# Howto - Distortion Measurements with REW

Bit better

#### Attachments

• motu vic better.png
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#### NicMac

##### Member
Paid Member
I don't think it looks so bad. Harmonics below -120 dB.
You should really change the Y-axis unit (from SPL to dBV) and tell RTA what the input level is (in the top box labeled "FS sine RMS") if you want sensible numbers for noise.

#### reddish75

##### Member
With coherent averaging enabled, at the moment I'm trying to learn how to use Rew properly, any help or tips would be gratefully received.

#### Attachments

• coh averaging.png
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#### triplej

##### Member
Now we're getting somewhere. 000079% THD / 0002% THD+N ! I find the motu is easily over driven. You may get even better THD figures around -30 dBFS

#### jschwender

##### Member
Has anyone identified these distortions as thermal distortion? It should look striking, as it must be a H2 harmonic with -90° phase. For a standard 0.6 W axial wire resistor with a Tc of 50ppm this H2 calculates to -130 dB if 1 kHz sin max power applied. If 10% power applied it should be -150 dB.

#### bohrok2610

##### Member
Now we're getting somewhere. 000079% THD / 0002% THD+N ! I find the motu is easily over driven. You may get even better THD figures around -30 dBFS
Noise related numbers are not valid when using coherent averaging. That is why they are displayed in orange.

#### triplej

##### Member
OK that makes sense.

#### reddish75

##### Member
If anyone could help with an vrms value for -30dbfs? Or the calculation to convert?

#### NicMac

##### Member
Paid Member
Calculate or us a web converter like this.
How did you determine that the FS sine Vrms is 2.75V?

#### reddish75

##### Member
Calculate or us a web converter like this.
How did you determine that the FS sine Vrms is 2.75V?

I used my dso scope with rms readings but used a 1x bnc to phono wire first with the scope set to 10x so ignore that reading.

Here ive used the correct 10x probe to measure 1vrms from the oscillator then connected it all back up, this is what I have now,

#### Attachments

• motu better.png
148.5 KB · Views: 26

#### triplej

##### Member
From memory it was only about 150mV RMS before the THD started to creep up. I guess the easiest thing is just reduce the level to optimum THD reading. Also saw somewhere the Vic Osc. has a minimum output of 300mV ?

#### trobbins

##### Member
The link in #1177 indicates it may be important to keep Victor's oscillator output set to max, and attenuate externally if noise level becomes a concern.

#### jschwender

##### Member
I used my dso scope with rms readings but used a 1x bnc to phono wire first with the scope set to 10x so ignore that reading.

Here ive used the correct 10x probe to measure 1vrms from the oscillator then connected it all back up, this is what I have now,
please can you please switch on phase information for the distortion display, i want to compair that to my motu.