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Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements

From: madengr
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Sat, 9 Aug 2014 18:45:58 -0700 (PDT)

I updated the repo with results for the N210+WBX:


Our single tone sweep results look very similar.  Your IMD plot should be in
dBc; plot the difference in power between one of the test tones and one of
the IMD3 products.  Mine don't do below -50 dBc since I have the input
attenuator on the analyzer set for the higher power level.  I need to reduce
it for the lower power levels. 

1) Yes, that would be a calibration factor.  If you measure 0 dBm on the
spectrum analyzer, and -20 dBFS in the FFT, then 20 dB is the calibration
factor, so -20 dBFS + 20 dB = 0 dBm.  Note when you change the RX gain, then
the calibration factor changes.  If you increase gain 10 dB, then you must
decrease the calibration factor 10 dB.

2) Using the FFT to manually measure level can be a problem if the tone is
split between bins, so it's better to use a coarse FFT.  Measuring all the
power in the channel at once may be preferable, and is like using a power
meter.  Use the complex_to_mag_squared, followed by
inegrate_with_decimation, then 10*log() +K, where K is your calibration
factor.  If you decimate down to 1 Hz, i.e. decimation_rate=sample_rate, you
can get very precise power readings.  The accuracy drops off with tone power
due to the wideband noise; just like a power meter.

You would have to look at the USRP FPGA block diagram to find out exactly
what is going on between the input and the FFT, but it is essentially fine
tuning with an NCO then many stages of filtering and decimation.  I'm sure
it affects the amplitude slightly as different filters are used for
different decimations, and the odd vs. even decimation.

3)  Compression is the drop in gain (not power).  For example look at this
table where the USRP TX gain is stepped in 1 dB increments and the output
power is measured:

USRP_TX_Gain_dB, Pout_dBm,  Gain = Pout_dBm - USRP_RX_Gain_dB
3.0, 8.0, 5.0
4.0, 9.0, 5.0
5.0, 9.9, 4.9
6.0, 10.7, 4.7
7.0, 11.3, 4.3
8.0, 12.0, 4.0 <<-- This is the P1dB
9.0, 12.6, 3.6

The P1dB is where the gain has dropped from 5.0 to 4.0.  The P1dB referenced
to the output is 12.0 dBm.  The P1dB referenced to the USRP TX gain setting
is 8.0 dB.  Notice it is also where the digit after the decimal point
repeats itself if the input is stepped in 1.0 dB increments; i.e it when
from 5.zero to 4.zero.  This is the quick n' dirty method of finding P1dB. 
If the input moves in 1 dB steps, all you need to monitor is the most
significant digit after the decimal point.

4)  I have not done RX testing.  My signal generators are non-synthesized
and have no digital interface.  I bought a new one on eBay and it should be
here next week, but I still need another for a two tone test, not to mention
the components to achieve proper isolation between the two.


Gayathri Ramasubramanian wrote
> Hi
> Thanks again fro your explanations. You were correct about the
> measurements
> I had sent in earlier. I checked with the power meter and the O/p power at
> 450MHz @ ampl 0.707 and TX Gain of 25 gives 11.5 dBm.
> I have done the single tone and two-tone tests for USRPN210 +WBX board at
> 3
> frequencies of 400 MHz, 900 MHz, 1800 MHz.
> The settings are similar to yours :AMPL 0.707, TX gain : varied from 0 to
> 31.
> Could you please check them and let me know if they are fine.
> Also I am trying to find the RX side linearity range. For this I connected
> signal generator to the RX port (RF2) of the USRPN210 and viewed the
> received spectrum on UHD_FFT.
> I have plotted the readings that I got.
> I could see that there is a difference of around 35 dBm between the
> reading
> on UHD_FFT and that on spectrum Analyser when the same input from signal
> geenrator was given to both (using a power splitter ) @ 400 MHz.
> This changed to ~ 31 dBm at 900 MHz and ~ 23 dBm at 1.8 GHz.
> Could you kindly clarify the below points regd this:
> 1) So it is good to assume *35 dBm as a calibration factor  @ 400 MHz 
> *for
> the USRPN210 +WBX device.  i.e in case we use USRPN210 +WBX device test as
> Receiver for testing and use UHD_FFT to plot the spectrum and take the
> reading of amplitude in dB and get a value of *25 dB* , would it to fine
> to
> say that 25 - 35 = -10 dBm is the actual power received by USRP to an
> extent ( not only on the RX port).
> So can we use the difference to be the calibration factor in the linear
> range?
> 2) The link
> http://www.ettusresearch.com/content/files/kb/application_note_uhd_examples.pdf
> says that "When the FFT(default) view is used, the x-scale is the
> frequency, and the y-scale is amplitude.The y-scale shows the amplitude
> with “counts,” and the values do not typically correlate to a specific,
> absolute power input. The amplitude read on the display is useful for
> approximate comparisons. The level for a given input amplitude will vary a
> few dB across frequency and from unit to unit. Also, receiver
> daughterboards provide various levels of amplification in their analog
> chains,which will affect the amplitude result in the FFT".
> Could you clarify what type of processing is done on the received signal
> from the point of reception till the display on UHD_FFT briefly in terms
> of
> scaling/ normalization.
> The basic idea is to find a relation between the amplitude value shown on
> UHD_FTT plot and basic power scale in dBm.( a factor to be
> summed/subtracted or multiplied or divided from UHD_fft reading to get
> real
> power value in dBm)
> 3) On further increase of input power in steps of 1 dBm or 0.1 dBm, there
> was a 1 dB drop of power. We assumed this was the point of 1 dB
> Compression
> for the receiver side of USRPN210. Is this understanding correct? This
> comes around *-8 dBm @ 400 MHz, -4 dBm @ 900 MHz and ~ +2 dBm @ 1.8 GHz*.
> 4) I have also tried two tone test on the USRPN210 +WBX device. The plots
> shown under Q4. The IIP3 point comes ~ 4 dBm when the factor of 35 is
> subtracted from the values got from UHD_FFT and is ~ 25 dB on IIP3 and 50
> dBm on OIP3 ( which was told to me by my advisor as not the normal values
> to expect)  . You said you plan to do the  one tone and two tone test for
> RX side for B200. Do you find any similarity in results. Kindly let me
> know
> if I seem to be going wrong any where.
> Kindly Clarify the above points.
> I eagerly look forward to your responses.
> _______________________________________________
> Discuss-gnuradio mailing list

> Discuss-gnuradio@

> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> Plots for the Questions.docx (47K)
> &lt;http://gnuradio.4.n7.nabble.com/attachment/49883/0/Plots%20for%20the%20Questions.docx&gt;

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