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Second Harmonic Distortion Measurement

K

knausz

New Member
Hi All,
How do I measure the percentage of a specific harmonic, rather than just the lumped such as THD? I basically want to measure only second or only third harmonic percentage.
I have a Heathkit 5258 distortion analyzer - is this possible with this device or a similar one such as an HP 334A?

Thanks,
Imre
 
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knausz said:
Hi All,
How do I measure the percentage of a specific harmonic, rather than just the lumped such as THD? I basically want to measure only second or only third harmonic percentage.
I have a Heathkit 5258 distortion analyzer - is this possible with this device or a similar one such as an HP 334A?

Thanks,
Imre
Click to expand...

THey lump all distortion products together.

You could use a selective 2nd harmonic filter for a specific frequency, a spectrum analyzer, or a frequency seletive voltmeter... you could just look at the distortion output provided by the instruments, and pay attention to the stuff at that frequency.
 
Short of a spectrum analyzer that only semi-easy way I can think of is to build say a 2khz stop band filter, either active or passive and install it between the amp output and the input to the distortion analyzer. Now take readings on the analyzer both with and without the the filter with a 1khz signal. The difference in THD could then to attributed to the 2nd harmonic. Really this is the kind of thing that spectrum scopes were made for.....still wish I owned one :thmbsp:

PS: Can anyone one explain why they call the 2nd harmonic the 2nd harmonic. If it's the second then that means the fundamental signal is also the first harmonic? how can a signal be a fundamental and also a harmonic. Makes no sense :D

Lefty
 
Lefty said:
PS: Can anyone one explain why they call the 2nd harmonic the 2nd harmonic. If it's the second then that means the fundamental signal is also the first harmonic? how can a signal be a fundamental and also a harmonic. Makes no sense :D

Lefty
Click to expand...
Quiet day there in Cally?:scratch2:
 
EchoWars said:
Quiet day there in Cally?:scratch2:
Click to expand...

Yea, raining all day and I'm sorting and marking a package of 1,000 1/4 watt resistors of 100 different values so I'm kind of in a harmonic mode :smoke:

Lefty
 
Lefty said:
PS: Can anyone one explain why they call the 2nd harmonic the 2nd harmonic. If it's the second then that means the fundamental signal is also the first harmonic? how can a signal be a fundamental and also a harmonic.
Click to expand...

So, you want an explanation as to why the 2nd harmonic is called the 2nd harmonic? In my understanding, it all comes down to that old buggaboo - math. It's defined the way it is to make conversations and calculations in the head easy to do. For example, the second harmonic is 2x the fundamental, and the third harmonic is 3x. Second is 2x, third is 3x, etc. Clever, huh?

You may have noticed I also used the term fundamental. This introduces the other way of talking about harmonic content, fundamental and overtones. The overtone progression may make more sense to you, but it does make the math less obvious. In short, "harmonics" are more used in the engineering world, and "overtones" are used in the musical world.

Here's a quick chart of how "harmonics" relate to "overtones":

"Harmonics"............."Overtones"
First harmonic.......=..Fundamental
Second harmonic...=..First overtone
Third harmonic......=..Second overtone
Fourth harmonic....=..Third overtone
Fifth harmonic......=..Fourth overtone
etc etc

DON'T CONFUSE HARMONICS AND OVERTONES - they mean different things.

Hope that's somewhat clear. If the first harmonic thing still bugs you, just accept it as a definition that the first harmonic IS the fundamental. Sorta like how we define 10 to the power of 0 is 1. Doesn't make sense, but that's how it is defined so the math works out.
 
You could buy two analyzers. The first would measure THD. Feed the filter (distortion) output to a second analyzer, setting the frequeency of the second to twice the source. The difference in the two THD readings will be 2nd harmonic.
There are some pretty cheap soundcards for computers that work with software to do RTA. Feed the distortion meter output to a card and it will give you the amplitude of the harmonics.
You can also look at the distortion output on a scope. If the wave looks like a sine of twice your source frequency (use both traces), then you have lots of 2nd. If it looks nothing like the source, then you have very low 2nd.
 
As I recall, only the odd order harmonics are calculated in THD. Anybody know why?
 
d3imlay said:
As I recall, only the odd order harmonics are calculated in THD. Anybody know why?
Click to expand...

That is not what I recall. Most simple DA circuits I've seen notch out only fundemental test signal only and then measure the resulting signal so all harmonics present should contribute to the total distortion value. :scratch2:

Lefty
 
Lefty said:
PS: Can anyone one explain why they call the 2nd harmonic the 2nd harmonic. If it's the second then that means the fundamental signal is also the first harmonic? how can a signal be a fundamental and also a harmonic. Makes no sense :D

Lefty
Click to expand...

analogous to: "the sound of one hand clapping" :zen:
 
I know it's not much use to knausz, but I have a wave analyser similar to that mentioned by tcdriver, though made by Marconi Instruments. It acts like a very narrow band tunable filter across the audio and supersonic ranges
and is good down to about 0.001% distortion. It's quite ancient - even has some germanium transistors in it but still works like a charm. See pic. It's the one in the bottom left hand corner.
 

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I have found some free software which uses an fft analysis to extract the THD and harmonics over a frequency range. It allows you to look at the harmonics individually. The program is called "Steps". It is part of a package which you can download at:

http://www.fesb.hr/~mateljan/arta/download.htm.

If you have a reasonable computer with a good sound card this is a good solution for distortion analysis in the audio frequency range.
 
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