Another Square Wave Request

Hi, been following the QA40x development for a while. I’m really interested! The two things holding me back are OSX (but I would can some Windows machine, just an expense) and the lack of reasonable square wave generation/analysis. Not a complaint, just another vote :slight_smile: .

I do appreciate that generating and analyzing the finest square waves is tricky business. However, I think it would be reasonable to generate passable square waves in the usual band limited audio domain.

Currently I use scopes and (analog) sig generators for this. These certainly aren’t the highest calibre squares, especially a sig gen that is probably about 60 years old… Nonetheless, they are still very useful for testing and tuning audio designs.

Above is a simple example of an amp with a resonance which I later traced to a specific transformer. Being able to generate this test (or sweep it, dreams) over the audio range with analysis views in time and frequency domains would very useful (to me).

I’d love to see this in a future version :smiley: .

Hi @garthvader, I don’t think you’d see what you hope to see. The issue is the sample rate of 192k is too low AND the built-in filters on the ADC/DAC are very good and very bandwidth-limited. In the plot below, you can see a 6Vpp square wave driven in the QA403 sampling at 192ksps. Note Gibbs phenom at the edges (which is the FIR filters doing their job) and the slew rate is limited to 79.8 kV/s, which is 79.8mV/uS. A 40 year old op-amp like the TL082 has a slew rate of 13V/us, which is 150X faster! If you want to know the slew rate of a modern amp, I think you need to be sampling at 10 MHz or more.

As a diagnostic tool for preliminary stability checks, you are correct it’s hard to beat a square wave. I have a 300 MHz scope (2Gsps) with a square wave generator that is primarily used for that. The internal rise/fall times of the combined generator and scope are about 6V in 90nS, or about 66V/us. The OPA1612 is around 27V/uS slew rate, so it’s it’s enough to measure that. The OPA1632 FDA is around 50V/us, so it’s getting tight there. The counterintuitive thing about slew rate is that it’s not tied to gain…Stability definitely is. But, in short, don’t give up on your current method! It’s likely to reveal things you won’t see on any audio analyzer with a sample rate below 10Msps.

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Just to add to this. I just went back to ARTA for some testing the other day.

The reason is simply I use all those waveforms ARTA has, especially triangle and the various pink and white noises. The QA can’t do those. So a more complex generator sure would be nice.

I often use triangle for biasing, easier to see clipping or the soft roll off of filters. And white and pink noise are great for full band RMS gain testing, esp the Periodic Pink of ARTA. I have no idea what that is but I use it all the time. Seems to give me more accurate frequency sweeps when there is phase or time distortion than the QA chirp test.

Hi @HawkerAMpT, I agree on triangle and noise options. The next version of the app (a ways out still) will have a generic low-pass filter on the generator output that can apply 10 or 20 dB/decade attenuation. If you take white noise and apply 10 dB/decade low pass that will give you pink, and a 20 dB/decade low-pass will give brown noise. I think both are helpful when driving a cabinet with a tweeter as white noise can be a bit harsh on HF drivers. The filters can be applied to any of the generators, including multitone.