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.

MBS = NBS


Why they put them letters right next to each other ?

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.


** The HP339A is a high performance, auto-nulling THD analyser, capable of resolving 0.001% distortion.

To test it you *must* use the internal low distortion oscillator and set the unit to auto null at say 1kHz. After a few seconds the reading should fall to around 0.001 or 0.002%.

Pegging the meter is not a fault, until the auto nulling system has finished its job large amounts of fundamental frequency get through the notch filter/s and the meter is simply showing you that fact.

It is very instructive when using a THD analyser to view the residual signal on a scope - so you know what it consists of.

Might be mostly AC hum, random noise, second harmonic or crossover distortion spikes - or a mix of some or all the above.



..... Phil

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

"It is very instructive when using a THD analyser to view the residual signal on a scope - so you know what it consists of. "

I assume that is what is at the "monitor" terminals. Is that correct ? Iff thsat is what it is I would like to have a scope connected there, or later when I get one, a spectrum analyzer maybe ?

"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. "

Very interesting. Once I got the thing to the point where I think it works I will have to look into that. An actual ay to calibrate it, though not really accurate it does prove it works. Maybe with a little bit of trouble it can be confirmeed down to a certain resolution.

The Wavetek has a fixed square wave (in fact all of them) and a variable output that is selectable. Couple of resistors should do it.

So 90/10 should read 48.4 %. That means 99/1 should read 4.84 % ?

Question - am I going by peak value or RMS/average or what here ?

On 8/1/2015 4:41 PM, wrote:
"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. "

Very interesting. Once I got the thing to the point where I think it
works I will have to look into that. An actual ay to calibrate it,
though not really accurate it does prove it works. Maybe with a
little bit of trouble it can be confirmeed down to a certain
resolution.

The Wavetek has a fixed square wave (in fact all of them) and a
variable output that is selectable. Couple of resistors should do
it.

So 90/10 should read 48.4 %. That means 99/1 should read 4.84 % ?

On a voltage basis, 100% square is 48.4, 10% square is 4.84, 1% square
is 0.484, 0.1% square is 0.0484, and so on. (On one definition of THD
it's slightly inaccurate at 10% because there's only 98% of the sine
power at that point, but it's the lower numbers you care about anyway.)


Question - am I going by peak value or RMS/average or what here ?

From a signal processing POV, the THD meter reading is the square root
of SINAD, i.e. the ratio of (everything except the sine wave)/(the sine
wave) on an amplitude basis, not a power basis like a SNR or SINAD. I
gave the definition in my last post.

The intention is to have a number you can read off on a scope, so it's
basically voltage. You can't usefully add up the amplitudes of
different frequency components, so the audio THD definition uses the
square root of the power rather than the voltage per se. You can think
of it as RMS distortion divided by RMS signal.

Cheers

Phil Hobbs




--
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

Phil Hobbs wrote:


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

** One needs a test that shows the instrument is doing the things a THD analyser is meant to.

1. Thoroughly remove the fundamental frequency.

2. Pass other frequencies up to 100kHz to the metering circuit.

Frequency sweeps from 20Hz to 100kHz will verify this - with the instrument set to notch a variety of spot frequencies in the audio band.

The notch should be sufficiently narrow at each test point that little attenuation exists at half and double the centre frequency and beyond up to a -3dB point at 100kHz.


.... Phil

On 8/2/2015 4:10 AM, Phil Allison wrote:
Phil Hobbs wrote:


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

** One needs a test that shows the instrument is doing the things a
THD analyser is meant to.

1. Thoroughly remove the fundamental frequency.

2. Pass other frequencies up to 100kHz to the metering circuit.

And

3. Measure distortion accurately.

The square wave/attenuator trick does all of that.


Frequency sweeps from 20Hz to 100kHz will verify this - with the
instrument set to notch a variety of spot frequencies in the audio
band.

The notch should be sufficiently narrow at each test point that
little attenuation exists at half and double the centre frequency and
beyond up to a -3dB point at 100kHz.

