Advice about gnd seems important, the screen I post show a 50hz, following screen show the same setup with a little variation: I connect the laptop (which is not isolated from earth) to the DUT via USB (my amp own a dac ) and then 50hz simply disapear.
Hi @Briks, glad you are up and running! A lot of isolated PSU (including for laptops) are isolated from an electrical safety standpoint, but they have a very strong 50/60 Hz component impressed on the both the ground and supply. That is, relative to earth, the ground might swing up +/-10V and the supply line might swing up +/-10V. But they swing together, so itās always the expected DC voltage. But for a DUT, it puts a larger burden on the CMRR of the DUT. So, what you are seeing and learning makes perfect sense.
Thanks very much for sharing this. And Iāll bet your pictures of your bench reminds everyone itās time to tidy up. Me especially 
Considering work bench of an analog guru like Jim Williams almost sure everyone here have margin regarding to cleaness 
Indeed my laptop PSU is also a isolated one, so all my setup is floating/isolated from earth, the DUT PSU is a high quality Meanwell medical grade.
I have a question regarding the THD calculation of QA403; how are managed harmonics upper 20Khz ? I make measure AMP THD versus frequency and results are strange:
Looks like harmonic upper 20KHz are simply ignored, because of the gap in THD between working frequency lower than 10Khz and upper. Normaly we expect from the DUT to have higher THD when frequency goes high ?
The screen show 2 run, one with FS 48k and the other with FS 192k in order to see if there is any difference.
Hi @bricks, you can right click on the THD or THDN button (aka the ācontext menuā) or tap and hold that button on a touch screen and that will open the Measurement Start and Meausurement Stop frequency settings. These are global and apply to RMS, THD and THD+N. If you want to measure harmonics beyond 20 kHz, then just increase this to whatever you need.
My workbench today is much more organized than the one shown above 
However, in the 90ās when I had my electronic repair business, my ājunkyardā part of my shop was not organized at all:
only moment I organize my bench is when I wait for PCB or component but in debug session no way to waste any time to clean
I like to be organized to a certain extent. I may not be organized to where everything inside a drawer is neat, but I pretty much know what is in the drawer. I recently organized nearly all of my resistors, capacitors so I can find parts when I need them. I had about 30years of parts from my electronics repair business that I had in parts bins, but not organized. The last was to go through all my transistors, diodes and ICās and sort them by their NTE cross reference # if there was not one I threw it away except for a few parts. I am retired and have to do stuff like that- it is nice not to have to go to the only electronics part store in town so much.
One Q. Would there be a swap deal if one, with a QA402, want to get a QA403 ? And what would that be ?
Hi @Thumb, thereās not a deal for those upgrading, but itās a bit of a moot point at this stage given the extended backorder on QA403s unfortunately
Hello everyone,
I got my QA403 and have to say what a delicious gear!
I was anyway expecting that it has an autoranger function that adapt the attenuator automatically to the signal feeded to the input.
Am I missing anything? I read the owner manual, but no mention on this.
Thanks.
Hi all,
I try to get one of those QA-403 for quiet some time already. I am checking the website on a regular basis if new stock is available and if there is an option to place the actual order.
I also registered for the newsletter for the latest news (of which I didnāt heared anything yet).
While searching the website somehow I got to this blog area and I just read a message posted less that a day ago of someone just receiving his/her QA-403 device.
No w I start to wonder if I āmissed the order trainā again.
Or maybe itās just a long-time-ago order and the person got rewarded for the long waiting
Kind regards,
William L (From The Netherlands)
Okay, no problem. Could you put me on the ordre list @Matt for one pcs. QA403 ?
Is there any possibility of autorange function in a future release?
Hi @Lgcm1982, that could be an option. Personally, I find there is so much to be learned by looking at THD and noise while manually switching input ranges Iām not sure Iād use it that much. With spectrum analyzers, the input range that is best for noise is usually not the input range that is best for distortion.
Where would you find it most useful in your work?
@matt Can you explain your comment about why the noise and distortion ranges are different for each measurement, or link to the information?
Hi @matt ,
I agree with You there is a lot to learn about the behaviour of a DUT when an Index changes with input range.
Anyway from my own experience at a certain stage of development, for example debugging, there is a need for āquantitativeā analysis rather than āqualitativeā.
For āquantitativeā analysis it is more user-friendly if you don 't need to care about the input range, so avoiding back-and-forth with your hand on the mouse.
I experienced several time the benefit of an autorange working with several commercial analyzers from fully analog to digital domainā¦ stand alone to PC based.
Of course I understand that an autorange is not that simple and has to take into consideration some metrics on amplitude (ex. Peak, quasi-peak, rms) and time integration.
Thanks anyway!
Hi @Dave,
As you increase the input level, the noise floor rises. This should make sense, because the dynamic range is the same. So, for example, when you are at +18 dBV max input, the noise floor is about -103 dBV. When you are at +12 dBV max input, the noise floor is at -109 dBV. That is 6 dB apart (-103 versus -109). But, the dynamic range of the converter is the same (18 + 103 = 121 dB and 12 + 109 =121).
Next, you have the self-distortion associated with the ADC. The ES9822 is quite good in this regard. But still, the closer you get to the max input level for your given full scale input setting, the more should will see harmonics increasing.
And so, the setting that is best for noise isnāt usually best for harmonics, and vice versa. In the case of what @Moto is working on in another thread (eg the limits of humankind on THDN), thereās an opportunity to make a lot of measurements: Some with very large FFT at high input level (to do a deep scrub on harmonics) and then a series of measurements near full scale to accurately determine the noise below the fundamental, and then between each harmonic, and then report a composite.
@matt is there any chance we could get a āmonoā capability like the e1da cosmos adc to add some performance?
Hi @Moto, I think there will be a checkbox added in the Edit->Settings dialog that allows you to specify āADC high performanceā mode and that will enable mono and harmonic cancellation. But itās still being studied.
Hi @Matt, this is a very useful of the qa403 possibility. Sometimes you can give up one channel for better performance on a single channel. I await this new possibility with much enthusiasm and hope this will be done soon. Thanks
