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Phil Hobbs

On 07/31/2015 11:18 PM, wrote:
One has come in to our realm. We do some high end audio so having
something like this desirable. It was bought surplus on the cheap.
It is not dead, but I am pretty sure it is not working correctly.
I've yet to completely learn to work the thing actually, and am
taking it slow.

First off, I DO know the basic tenets of a distortion meter. It
filters out the fundamental frequency and measures what remains. Not
quite rocket science. Unfamiliar territory for me indeed, but I
should be able to understand this.

I am looking for failure modes, tips on operation and basic self
checks and so forth. I have only gotten so far in the manual but I
am on it. The way I see it, these things you take a sine wave, put
it through whatever and calibrate the distortion meter somehow. Like
set it to full scale, flip the switch and you get the distortion
reading.

I cannot get it to act right, at least what I think shoud be right.
I have actually confirmed that it does detect distortion. Instead of
the onboard oscillator I used my Wavetek 111. Not the greatest but
good enough for what I wanted to do. I got the thing to give a
useful meter indication and switched waveforms. I switched to
triangle from sine and the needle didn't go up. OK, now realy I am
not sure if the peak/RMS value of a triangle wave is less or more
than a sine wave. And I fully expected the square wave to peg the
meter, which it did.

Until I hit this other waveform on it, looks like _/|_/|_/| and saw
the needle rise could I be sure this thing is anywhere near being
able to really measure distortion. I don't care what, that half
sawtooth with the 50 % dead time will deflect any measuring device
less whether it measures average, RMS, peak or smegma. PLUS the
thing is only positive going, so the peak value is half. And yet the
needle moved up.

I consider that a good sign. The Wavetek 111 info is available at
BAMA if you want to really see the waveform. And it is that waveform
like, exactly.

Another thing I am noticing is that when you change certain settings
the needle pegs and takes a few seconds to settle back down. Now,
with HP I am not impressed. some of their designs, certain other
things and their ideas of the human interface, I think suck. but the
fact is they built this, not me and no matter what I think about
some of their dowside, I do not bleieve they designed the thing so
it would peg the meter on things like range changes. Mode changes.
Like from input to distortion.

We did a quick visual on it, there is nothing spilled inside, no
burn marks, bulging caps or anyhing of the sort. Of course this is
going to be done again but we stuck the lid back on to see how it
works.

Other thing is that it has some broken knobs. I can deal with that
until some come along, as long as the thing works.

But really, ike known good test ways ? I am not talking MBS here,
just a basic test. Like if I switch from sine to square, is there a
number that it should read like 50 % or something ? I mean like at a
standard say 1 KHz and a specified rise/fall time. Can we get close
somehow ? Like if you buy a used ohmmeter off someone that is
supposed to work you would take a resistor and check it to see the
thing actually works.

Actually a test like that is what I am really after. And of course
whatever else might help this thing stay out the dumpster.
Calibration attempts later.

One simple method is to wire up a comparator to turn the internal
oscillator signal into a square wave.

The fundamental component of a 1V amplitude square wave (2V p-p) is 4/pi
volts, and the THD is everything else. The THD is sort of a voltage
ratio (it's the square root of the power ratio), but whatever it is, the
impedance divides out, so for simplicity let's figure based on 1 ohm
impedance.

Total power = (1 V)**2 / 1 ohm = 1 W

Fundamental power = (4/pi)**2 / 2 = 0.81 W

Thus

Harmonic power = 0.19 W

and

THD = sqrt(0.19/0.81) = 48.4% (*)

You can take that and add it to the original sine wave via voltage
dividers (a decade resistance box would be useful), and calibrate the
meter down as low as you like. 90% sine + 10% square = 4.84% THD, etc.

Watch out for the comparator pushing crap back out its input, which can
be a problem at very low levels.

Note that because they use a notch filter, these meters measure
THD+noise, not just THD.

Cheers

Phil Hobbs

(*) Another definition of THD is sqrt(harmonic power + noise
power)/(total power), which for a square wave would be sqrt(0.19/1) =
43.5%. Doesn't make much difference at tolerable distortion levels.

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net