(I actually own an HP 339A but haven't used it for anything serious.)

Cheers

Phil Hobbs


--
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

Phil Hobbs wrote:
Phil Allison wrote:




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

** One needs a test that shows the instrument is doing the things a
THD analyser is meant to.

1. Thoroughly remove the fundamental frequency.

2. Pass other frequencies up to 100kHz to the metering circuit.

And

3. Measure distortion accurately.

The square wave/attenuator trick does all of that.


** Like hell it does.



Frequency sweeps from 20Hz to 100kHz will verify this - with the
instrument set to notch a variety of spot frequencies in the audio
band.

The notch should be sufficiently narrow at each test point that
little attenuation exists at half and double the centre frequency and
beyond up to a -3dB point at 100kHz.

(I actually own an HP 339A but haven't used it for anything serious.)


** That is very obvious.


.... Phil

On 8/2/2015 8:33 AM, Phil Allison wrote:
Phil Hobbs wrote:
Phil Allison wrote:




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

** One needs a test that shows the instrument is doing the things a
THD analyser is meant to.

1. Thoroughly remove the fundamental frequency.

2. Pass other frequencies up to 100kHz to the metering circuit.

And

3. Measure distortion accurately.

The square wave/attenuator trick does all of that.


** Like hell it does.

Really? Why not?

If the indicated distortion gets down to the noise floor, the
fundamental has to be gone to that level, which satisfies #1.

If the indicated reading is correct and stable over frequency, then the
notch isn't attenuating the third harmonic. Some more asymmetric
rectangle would get you the second harmonic, but those are a bit more
difficult to calculate, especially for a technician. So #2 could be
improved, but is probably fine if the filter is actually working.

And nothing in your method addresses #3, the accuracy question, whereas
the THD of a square wave is known very accurately, and an attenuator
allows you to make accurate distortion standards down right into the
mud. What's not to like?

Cheers

Phil Hobbs


--
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

On Sat, 01 Aug 2015 13:32:53 -0700, jurb6006 wrote:

"It is very instructive when using a THD analyser to view the residual
signal on a scope - so you know what it consists of. "

I assume that is what is at the "monitor" terminals. Is that correct ?
Iff thsat is what it is I would like to have a scope connected there, or
later when I get one, a spectrum analyzer maybe ?

The 339A manual is on the BAMA site at edebris.com

Phil Hobbs wrote:

Phil Allison wrote:


** One needs a test that shows the instrument is doing the things a
THD analyser is meant to.

1. Thoroughly remove the fundamental frequency.

2. Pass other frequencies up to 100kHz to the metering circuit.

And

3. Measure distortion accurately.

The square wave/attenuator trick does all of that.


** Like hell it does.

Really? Why not?

** Passing test 2 is essential, it shows the instrument complies with the definition of THD+N used in audio.

Unlike your test, it identifies if the notch has the required narrow bandwidth and that the high frequency response is correct.

Your test, if done at 8kHz or above, will show a significant under reading error on a faultless instrument due to progressive attenuation of the harmonics.

BTW:

A *far simpler* test of overall THD reading accuracy is to mix a small sine wave signal of known amplitude from another oscillator with the internal one. Long as the new frequency is set clear of the notch filter's effect, the correct reading in percentage is easily calculated.



..... Phil

"due to progressive attenuation of the harmonics. "

Seems you have answered a question I have not yet asked. Like, if the reading is to be considered valid there has to be a specified bandwidth otherwise you are excluding the meaningful or including the useless. Like, you do not need to know the content at 4.8 GHz.

So 100 KHz is the cutoff then ? I never checked. Seems reasonable, that would be the fifth harmonic of 20 KHz. If anyone can hear that we should all line up to kiss their ass and give them time to advertise the tickets.

Anyway, my buddy took it home today. It is his actually, but we just umm, don't care which place a certain piece of equipment is at all the time.

