[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Discuss-gnuradio] USRP downconversion

From: Eric Blossom
Subject: Re: [Discuss-gnuradio] USRP downconversion
Date: Thu, 28 Apr 2005 14:39:14 -0700
User-agent: Mutt/1.5.6i

On Thu, Apr 28, 2005 at 02:12:37PM -0600, Viktor Przebinda wrote:
> Hi,
> I have a simple question on the USRP. When setting fpga digital 
> down-conversion frequency to zero via set_rx_freq method why do 
> I get nonzero (and changing) values in the imaginary channel? 
> Seems like these should all be zero since sin(0)==0 ??
> Thanks,
> Viktor

What's happening is a complex multiply of the incoming I/Q with
cos/sin out of the NCO (really all done together in the CORDIC).  This
effectively rotates the incoming I/Q signal.

What you're seeing is that the phase register in the NCO has a
non-zero, but constant value (freq == 0) and as such is rotating the
input I/Q by a fixed angle on each sample.

Right now there's no way to zero the phase register, but you can see
this behavior by cycling power on the USRP and then setting the
frequency to zero as the first thing (or don't set it at all).  In
this state Q will be a constant zero.


reply via email to

[Prev in Thread] Current Thread [Next in Thread]