[USRP-users] Capturing signals at the carrier

Marcus D. Leech mleech at ripnet.com
Wed Sep 18 17:04:56 EDT 2013


On 09/18/2013 04:58 PM, Cochenour, Brandon M CIV NAWCAD, 4.5.6 wrote:
> I have an application where I want to transmit a single frequency and measure it's magnitude and phase.  My Tx baseband data is constant (DC), and I've verified that the output of my N210 with WBX is simply the carrier frequency I set.
>
> However...upon receiving...I get zero signal.  Nothing on I and Q.  If I put a bit of a tone on my Tx (a few hundred Hz - kHz), all is good.  It's as if something is AC coupled after the demodulator.  The ADCs?  Something on the digital side?
>
> This result was a bit surprising.  It seems to suggest that if I were to capture a random chunk of spectrum somewhere and convert it to baseband...that I'd always miss the little bit around the carrier (or middle of the spectrum).  Is this true?   Should I be resigned to having to do the extra processing to get my phase information at an IF instead?
>
> ---
> Brandon
>
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The DC-offset-removal loop within the FPGA is probably removing your 
carrier.

You can disable that, or use offset tuning to move DC to somewhere 
outside your passband:

http://files.ettus.com/uhd_docs/manual/html/general.html#two-stage-tuning-process


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