But now I want to get more specific on this mixing a square wavee into the sine wave thing to do an ersatz calibration check. Right now it is reading pretty high distortion on its internal oscillator, and actually reading lower on my Wavetek. I haven't been to all the ranges of the internal oscillator yet, nor on the Wavetek. At 1 KHz, the Wavetek read like 0.48 %. the internal oscillator was like 27 %. This does not seem right, maybe a ground problem or bypass caps or something because it just doesn't seem all that likely. I would like to have a distortion meter to check the oscillator on this distortion meter. Yup.

The wavefoprm looks alright by eye. I did not try nulling it with the Wavetek nor doing the Lissajous thing to see if I oculd get a decent circle. but it looked alraght and back then I did see a decent sine wave. the high distortion reading came after it had an all night burn in.

Then, unfortunately it was time to go.

He has to learn how do run the thing. Hey, I didn't know initially. I know the theory behind it but operation is another thing. I got it figured out pretty much and next will come the finer points. Like, yeah it measures, but does it measure accurately ?

Next question. This thing FILTERS, not really nulls out the fundamental. this allowws it to accurately measure through a system that has phase shift. Otherwise an OP AMP would make the best distortion meter. So it gets lovcked to that frequency by something that resembles a PLL at least remotely.

Now, if I have 1 V P-P sine going in there at 1 KHz, if I took another generator, not locked or anything to the fundamental one, and injected a 2 KHz sine wave into it, the distortion meter should read that accurately, correct ?

I mean I am putting in a controlled amount of the second harmonic, but it is not locked in any phase relationship and in fact is not even frequency locked.

WILL THAT WORK ?

That thing wiht the mixing a square wave with the sine wave, I want to try that. But the ratio, is it RMS voltage or P-P ? A sine wave makes it easy because it is the same as the fundamental. Adding it at the second harmonic at 10 % the amplitude clearly should read 10 % on the meter, correct ?

In fact, now that I think of it I shold be able to add ANY frequency to it not at the fundamental and the meter should read that. Read the dB scale or the percentage, whatever snokes ya up. (yeah, I hear some are starting to describes THD in -dB instead of percent)

Anyway, now that I think of it, all I need is a second generator.

Interesting.

wrote:

"due to progressive attenuation of the harmonics. "

So 100 KHz is the cutoff then ?


** It's the typical -3dB point of the metering circuit.

I never checked. Seems reasonable, that would be the fifth harmonic of 20 KHz.


** Correct - 2nd, 3rd & 5th harmonics plus noise are read by the meter when if 20kHz is the fundamental. By convention, this has long been is deemed adequate to quantify non-linearity in audio circuits.


Next question. This thing FILTERS, not really nulls out the fundamental.

** A notch filter IS a filter, adjusting the centre frequency to match an incoming sine wave is called " nulling". The 339A has a notch filter.


So it gets lovcked to that frequency by something that resembles a PLL at least remotely.


** The auto nulling feature tracks the incoming frequency, long as it varies slowly. Without it, small temperature drifts in the oscillator and notch filter makes manual nulling very tedious or impossible when dealing with low THD percentages.


Now, if I have 1 V P-P sine going in there at 1 KHz, if I took another generator, not locked or anything to the fundamental one, and injected a 2 KHz sine wave into it, the distortion meter should read that accurately, correct ?

** Yep - a pair of say 10kohms in a Y arrangement allows the signals to combine at the input to the 339A. The level of both will be cut in half, of course.


I mean I am putting in a controlled amount of the second harmonic, but it is not locked in any phase relationship and in fact is not even frequency locked.

WILL THAT WORK ?


** Yep.


Anyway, now that I think of it, all I need is a second generator.


** Best set the level of the second generator to not more than say 10% of the main one, so the auto-nulling system is not confused.

..... Phil

Now I notice the onboard oscillator goes up to 110 KHz. Also, I notice that the bandwidth is only 110 KHz. This should mean that it really cannot measure any distortion on the 110 KHz from the oscillator. At 55 KHz it would only pick up the second harmonic which is pretty much useless.

