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Re: [Discuss-gnuradio] Phase Lag in USRP Output Signal

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Phase Lag in USRP Output Signal
Date: Thu, 03 Jun 2010 10:54:15 -0700
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On 06/03/2010 10:36 AM, Balijepalli, Arvind wrote:

Matt, Thanks for your reply. The problem seems to be mostly solved
now, I'll explain below, but to first respond to your
suggestions/questions. I changed the DAC to accept non-interleaved
data, which I presume should be at 64 MS/s. I setup the interpolator
on the Tx side to 2x to then bring it up to 128 MS/s. I am not sure
how to disable those mixers exactly. The only coarse-fine settings I
see are for gains (maybe these somehow control the mixing?) which I
have set to 0.

The way I fixed the phase lag I was seeing was to setup the DAC to
accept 2s complement data and then provide it signed integers. I
changed the verilog code where I assign the output from

I've never used anything other than 2's comp, and I don't think there is any reason to use anything else.

Unless I am completely missing something which is possible, the DAC
seems to like signed integers. With this change, the latency between
the inputs and outputs is approximately 400ns, with timing jitter
less than ~4ns.

That is good. The ADC itself has pipeline latency, so what you are seeing is about right.

On a related problem, I see that the USRP is loading the input of the
function generator. I have turned off buffers and stages of the ADC
that I don't use according to the ADC documentation. Also, my input
signal is never greater than 100 kHz, so I don't think this is an
impedance matching problem.

The LFRX has a 50 ohm termination on its input, so when you connect it to a signal generator with a 50 ohm output impedance, the voltage goes down by half vs. open circuit. This is the appropriate behavior.


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