QA403 generator spike

I just want to notice that when you increase QA403 generator from 7dBV to 8dBV there is a huge spike on output signal.

Best regards.

Hi @bmsorin, thanks very much for reporting this. This is when generating only in idle mode, is that right?

I think what is happening is this: Anyplace you have relay boundary in the output (8 dBV, -2 dBV, -12 dBV) the sine amplitude sent to the DAC needs to be in sync with the DAC attenuator setting. So, you are at 7 dBV and you are sending to the DAC an (almost) 0 dBFS sine. That is, the sine is at ~0 dBFS and the attenuator is at 10 dB.

And then you switch to 8 dBV output, and the relay attenuator goes to 0 dB attenuation, and the sine level goes to roughly -10 dBFS. What happens in the code is the relays always switch immediately (to maximize the settling time). And so, at 7 dBV you have a 0 dBFS sine in the various DAC buffers (USB PC side, USB firmware side, I2S buffers). And then the relay switches to 0 dB, and suddenly that 0 dBFS sine is playing with a 0 dB attenuation, getting you very close to the full scale output of the DAC.

There are probably a few different approaches to take here:

  1. Keep as is. But the momentary full scale for 200 mS isn’t good.
  2. Don’t change the relay until the old data has cleared. This will give a momentary 10 dB dip in output. But a momentary dip is better than momentary full scale.
  3. In idle mode, keep the output attenuator at 0 dBFS all the time. This will give perfect transition between steps, but delivers excess noise and lower levels.
  4. Stop the idle generation when transitioning from 7 to 8 dBV and then quickly restart. This is similar to 2), but instead of a 10 dB dip in amplitude, you get a full period of silence.

I think #2 might be the best. What do you think?

Looking at this more, I think the next release will do #3. This will give seamless amplitude transitions across the full range by locking the DAC output attenuator at 0 dB.

Hi Matt, yes, idle mode. I test an auto ranging device, and at 7-8dBV step the device switching relay to more that it should, testing with scope reveal what i post.
I think #3 will be best option. One think i have in mind right now, testing RIAA premp will be at limit.

Can we have options (2,3,4) then for the attenuator behaviour switching attenuator when in idle mode? One of the excellent things about the QA-403 is the excellent D/A signal generator and its low noise and distortion. It can virtually replace a standalone sig gen as it stands.

Sure, the behaviour is not ideal as it stands, but couldn’t you employ muting in the D/A itself during that particular transition? D/As have integral ramp up/down muting don’t they?

I also see there’s some strange glitches in the waveform at the start of the 7-8dBV ‘event’.