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#1 2018-May-11 11:17:15

FoufiDoumi
Member
Registered: 2014-May-05
Posts: 14

Received noise features - WARP v3 - WARPLab 7.7.1

Hello everyone.

I'm posting this to find some help in figuring out why the received noise isn't zero mean.

First off, we have here 2 WARP v3 nodes and I'm working on them for experiments concerning signal detection. Most of noteworthy sensing techniques that do not require synchronization are based on the assumption that the noise is white and zero mean. Each of the receiver on the nodes has its own point around which the noise appears to be detected, and all I'm wondering is why. I'm in no position of understanding the specs of the several modules on the boards. I do have some pictures though to help visualize the issue:

Here you see the same receiver (A on the receiving node) in an attempt to measure thermal noise by only enabling the buffer and without enabling the receiver, with a terminator or an antenna attached. In such small energy scale the offset from (0,0) is precisely visible.
https://imgur.com/3iU96fd

Also as you can see in the next picture, each antenna has its own offset from (0,0) with blue in the back being antenna A, green = B, red = C and light blue = D.
https://imgur.com/a/2LvN6VM

I just would like to know how to explain this for documentation reasons. (Correcting it post-reception is a simple thing.)

Both nodes have CM-MMCX on. I don't know if that has anything to do with this though.

Thank you very much in advance.

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#2 2018-May-11 11:44:41

murphpo
Administrator
From: Mango Communications
Registered: 2006-Jul-03
Posts: 4924

Re: Received noise features - WARP v3 - WARPLab 7.7.1

A small DC offset at the ADC input is normal, the result of DC biases in the RF circuits, component tolerances, and component value variations with time and temperature. The usual model assumes thermal noise is added in the LNA. In the MAX2829, after the LNA the analog signal flows through the mixer, baseband amplifiers and output drivers. The WARP v3 Rx circuit feeds the MAX2829 analog outputs through an ADC driver amplifier, then into the ADCs. Each of these circuits can add a DC offset. As long as the cumulative offset is much smaller than the Rx signal (i.e. small enough to avoid degrading dynamic range), the DC offset can be removed/ignored in the Rx PHY with little impact on performance.

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#3 2018-May-11 12:06:32

FoufiDoumi
Member
Registered: 2014-May-05
Posts: 14

Re: Received noise features - WARP v3 - WARPLab 7.7.1

Thank you so much for your detailed explanation and for replying so fast!

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