2011/10/27 Marcus D. Leech <mle...@ripnet.com> > > Well, that sounds like the lazy solution, intermodulation products are > bad, so just throwing the transmitter power away is not what I'd prefer. > > > But what it points to is an *analog* issue, entirely independant of the > CORDIC (which, as I observe, isn't likely involved in the test cases > at hand here). > > Analog gain elements (including DACs) have operating regions over which > they're linear, and operating regions over which they're not > linear. If you drive any amplifier near its maximum operating point, it > will start to become non-linear to one degree or another. I'll > let Matt or one of the other engineering folks at Ettus comment further, > but I personally am totally unsurprised when things start to > become non-linear near the nominal maximum operating point. > > > > Is there any way of finding out what the resolution is? We haven't been > able to track it down for the RFX2400 board, > but this sounds like a nice way to test if it _is_ the CORDIC. > > > Look at the tune_result_t from tuning: > > > http://files.ettus.com/uhd_docs/doxygen/html/structuhd_1_1tune__result__t.html > > If the actual_dsp_freq is 0, then the CORDIC wasn't involved. > > I tuned to an even number of MHz, which on all of the synthesizers *should* > yield 0 CORDIC frequency. > > But maybe Josh can add a feature to 'uhd_usrp_probe' to display the PLL > resolution (although in some cases, > it may change with target frequency range, I think). > > Thank yo very much for this, It is really usefull, and it furthermore confirms what we have observed. At 2.4GHz there is no problems, when we go 300 kHz up, we start seeing the problems. (see images attached)
This is further collaborated, with the fact that we can find "good" frequencies up through the entire band. > > >> Only problem there is that there is a 55 dB loop back between the in and > output of the RFX2400 board, so two different radios are needed. > > You're talking about the combined isolation of the two RF switches in > the path between the TX and RX? That's adequate attenuation > for the tests I'm suggesting. > > I think I prefer our measurement equipment at the University > > > We have observed this as well, but as described before we do not find > this to be the correct solution. > > I'm keen to hear what your "correct solution" is to the problem of > non-linearity in off-the-shelf analog gain devices. I suspect the solution > won't be in the digital domain, but I'm always willing to be surprised. > > I have implemented the cordic in vhdl now, this should reduce the phase error (which is also mentioned in the pdf you referenced) because of the ability to increase the CORDIC stages. I am now just waiting for a Xilinx license. Best Paul -- * - - */* -/* * -/* - * */- * * */*/- */- * */* */- * * -/*/- */* - - *- */- - */- -/* -/* */* - * */* - * - * -/- * - */- - -/- -//
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