opamps), needs no component sorting or selection, and is completely adjustment-free.
It works on a single, non-regulated supply, and is easy to use, making it ideal for educational/didactic purposes.

It has three ranges: 100%, 10% and 1%. On the 1% range, the measurement floor and usable resolution is 0.01%.

The indicator is simply a digital multimeter in the 2V range, the full range voltage being 1V.

The circuit is based on a SVF (state variable filter), making it highly tolerant to components inaccuracies: they have no impact on the depth of the null, only on the exact frequencies.
Compared to a double T, it is a huge improvement, and it makes frequency variation easy with any low-cost stereo potentiometer.
With the values shown, the frequency range is 500~5000Hz, this can easily altered or augmented by changing/switching the integrator's capacitors.

The discrete precision rectifier renders operation possible from 30Hz to 30KHz (harmonics up to 150KHz).

The first version has a discrete residue amplifier.
The second schematic is annotated with DC quiescent voltages, and the third shows a fully integrated version.

Some pics of my prototype follow.
Attached Images
File Type: gif DistoGR2.gif (30.6 KB, 633 views)
File Type: gif distann.gif (36.2 KB, 594 views)
File Type: gif dist3.gif (32.0 KB, 581 views)
File Type: jpg 100_5163.JPG (312.0 KB, 571 views)
File Type: jpg 100_5166.JPG (289.3 KB, 545 views)
File Type: jpg 100_5172.JPG (275.0 KB, 213 views)
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Last edited by Elvee; 25th February 2012 at 09:51 AM.
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Old 25th February 2012, 10:33 AM   #2
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Interesting. Add a couple of switchable attenuators and it would be dead handy for quick power amp testing.
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Old 25th February 2012, 11:17 AM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by pinkmouse View Post
Interesting. Add a couple of switchable attenuators and it would be dead handy for quick power amp testing.
The input potentiometer acts as a variable attenuator, the input level can range from less than 1.5V to 100V, that is sufficient for 99% of power amplifiers.
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Old 25th February 2012, 02:25 PM   #4
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Doh!

What about input protection for incorrect settings. Diodes to the rails or suchlike.
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Old 25th February 2012, 05:27 PM   #5
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by pinkmouse View Post
Doh!

What about input protection for incorrect settings. Diodes to the rails or suchlike.
With R1 and C1, it could probably survive momentarily a direct connection of the input to the 230V mains.
Not something I'd recommend, but rugged enough to survive "normal" mishaps.

Some instructions for those unfamiliar with manual THD meters:

-Connect input to the A.U.T. and output to a DVM, range 2V, select "Set level" and adjust the level to read ~50% (0.5V)
-With the main tuning pot P2, search for the maximum amplitude. When done, adjust level to read exactly 1.000V.
-Select "100%" and adjust P2 for a minimum reading
-Switch to "10%" and adjust fine tune P3 for minimum
-If necessary do the same in 1% range

The output is linear, and 1V=100%, thus a reading of 110mV in the 10% range is equivalent to 1.1% distortion.

Note that the rectifier is not a true rms, and for high levels of multiple harmonics, the reading will be slightly underestimated, but at "normal" distortion levels, this effect is negligible.

For example, a pure triangle wave reads 11.2% instead of the actual ~12% it should, but this shouldn't be a problem as this instrument is not destined to be used in metrological applications
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Old 25th February 2012, 06:59 PM   #6
Frex is online now Frex  France
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Quote:
With R1 and C1, it could probably survive momentarily a direct connection of the input to the 230V mains.
Not something I'd recommend, but rugged enough to survive "normal" mishaps.
I'm not sure that your 22k input potentiometer survive on a 230V main direct connection... (it must dissipate ~2.4W).

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Old 25th February 2012, 08:44 PM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Frex View Post
I'm not sure that your 22k input potentiometer survive on a 230V main direct connection... (it must dissipate ~2.4W).

Frex
Certainly not for a long time, in this sentence:
Quote:
With R1 and C1, it could probably survive momentarily a direct connection of the input to the 230V mains.
the important word is momentarily: the time to realize there is a funny smell around, and dive to pull the plug....
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Old 26th February 2012, 07:31 AM   #8
Frex is online now Frex  France
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Hello,

You are right Elvee, but you probably need to unplug it very faster !
So, about your design, did you have done some Bode plots to show notch depth and width in it's frequency setting range ?
Often, graphs speak more than words.
Regards.

FRex
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Old 26th February 2012, 08:57 AM   #9
Elvee is offline Elvee  Belgium
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Here you are:
First a global outlook with the frequency adjusted at 1KHz, marker at 2KHz (gain error = 0.4dB).
Then a zoom at the two frequency extremes.
And finally, the impact of random component deviations on the depth of the null.

Surprisingly, the fundamental rejection performance of the physical prototype is significantly better than predicted by the sim.
Perhaps caused by parasitic capacitance of the potentiometers, I don't know exactly
Attached Images
File Type: jpg SVF1.jpg (175.7 KB, 230 views)
File Type: jpg SVF2.jpg (182.4 KB, 171 views)
File Type: jpg SVF3.jpg (187.5 KB, 113 views)
File Type: gif SVFmc.gif (50.4 KB, 116 views)
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Old 26th February 2012, 09:23 AM   #10
Frex is online now Frex  France
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Hi,

Interesting, but when i had speak about Bode plots, it was real world measurements (done with sound card for example), not simulations.

Frex
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