Hello All. I’m new here and am just taking time today to get the feel of my new QA403. In the process of working through the quick-start guide, I noticed that my noise and distortion measurements were always four or five dB worse than expected. I’ve also carefully recreated the settings used in the product specifications and found worse-than-noted performance. Finally, I tried it on a laptop in a remote location to see if the noise was environmental and performance was in spec. FFW, after a lot of experimentation with different cables and ports physical locations, I’ve began to think that this degradation in performance is voltage-related. The degradation in performance on my desktop shows as 4.564V on the bottom bar of the software, whereas it shows around 4.8V on my laptop, using the same cable.
Sorry for the long-winded perspective above. The data sheet and getting-started guide certainly both reference the need for adequate power, but what would that minimum be? The software is reporting that 800mA is used on either computer. Is there a minimum voltage?
Hi @RobbN, the voltage that is reported on the bottom of the application is also color coded. So, if above 4.6, you are fine. But if below 4.6 it will turn orange, and if below 4.5V it will turn red. Both should indicate you are probably seeing conditions that mean your unit might not be meeting spec. The reason this happens if the incoming USB voltage is boosted to create +/13V for the internal rails. But if the incoming voltage isn’t high enough, then the 13V can’t be generated, and instead you end up with an unregulated rail (because the LDO is in dropout) and thus the noise of the LDO is higher than spec and thus the noise of the analyzer is higher than spec.
The reported voltage measurement is made at the entry to the QA403. The surest way to solve it is to use a heavy gauge cable (with 24g AWG power conductors) and/or a powered USB hub.
Thanks much for the quick reply. The average person who sees colors normally would surely have noticed these changes in color on the voltage readout. I can only speak for myself, but as one colorblind user, I’m sure that I could see that change if I were looking for it, but I don’t pay any attention to my unreliable senses there. Wouldn’t hurt to add this detail into your manual. As a sanity check–to see if I missed this somewhere in your documents, I see now that it is in your getting-started doc under “USB cable”. Sorry I missed that.
Hi @RobbN, noted thanks. If you are ever not sure about the threshold, you can hover the mouse over the USB Voltage status and it should show you a “tooltip” message if the voltage is lower than expected.
Hi, sorry for resurrecting this topic but I think it’s a better idea than starting a new one…
I got a 5-meter USB cable (20 AWG, 25 AWG) so it meets the requirements. I don’t use any hub and plugging it straight into my desktop I get 4.7(ish)Volts/ 0.8- 0.9A (simple loopback connection FWIW) so I guess am all good. Am I?
My question is do I get any benefit by using a hub? Quite frankly I don’t need a hub so if I decide to get one it’ll be only for the shake of gaining a bit extra voltage on the QA403. Is it worth it? Are there any other benefits of using a hub?
Hi @peppermint, you’ll see the USB voltage turn orange when you are getting close and then red when you are under the safe limit. As long as you aren’t seeing the warning colors, you should be fine and there’s no benefit to using a hub…
Thank you for clarifying Matt. I’m aware of the orange/ red warning but I wasn’t sure whether there’s any difference between say 4.69V and 4.99V (i.e as long as there’s neither orange or red warning).
Hi @peppermint, there shouldn’t be a difference between the levels. You can make a loopback THD or THD versus level measurement with the unit powered at 4.7V, 4.8V and 5V, and then again at 4.6 and 4.5V, and compare. The problems will surface at higher amplitudes in loopback because the supply rails will start to suffer from dropout at lower USB voltages.
