Re: [Discuss-gnuradio] USRP N210 Benchmarks.

[Marcus D. Leech]

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).

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.

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.

-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org