[USRP-users] signal stability at beginning of streaming

Marcus D. Leech mleech at ripnet.com
Wed Oct 7 09:24:18 EDT 2015

On 10/07/2015 02:19 AM, Francois Quitin via USRP-users wrote:
> Dear all,
> I’m using a USRP-N210 with a RFX2400 daughterboard, and when I 
> initially start streaming I see the amplitude of the I and Q 
> components first saturating before they reach a stable level (see 
> attached image, I should be receiving only noise).
> I don’t observe this behavior with the WBX daughterboard, so I’ll 
> assume it’s some of the amplifiers stabilizing initially? Is there any 
> way to avoid this behavior in the RFX2400 (I have an application where 
> I need to stream short non-contiguous bursts, so I cannot afford to 
> wait until it stabilizes for every burst).
> And by the way, should I also expect to see this behavior in the 
> CBX/UBX daughterboards?
> Thanks!
> Francois
Analog hardware has its non-ideal behaviors, and at turn-on is one of 
those times where it's more obviously non-ideal.

There are several things going on that can affect signal quality at 
start of streaming:

    o   Enabling of various analog elements
    o   PLL instability --- are you tuning every time you stream a burst?
    o   DC offset correction -- it takes a while to reach equillibrium
    o   DSP filter discontinuities

You can either stream continuously, and use timestamps to decided which 
parts of the stream to pay attention to, or you can schedule
   your RX bursts a little bit ahead of when you need them, and use 
timestamps to decide what to pay attention to.

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