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

[Matt Ettus]

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.

Matt