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AGC gain and ADC bits in NUT4NT

Posted: Fri Feb 16, 2018 9:40 am
by Micaela
Dear all,

I am using the Nut4nt - board with nt1065‎. I have a couple of questions about the AGC gain and the number of bits of the ADC.
About ADCs, I was wondering if it should be possible to allocate all the 8 bits (2 per channel) actually to one channel only, as using an 8-bits ADC, but I I have seen in the user manual that each channel has one 2-bits ADC, so I guess not. Can you confirm this?
About the AGC, I have found in the user manual that there are basically 2 AGCs: RF GC and IF AGC. The former is set in manual mode by default, while the latter in automatic mode. This is done during the configuration phase.
Do you think it should be possible to change the gain of RF/IF AGCs at run-time, while grabbing data?

Thanks in advance for your attention and support!

Best Regards

Micaela Troglia Gamba

Re: AGC gain and ADC bits in NUT4NT

Posted: Sun Mar 25, 2018 11:37 pm
by itsar
Dear Micaela,

Thank you for asking and using our board! Sorry for delay, you question is the first one for months (thanks to Maurizio for kicking me).

It is not possible to allocate all 8 bits of ADCs to one channel. If you need to increase the number of bits, you can sum all channel's signals. The number of bits will be increased by one for each sum, so if you sum all four channels, you will have 4 bits sampling. You need to be careful about phase difference of channels, it needs to be near zero. If it is not, you need to know it exactly and correct each signal before summing. To be accurate, effect depends on channel's noise correlations, but in our case I think you will have near 6 db SNR increasing and real 4-bits sampling.

About AGC, you can change it at run-time.

Thank you and sorry again for delay,

Re: AGC gain and ADC bits in NUT4NT

Posted: Mon Mar 26, 2018 5:54 am
by Micaela
Dear Igor,

thank you very much for your answer, and don't worry for the delay: it was not a (so) urgent topic! ;)

The details you provide are really precious. I just want to be sure to have correctly understood. If I sum all 4 channels, I will get a signal represented on 4 bits, but in doing this I will lose 3 levels, right? I mean, starting from the 2-bits signal of each channel with 4 levels {-3,-1,1,3}, the signal obtained by summing all the 4 channels, will be on 4-bits but with 13 levels {-12, -10, -8,- 6,-4,-2, 0,2, 4, 6, 8, 10, 12} instead of 16. Am I missing something? Anyway, I guess this is not a problem.
About the phase difference, I am not sure to have understood. For example, if I configure all 4 channels on L1 using PLL A for all, they will be synchronized with the same clock, but there could be a difference of a certain number of samples (clocks) due to the 4 different HW chains. It is so? Is it fixed according to the configuration? I mean, I should estimated it just once for that particular configuration, right?

For the AGC, good to know that the gain can be changed run-time! :D

Thank you again for your support!


Re: AGC gain and ADC bits in NUT4NT

Posted: Tue Mar 27, 2018 3:23 am
by itsar
Dear Micaela,

Thank you for patience about delay!

I think, you are right about the number of levels, but it is not so significant Big thing is signal gain to noise gain ratio. We know that signals are correlated in all channels, so the signal gain will be 12 db. If noises are uncorrelated, summing gain will be only 6 db. So SNR improvement will be 6 db.

The phase difference I meant is mainly in RF stage: in NT1065, in RF chain, in power divider (external to NUT4NT board), cables e.t.c. It needs to be compensated to have 12 db of signal gain.

I hope it helps.

Thank you for your questions!