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

From: Balijepalli, Arvind
Subject: Re: [Discuss-gnuradio] Phase Lag in USRP Output Signal
Date: Thu, 3 Jun 2010 14:19:01 -0400

On Jun 3, 2010, at 1:54 PM, Matt Ettus wrote:

> 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.
Yes, I would expect that as well. However, what I see is that the voltage on 
the scope, does not go to zero when the function generator output is off unless 
I also disconnect the USRP.  This tells me that the USRP is somehow loading the 
function generator.

> Matt


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