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 Arvind _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
