Distorsions...

[GOULAS]

Hello. Please, why do i have such distorsions at 12K6 hz?

 

[John Mulcahy]

Probably because the reference is the harmonic frequency. The 2nd harmonic reference is at 25.2 kHz which is likely after your measurement rolls off so the fundamental there is very low. You can choose whether or not to use the harmonic frequency as the reference in the graph controls, the help explains why that may be helpful.

 

[GOULAS]

That is true. You are the best.

 

[GOULAS]

Please could you tell me when we must use "Plot harmonics at the harmonics frequency"?

 

[John Mulcahy]

Plot harmonics at the harmonic frequency changes the way the harmonic orders for sweep measurement distortion are plotted - rather than plotting the distortion at the frequency of the fundamental it is plotted at the frequency of the harmonic, so (for example) the 2nd order harmonic distortion level for 1 kHz would be plotted at 2 kHz, where the distortion occurs. This makes it easier to correlate harmonic levels with the level of the fundamental at the harmonic frequency and also helps distinguish the effects of external noise, since noise affects all harmonics at the frequency at which it occurs. The THD trace is not affected by this control, it will continue to show the THD value corresponding to the harmonic levels at their fundamental frequency. However, when plotting harmonics at the harmonic frequency the level of the fundamental at the harmonic frequency (which in this graph mode is the cursor frequency) is used as the reference, so Use harmonic frequency as ref will be selected and the control for it disabled. If that was not previously selected the THD trace will update.

 

[GOULAS]

I did not understand. Maybe with this exemple :

1/ without "plot", frequencies start at 100hz. At 1khz H3=0,026%.

 

[GOULAS]

2/ With "plot", at 1khz H3=2,22%.

Please Mister John, could you tell me which is the real distorsions : 1/ or 2/ ?

 

[John Mulcahy]

In the second case you would need to move the cursor to 2 kHz to see the 2nd harmonic level for 1 kHz fundamental, 3 kHz for the 3rd etc.

 

[GOULAS]

I am lost. Is the "plot" shows where the H3 appear on the fundamental in this example?

 

[John Mulcahy]

If you choose "Plot harmonics at the harmonic frequency" then the 3rd harmonic of 1 kHz will be plotted at 3 kHz. The whole 3rd harmonic graph is shifted 3 times higher in frequency than when you do not select that option. The graph legend show figures for the cursor position, so if you have "Plot harmonics at the harmonic frequency" selected and the cursor is at 1 kHz the 2nd harmonic figure will be for 500 Hz and the 3rd harmonic will be for 333.3 Hz. Don't use "Plot harmonics at the harmonic frequency" unless you have some specific need to do that.

 

[GOULAS]

I will never use "plot...". And thank you a lot John.