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Behringer UMC 202HD for measurements
Behringer UMC 202HD for measurements
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Old 27th August 2019, 06:09 AM   #31
trobbins is offline trobbins  Australia
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REW allows option to adjust test frequency to be related to fft frequency - is that an Arta option you are using?
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Old 27th August 2019, 09:01 AM   #32
MagicBus is offline MagicBus  Greece
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I downloaded REW yesterday and I'll need some time to find my way with it. At the moment, the only way to make it work is to check "lock frequency to FFT". I don't know what it really means. I haven't seen that in ARTA. First impression is that REW allows for more detailed measurements but the configuration is not for beginners.
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Old 27th August 2019, 09:05 AM   #33
phofman is offline phofman  Czech Republic
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It's quite simple - in configuration choose your sound device, start generator, start RTA and set FFT length in config dialog (the gear icon in upper-right corner of RTA window).
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Old 27th August 2019, 09:32 AM   #34
MagicBus is offline MagicBus  Greece
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OK! I got some graphs by just sniffing around. First pic is the best I could do so far. "Lock frequency to FFT" checked. The "peak" is between 7 and 8 harmonics. REW doesn't give it a name or measurement. Second pic is the "peak" alone. Third pic is with "lock frequency to FFT" unchecked.
Attached Images
File Type: jpg FFT.JPG (229.6 KB, 142 views)
File Type: jpg peak.JPG (219.5 KB, 143 views)
File Type: jpg unlock FFT.JPG (216.1 KB, 137 views)
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Old 27th August 2019, 09:37 AM   #35
phofman is offline phofman  Czech Republic
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Perfect. The last picture is typical buffer underrun, just click on Reset averaging. Also you might want to select averaging forever in RTA settings.
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Old 27th August 2019, 09:43 AM   #36
MagicBus is offline MagicBus  Greece
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Thanks! I need to spend a little more time with it. Seems very interesting!
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Old 27th August 2019, 12:00 PM   #37
JohnPM is offline JohnPM  United Kingdom
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Quote:
Originally Posted by MagicBus View Post
OK! I got some graphs by just sniffing around. First pic is the best I could do so far. "Lock frequency to FFT" checked. The "peak" is between 7 and 8 harmonics. REW doesn't give it a name or measurement. Second pic is the "peak" alone. Third pic is with "lock frequency to FFT" unchecked.
Lock frequency to FFT means REW adjusts the generator frequency so that it is an FFT bin centre. That means it is periodic in the FFT length and can be analysed without a window (or with a Rectangular window, which is the same thing). If the test signal is not on a bin centre (or an external generator is being used) a window must be used, such as Blackman-Harris 7. Your third plot shows the effect of using a Rectangular window with a signal that is not periodic in the FFT length.
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Old 27th August 2019, 12:35 PM   #38
phofman is offline phofman  Czech Republic
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John, are you sure that the third plot is effect of the disabled FFT tracking? I always get that chart when discontinuity of input data occurs (e.g. switching fft option + resetting generator, or buffer underrun, or java does not make the calculation in time for new data in 192kHz), must reset the averaging to get back to proper result. I never use the FFT tracking checked.
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Old 27th August 2019, 12:39 PM   #39
JohnPM is offline JohnPM  United Kingdom
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John, are you sure that the third plot is effect of the disabled FFT tracking?
Yes, that's how rectangular windowing looks on non-periodic signals. A signal interruption would have a similar (though transient) effect. Locking the generator to the FFT length allows maximum frequency resolution by using a Rectangular window so no window spreading.
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Old 27th August 2019, 03:02 PM   #40
MagicBus is offline MagicBus  Greece
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Quote:
Originally Posted by JohnPM View Post
Lock frequency to FFT means REW adjusts the generator frequency so that it is an FFT bin centre. That means it is periodic in the FFT length and can be analysed without a window (or with a Rectangular window, which is the same thing). If the test signal is not on a bin centre (or an external generator is being used) a window must be used, such as Blackman-Harris 7. Your third plot shows the effect of using a Rectangular window with a signal that is not periodic in the FFT length.

Hi JohnPM,

The more I use REW the more I appreciate it, great work! Indeed, using Blackman-Harris 7 doesn't require to lock frequency anymore. So, I had my first IMD measurements. It tells you all harmonics with numbers and it leaves aside any artificial peaks not related with the signal. This is a great feature IMHO. REW stays!
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