I’m wondering if it is a simple calculation to determine how many dBs of headroom a notch adds to the system.
Suppose the QA403 can measure my DUT down to -120dB THD+N but the true THD+N is lower (all hypotheticals)If I add a 1KHz notch with 20dB of suppression and 20dB inbuilt gain, is there a way of estimating the next floor down to which I can measure?
Hi @Michael1, I think given a pure source, the best the QA403 ADC will do is around -114 dB THDN or so. And with a notch you can probably get to -123 dB or so. Sometimes a little better, sometimes a little worse. When measuring all this, everything needs to line up just so wrt apmplitudes. That is, you will get the best number at a given amplitude +/- 1 dB or so.
Ah yes, I couldn’t quite recall the actual limit of the QA403, so I gave it the benefit of doubt. -114dB was the limit I measured pre-notch. I’m now measuring with notch (30dB notch, 20, or 26,dB in built gain), and I’m now getting readings of -124dB, pretty much as you say.
Really, my question is what is the reason for the limit I (might) be hitting now? Would a deeper notch help, versus more overall gain?
Hi @Michael1, it’s hard to know. If you are at -124 dB for THD+N, you are pretty much at state of the art levels. Hard to know where to go next. Are you limited by harmonics or noise? If noise, then I’d look across everything to see how to shrink resistor values. If harmonics, then you’d want to figure out the contribution of your source and the analyzer. There’s some interesting work done by a guy on DIY audio for canceling harmonics in measurement equipment. It’s a big, big job. But I think fertile for finding the next 3-6 dB or so.
Congrats on getting where you are! Please feel free to share your setup as I’m sure others would like to see the pieces along with me!
Step 1: spend over £70 on a single DAC chip - the ES9039SPRO! I suspect the team at ESS should take most of the credit.
I’m using the QA403 teamed with a Cosmos APU for measurements. The latter has a 30dB notch with 20 or 26dB inbuilt gain. It also has a separate low noise pre-amp with 34 or 60dB of gain. It’s good at what it does, but I much prefer the QA kit.
I’ll need to do some more experimenting with what I’ve got, then. Of note, the DAC has configurable THD compensation for the 2nd and 3rd harmonics. I haven’t fully explored this feature, but a quick look seemed to demonstrate that I could make the second harmonic nigh on disappear fairly easily, but the 3rd harmonic, which, at baseline, is always slightly larger than the second harmonic, doesn’t budge no matter how much a adjust the coefficients.
Thanks for the additional details, @Michael1. Something that might be informative is swapping the + and - inputs on the QA403. Theoretically, the distortion generated by the 403 should be the same regardless of polarity. And of course your setup should be the same regardless of polarity. So, if you look at THD normal and inverted, it should be the same. BUT! what is likely happening is you have some 2H contribution from the DUT and the anlayzer, both with different phase and magnitude. When you swap in the inputs, your DUT phase will be inverted and you can get a sense of how much of the 2H you are seeing is from interaction rather than just the DUT or just the analyzer.
Also, the N+D and N-D measurements were added to help understand if you were noise are harmonic limited. If you can do plots, please share those numbers too.