Now that the thing seems to be acting right, when it comes back it will be time to see about accuracy. It actually belongs to my partner but we kinda don't care who has what as long as we got the basics. He has frequency counters, and this. I have everything else. He lacks a really good function generator, well he decided to tinker with it. There's another story that might warrant a thread. I did a workaround that made it work almost perfectly but this guy is a fanatic about measurement and things like that. I can start the thread but any real working on it will have to wait until I figure out what went wrong. I think he has some wires connected wrong. They are all individual with almost no markings, though some of them have a little paint on them. Few.

So basically this thing will be back. It was here a few days. Probably in a couple of weeks after he tinkers with it.

Anyway, it is good to know that I can check this thing. If the readings are within reason I should be able to trust it enough for the kind of audio I do. See I will work on high end and vintage stuff, but I do repairs, not restores. I don't go changing every capacitor in a unit with those high resolution types. I have tried to explain that to some people and some understand yet others claim thy can hear the difference.

The way I see it is if you can hear it I should be able to measure it. In fact that claim was made and proven by Bob Carver. He, in one act (well a series of the same) not only proved that the regular IHF specs don't tell the whole story, but also put some of the audiophoolery to rest. It IS measurable, and what I say is that if it IS measurable then what is not measurable should not be hearable.

Then it gets to the realm of psychoacoustics. Then it also gets to the point where people might WANT some distortion. Electric guitarists are certainly not afraid of distortion.

Anyway, back to this. Since it is a notch filter based device we know it is measuring THD + noise. Anything outside of the notch counts.

I have contemplated a way to measure that without such a device. (yeah, now we got one and won't need it HA !) I have, in the past, taken and nulled out the input and output of an amp using the add function on the scope, and usually having to invert one channel. The scope will display THD+N but will also indicate if there is any phase shift. You start getting up near the upper limit in frequencies, many audio amps have considerable phase shift.

To solve that, first of all I do not want to tamper with the signal coming back. But the sine wave going in, where it goes either to the DUT or the scope, I could shift the phase around a little with a cap and resistor. Bottom line, in an amp the phase is going to lag of course. So all I have to do is lag it a little bit where it goes into the scope or whatevr actually does the nulling oin this case.

That still leaves the problem of deriving an RMS measurement out if it which means nulling it externally and using a true RMS meter. I have a Fluke down there. I just checked and it says it is down -3 dB at 200 KHz.

So I can use the scope to get the best null in both aplitude and phase with a pot and a cap, and then what the Fluke sees on there in RMS should be comparable to the total amplitude and then we can derive percentage easily. In fact using the KISS theory just keep it at like 10 volts RMS or something that makes it easy to divide. (when possible of course) Make it read 10 on the Fluke and then switch the source to the nulled signal.

Now this is nulled electronically, not with a tuned circuit.

Except for the notch filter, that 339A is just a generator and an oscillator. Question is how hard will it be to get a comparable accuracy as opposed to something like the 339A ? One thing about it is that any distortion in the source oscillator is also nulled and is taken out of the picture. This should lower my measurement floor if it works.

Another question, should you decide to field, and able, is what about PURE THD ? I mean pure THD without the noise. The only way I can figure is to exploit the randomness of the noise by somehow recording the output and synching it somehow to get an average reading from a number of cycles.

The problem with that is the only real way to do it is digitally or BBD. If I want a low measurement floor there, that is going to have to be one hell of a BBD.

But is there another way ? They routinely say THD, not THD+noise. Either there is a way to null out the noise, or they are using signal levels that make the noise insignificant. I don't see how this would be easy on something like a moving coil phono preamp. Very few, but some of them are complimentary push pull and in those you might want THD measurements at the lower levels. In the ones that just use single ended stages not so much. Mostly you would want to use higher levels anyway because the THD levels will be higher then. It is just that it is not true of the fully complimentary ones.

Sorry this is getting so damn in depth but I guess that's the way the rock rolls.

wrote:



Another question, should you decide to field, and able,
is what about PURE THD ? I mean pure THD without the noise.
The only way I can figure is to exploit the randomness of
the noise by somehow recording the output and synching it
somehow to get an average reading from a number of cycles.


