We recently bought the QA403 for various purposes, one being the measurement of a diy DAC PCB.
I’ve encountered an annoying issue when changing the attenuation of the QA403. When changing the attenuation from 0dB to 6dB, the RMS voltage doubles. This goes on for each subsequent increase in attenuation. Also, the RMS voltage of the signal at 0dB doesn’t match the measured voltage using an oscilloscope, so I’m having a hard time trusting my results.
In the image below, on the left an RMS voltage of 786.5mV was measured, which matches the oscilloscope measurement. On the right, the attenuation was changed to 12dB and the measured Vrms is 1.516V.
Note: I’m using the I2S output of the Qa403 to drive my DAC. For some reason, the generator output value also doesn’t match with the actual output of my DAC. No gain stage was added so this should be equal to the output dBV depicted in the right upper corner. As this is annoying, it is doable to work with.
Currently I’m running the latest firmware 1.198, but initially on firmware 1.194, I didn’t notice this. I tried changing between various firmwares but the issue still persists.
Sometimes when restarting the application and unplugging the analyser, the issue is gone, but lately it consistently present.
Lastly I noticed that after a while, the attenuation relays stop clicking when going from i.e. 18dB to 42dB…
Is this normal behaviour or is there something wrong with my analyser?
Hi @Simone, this should not be happening, but it does look like something is getting into an odd state with I2S enabled. Can you please try this (without I2S enabled):
Put your QA403 into loopback (L+ OUT to L+ IN, short L- IN, and R+ OUT to R+ IN, short R- IN)
Do a File->New Settings to put the analyzer into a known state
Click on RMS measurement (in Measurements control group) to add RMS measurement tile
The default output level from a File->New Settings is -10 dBV. And then change your full scale input from 0 to 6 to 12 to 18 to 24 to 30 to 36 and finally 42. At every full scale input range you should see -10.0 dBV reported.
For some reason, the generator output value also doesn’t match with the actual output of my DAC.
Note that the when using the front-panel I2S, the generator settings higher than 0 won’t make sense because you are specifying dBFS. And also there is no association between the L+ output voltage and the output voltage of the device you are driving via I2S. The max voltage from your I2S device is whatever you say it is.
Lastly I noticed that after a while, the attenuation relays stop clicking when going from i.e. 18dB to 42dB…
You should always hear a click when going to 18 to 42 dBV full scale input and back (ie 42 to 18 dBV).
But, if I2S is enabled, then I do see something is stopping the input relay from switching. Let me check more on this.
