Is there anything special I should be aware of when using the QA402 as a basic spectrum analyzer (to measure an audio generator’s distortion, for example). Right now I would set the frequency of Sig Gen 1 to the frequency I would be looking at and the highest attenuation to start, and maybe the start & stop frequencies ? The devices I would be looking at would not be ultra low noise sources…
Is there anything special I should be aware of when using the QA402 as a basic spectrum analyzer
Hi @VAR, a few things to keep in mind are:
- You’ll probably want HANN windowing unless you can lock the source to the QA402 some how.
- Make sure you have selected Use L Peak if you want to make a THD measurement.
- Make sure you have the right input level selected. You can check a few levels. Note that with a -10 dBV input level, the 0 dBV Full Scale Input shows poor THD (-96.38 dB) but good noise minus distortion (-113.34 dB)
At +18 dBV Full Scale Input the same signal shows -112.91 dB THD and -96.98 dBV noise minus distortion.
At +12 dBV Full Scale Input the same signal shows -118.36 dB THD and -102.97 dBV N-D.
Now, since we can’t see the harmonics it’s important to understand the harmonic reading is really just the noise noise at the harmonic frequencies summed. So, you need to increase your FFT size until you see the harmonics surface. The plot above is 32K. If we bump to 256k we get the following. Note the THD has improved a bit, but the noise hasn’t. That is expected. However, we’re still not seeing the harmonics peek out of the noise. So, next let’s go to 1M point FFT
With a 1M point FFT, we can see the 2H and 3H clearly now. And their levels are at about -129 dBc according to markers. Since there are two of them at roughly the same level, their combined level is about 6 dB stronger than shown. So, that would degrade the -129 figure 6 dB or so to around -123. But there’s also something out at 9 kHz. In short, the -120.67 figure we’re seeing here looks legit in that it is dominated by tones and not noise at the harmonic frequencies.
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Thanks for the tutorial/explanation, Matt. I was not aware of trying to match the QA402’s source level with with that of my source’s level, if I am understanding correctly, and would not have selected L Peak for THD options.
Hi @VAR, If your Gen1 was set to 1 kHz and your source was 1 kHz, then it would have just worked out and you wouldn’t have needed to change anything. In the next release later this week, the fundamental marker (green F at bottom of the display) will show up when using an external generator (currently it only shows up when Gen1 is the THD source) and so you can easily verify the correct fundamental has been chosen. The arrow shows the “F” and from that you know the fundamental the system is using for it’s calculations.
That was indeed the case. I guess having the GEN output set to the level of my source is not really important from the little playing around I did… Is there a reason you like to show THD as dB and not % ? I prefer % since that is what I am used to looking at in spec sheets. Also, do you have to clear all markers or can you clear just a particular one ?
Is there a reason you like to show THD as dB and not %
Hi @VAR, I think % made a lot of sense when distortion levels of -60 dB (0.1%) were common in amps, and -80 dB (0.01%) was state of the art. The readability of those numbers is fine.
But when state of the art distortion is -120 (0.0001%) or even -140 dB (0.00001%) I think the number of leading zeros really hampers the readability I guess PPM could be another measurement to get rid of the zeros.
I prefer % since that is what I am used to looking at in spec sheets.
TI usually shows both, which seems increasingly common. Personally, I think it’s time to retire the %. Take a look at the OPA1612 data sheet…I think the % numbers almost require a straightedge to sort out. Five leading zeros followed by an 8? Alternately, that could be 8 PPM, which might be better. But -142 dB is so…perfect compared to 0.000008%. My $0.02 anyway 
Thanks for the explanation, Matt- that makes perfect sense. Most everything that I measure will be older, 80’s & 90’s gear, so % is just fine, and if I recall, we can’t typically 1% THD except on a good day…?