** Digital scopes can do that job, by averaging a number of screens.

When the scope is synched to the test frequency (using ext trig), it is also synched to the harmonics. So averaging 8 or 16 screens removes much of the random noise and hum.

.... Phil

"** Best set the level of the second generator to not more than say 10% of the main one, so the auto-nulling system is not confused.

..... Phil "

Hmmm.

I just sent an email to my partner on this, thinking about going up to 25 %.. If I let it lock, or tun or whatever, would I gt away with those higher levels ? Or are you talking about different units or something.

This is askable because this thing has a scale to read up to 100 % THD.

The way I am reading this fromm you is that the tracking circuit may be upset by frquencies too cloes. Would phase locking them help ? I can fuly understand a 1.4 KHz square ave assauling the tracking circuit, that is obvious..

But then, IF that freqquency rsponse of the detector is realy flat it doesn't matter what I put in there as ong as it is far awat fro the fundamental. What if I use 1 KHz for the fundamental and 5 KHZ for the induced signal ? (from the external oscillator) Surely that should not confuse it.

What's more, ou probably don't want a test " pollutin" frequency any closer than second harmonic. I can see that if you are testing the veracity of the filters themselves, but for most THS measurements doesn't matteer. However I intend to beat on the thing.

We got scopes, and I might just be using them all when this happens. In fact it might be a nice to have a high Q 1 KHz tuned circuit to eliminate any THD in the HP oscillator. I am not holding my breath.

We'll see how it goes. I am finally up on working the thing and next we scrutinize. For the audio business, when I test something on it, usually I can just compare channels. But it is nice to have at least a semi-accurate set of absolutes.

Update :

https://dl.dropboxusercontent.com/u/29948706/33901.jpg

Now have calibrated distortion in via a matched set of resistors and an external generator. It measured 0.055 % on its own oscillator. I had a 10K rom that nd a 10K from the HP generator with 600 ohm output impedance.

I calibrated, which is to set the level so the needle goes to 100 % and then you switch the switch.

We found that even moving around the room affected the reading when we were reading that 0.0055 %. Also, because of my slight (only one order of magnitude) error, we were measuring 10 KHz, not 1 KHz. Of course all the filters were off, but I do want to try those filters.

I started to get close to the null frequency with the external generator. I saw how the response dropped.

The scope on the left was connected to the monitor, where you can see the TRMS meter input. I am about to chew through these ropes and get a spectrum analyzer to hook up there. The scope to the right has the original HP 339A output going to one channel and the output of the other HP generator sitting on top to the other channel. Thus it is easy to compare the amplitude of their waveforms.

When the amplitude was 25 %, the distortion meter read 25 %. I mean when the internal HP was 3 volts, and the external was 1.5 volts it read 50 % distortion. Since the TRMS convertor is after the input attenuator, is it not reasonable to assume that it will be accurate at different ranges ? that is determined by resistors just like every, and I mean every other thing on the planet.

Well there are exceptions...

On 8/12/2015 4:01 AM, wrote:
Update :

https://dl.dropboxusercontent.com/u/29948706/33901.jpg

Now have calibrated distortion in via a matched set of resistors and
an external generator. It measured 0.055 % on its own oscillator. I
had a 10K rom that nd a 10K from the HP generator with 600 ohm output
impedance.

I calibrated, which is to set the level so the needle goes to 100 %
and then you switch the switch.

We found that even moving around the room affected the reading when
we were reading that 0.0055 %. Also, because of my slight (only one
order of magnitude) error, we were measuring 10 KHz, not 1 KHz. Of
course all the filters were off, but I do want to try those filters.

I started to get close to the null frequency with the external
generator. I saw how the response dropped.


THD is a ratio of RMS voltages, so 1.5V vs 3V is 50% THD, not 25%.

Cheers

Phil Hobbs



--
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

On Wed, 12 Aug 2015 01:01:08 -0700 (PDT) wrote in
Message id: :

Update :

https://dl.dropboxusercontent.com/u/29948706/33901.jpg

404.