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Voyager
Voyager
656 Views
Registered: ‎02-17-2009

RFSoC ADC and DAC full scale levels 6dB low

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Hello,

Using the RF Analyzer we are finding on a ZU28 that the ADC goes over-range when the received digital signal is at about -6 dBFS and it becomes non-linear if we try to increase signal any further. We are also finding that at 0 dBFS the DAC output seems to be about 6 dB below the expected level. 

Here is our configuration:

Screenshot - 05-Oct-2020 , 6_13_16 PM.pngScreenshot - 05-Oct-2020 , 6_14_55 PM.png

We tried enabling QMC gain but while it increases the absolute level we don't seem to gain any of the lost dynamic range. Are we missing something?

Thanks,
/Mikhail

 

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Community Manager
Community Manager
561 Views
Registered: ‎08-30-2011

Hi @mmatusov 

What kind of signal is tested here? If it is a bandwidth signal, -6dBFS would only be a signal power on one bin and it might go over-range due to the PAPR. 

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Voyager
Voyager
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Registered: ‎02-17-2009

Hi @zhendon ,

The signal is a simple CW.

/Mikhail

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Community Manager
Community Manager
500 Views
Registered: ‎08-30-2011

Hi,

If you are monitoring the DAC output with XM500 daughter board and looping DAC output back to ADC, you need to consider the board insertion loss + signal frequency roll-off which might be 6dB (depending on your signal frequency).

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Voyager
Voyager
471 Views
Registered: ‎02-17-2009

Hi @zhendon ,

We are using a 3rd party board with the analog input/output circuits based on Xilinx reference designs. But I don't think this is relevant for our discussion, at least not with regards to the ADC side. Here's a capture of the input CW at a level of 0.1 dB below of the ADC Over Range interrupt threshold. As you can see the amplitude values are barely above 16000. I would expect them to approach 32000 for a 16-bit signal close to the full scale. 

Screenshot - 2020-10-20 , 11_28_42 AM.jpg

Thanks,
/Mikhail

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Community Manager
Community Manager
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Registered: ‎08-30-2011

Hi @mmatusov 

I can see from your screen shot that you are enabling mixer so there is a 3db attenuation included in mixer output. You can find a detailed description of this part in PG269. 

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Voyager
Voyager
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Registered: ‎02-17-2009

Hi @zhendon ,

Yes, the mixer is enabled but we are in the R2C mode, so according to PG269 the mixer output attenuation is 0 dB. Am I wrong?

Thanks,
/Mikhail

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Observer
Observer
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Registered: ‎05-08-2013

Hi @zhendon,

Another test we performed was to disable the fine mixer, we used the coarse mixer only, and we still see the same issue.  This suggests the 3db attenuation you pointed out is not causing the low level.  Do you have other ideas about what could be causing this?

Thanks, Colin

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