Regd: IIP3 and OIP3 measurements for USRPN210

GNU Radio
1

Hi

My project is dealing with Tx/Rx Characterization. I wanted to know if
anyone has tried finding the IIP3 and OIP3 for the USRP N210 with WBX
and
SBX daughter-boards.
I did an experiment on Tx side ( USRP behaving as a TX, with the GRC on
host generating the two tones & output from USRP goes to an attenuator
of
10 db before going to a spectrum analyzer) and got the following
results.
(as in the excel sheet attached.)
I am planning to do this for the RX side using Two signal generators
instead of the GRC flow graph. However I had a few questions:

  1. I found the IIP3 for SBX to be given as 0 dBm & for WBX as 50 ~ 100
    dBm
    on receive, What is it for Tx?
  2. Usually IIP3 and OIP3 are said to have more importance on the RX
    side,
    to measure non-linearity. Do IIP3 and OIP3 hold same significance on TX
    side? if not, what could be judged as a good measure of non linearity
    for
    TX side.
  3. In this test, When I increase the amplitude of the signals, o/p
    varies.
    But for SBX the Inter-mods appear at 250mV while for WBX it is ard
    550mv,
    and here the plot is very unsteady, and the NF is seen as curving.
    however if I do the test with attenuation as 30 dB before the spectrum
    analyzer, The plot has a steady NF. Also after a certain point, I see a
    lot
    of spurious signals. Is there a specific amplitude range I should
    constraint myself to, to get the IIP3 and OIP3 points.

Thanks
Gayathri

2

Hi-

Comments Interspirsed

On Wed, Apr 16, 2014 at 4:22 AM, Gayathri Ramasubramanian
[email protected]wrote:

instead of the GRC flow graph. However I had a few questions:

  1. I found the IIP3 for SBX to be given as 0 dBm & for WBX as 50 ~ 100 dBm
    on receive, What is it for Tx?

IIP3 is really only relevant to receivers. OIP3 is the relevant metric
for
a transmitter. I’m not too familiar with the WBX, but a receive IIP3 of
50
to 100 dBm is way to high. Conversely, your attached spreadsheet seems
to
imply that the actual IIP3 is on the order of -50 dBm, which is too low.
I
suspect you’ve got some unit conversion things going on. Why are your
measurements in mV? dBm would be a more useful measurement.

You described your transmit test configuration. How are you testing
receive?

  1. Usually IIP3 and OIP3 are said to have more importance on the RX
    side,

to measure non-linearity. Do IIP3 and OIP3 hold same significance on TX
side? if not, what could be judged as a good measure of non linearity for
TX side.

Are you comfortable with the concept of IIP3/OIP3? IIP3 isn’t really
relevant to transmitters, but OIP3 is a relevant measure. There are a
whole
handful of relevant transmitter metrics that have been defined. What you
use depends on the system that you’re using.

  1. In this test, When I increase the amplitude of the signals, o/p varies.
    But for SBX the Inter-mods appear at 250mV while for WBX it is ard 550mv,
    and here the plot is very unsteady, and the NF is seen as curving.
    however if I do the test with attenuation as 30 dB before the spectrum
    analyzer, The plot has a steady NF. Also after a certain point, I see a lot
    of spurious signals. Is there a specific amplitude range I should
    constraint myself to, to get the IIP3 and OIP3 points

I tend to set up measurements of IP3 so that the interfering signals
(the
IP3 tones at +/- deltaF) are 6 to 10 dB above the measurement noise
floor.
That ensures that the IP5 products aren’t also being generated. If you
get
close to the point that the receiver or transmtter is going into
compression, then your IP3 measurements are going to be masked by other
nonlinearities.

Dave

3

Gayathri,

You can find IIP2, IIP3, and NF data for the N210 with WBX (on the
receive
side) here:

http://files.ettus.com/performance_data/wbx/wbx_imd_and_nf_vs_gain.pdf

Please let me know if you have any questions about the data.

Thanks,
Matt

On Wed, Apr 16, 2014 at 10:22 AM, Gayathri Ramasubramanian