THD 0.0001% [Design]

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From: Vladimir Vassilevsky on 27 Jul 2010 10:48

I just tested an active filter with THD ~ 0.0001%. Wow. This filter is
intended for distortion measurements.

There are two key things about it:

1. Using good quality film capacitors at the swings of less then 1% of
the rated voltage.

2. All opamps should be in the inverting mode.

Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

From: Jan Panteltje on 27 Jul 2010 11:14

On a sunny day (Tue, 27 Jul 2010 09:48:37 -0500) it happened Vladimir
Vassilevsky <nospam(a)nowhere.com> wrote in
<v42dnUnUC9wgbNPRnZ2dnUVZ_qadnZ2d(a)giganews.com>:

>
>I just tested an active filter with THD ~ 0.0001%. Wow. This filter is
>intended for distortion measurements.
>
>There are two key things about it:
>
>1. Using good quality film capacitors at the swings of less then 1% of
>the rated voltage.
>
>2. All opamps should be in the inverting mode.

How do you measure the distortion?

From: Jim Thompson on 27 Jul 2010 11:24

On Tue, 27 Jul 2010 09:48:37 -0500, Vladimir Vassilevsky
<nospam(a)nowhere.com> wrote:

>
>I just tested an active filter with THD ~ 0.0001%. Wow. This filter is
>intended for distortion measurements.
>
>There are two key things about it:
>
>1. Using good quality film capacitors at the swings of less then 1% of
>the rated voltage.
>
>2. All opamps should be in the inverting mode.
>
>
>Vladimir Vassilevsky
>DSP and Mixed Signal Design Consultant
>http://www.abvolt.com

Surf around for papers on compound connections of OpAmps that
dramatically improve GBW product.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Spice is like a sports car...
Performance only as good as the person behind the wheel.

From: Spehro Pefhany on 27 Jul 2010 11:36

From: Fred Bartoli on 27 Jul 2010 15:04

Vladimir Vassilevsky a �crit :
>
> I just tested an active filter with THD ~ 0.0001%. Wow. This filter is
> intended for distortion measurements.
>
> There are two key things about it:
>
> 1. Using good quality film capacitors at the swings of less then 1% of
> the rated voltage.
>
> 2. All opamps should be in the inverting mode.
>
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultant
> http://www.abvolt.com

Well, a few weeks ago I just measured levels just 20 times below that at
50kHz. OK, I cheated a bit since it was only H2 that I needed to measure :-)

Seems impossible, but doing it the right way it's not that difficult...
It's even deceptively simple once you've figured how to do it.

The real fun is to generate the signal and make sure that your
measurement is not pure illusion :-)

Again, H2 only made that easier (a ladder of notch filter does the job
pretty well). The funny thing was, for the first cut experiment, to
tweak the RLC filter distortion with magnets to set the inductor (last
stage) cycle mid point to a pure odd function this cancels any even
component.
A small NeB magnet at half a meter distance did the job but you had to
be really careful to not blow the distortion up to the ppm levels :-)
(RM8 cores)

--
Thanks,
Fred.

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