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

From: Gayathri Ramasubramanian
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Sat, 9 Aug 2014 01:00:30 -0700


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
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.

On Wed, Aug 6, 2014 at 11:12 AM, madengr <address@hidden> wrote:
1)  I have not calculated P1dB or IP3, yet.  I will have to do that.  I find
it's easier to look at a graph of IMD3 levels since it is more intuitive
that working from the IP3, which is more useful for calculations.

2)  0 dBFS would be the DAC swinging full scale; i.e. -1 to +1 float value
(DAC voltage or current if you want to think of it that way) from GNU Radio.
That is the maximum power you can get from the DAC.  So to drop the power by
1/2 (i.e. -3 dB), the amplitude must drop to 1/sqrt(2)=0.707.  You always
need to stay backed off from full scale, so -3 dB is a good point.  You
don't care about the power from the DAC, only the power from the DAC +
amplifier.  The DAC should be very linear compared to the amplifier, and it
is since the IMD really drops off with amplifier gain setting.

Your measurements should be dropping in 5 dB steps; they are not.  You can
either decrease your video bandwidth, or better yet remove that 60 dB
attenuator.  Most spectrum analyzers can handle 30 dBm with the input
attenuator at maximum.  60 dB is too much, not to mention it's probably not
exactly 60 dB.

Let's say you have two tone,  each is 0 dBm, at 100 Hz and  101 Hz.  The the
total (mean square) power is  3 dBm; i.e. measure each tone individually and
sum the power.  At the beat frequency of 1 Hz the amplitudes add
constructively and destructively; ie. doubling and going to nothing.  That
amplitude doubling gives an instantaneous (peak) power of 6 dBm, then it
disappears to nothing, hence the average is still 3 dBm.  So for two tones
the peak power is 3 dB over average power, or 6 dB over that of a single
tone.  That's why I set each tone power to -9 dBFS, so the peak power is -3


Gayathri Ramasubramanian wrote
> Hi.
> Thank you for your detailed response. it helps makes things clear.
> Could you also clarify the below questions
> 1. While you increased the gain were you also looking for 1 dBCP in one
> tone and IIP3 or OPI3 point in the two tone test. If so, what values did
> you get for the B200 USRP?
> I would like to get a reference for USRPN210+WBX test. Usually the Ettus
> web specifies values only for the daughterboard like WBX is said to have 5
> to 10 dBm on IIP3. So we don't know for the entire device set.
> 2. Also in you notes, you have said " The one tone TX test consists of a
> 0.707 amplitude tone at 100 kHz offset from the center frequency. The tone
> amplitude is -3 dBFS, which is half power from full scale of the DAC."
> "The two tone TX test consists of 0.304 amplitude tones with 25 kHz
> spacing, offset 62.5 kHz from the center frequency. Each tone amplitude is
> -9 dBFS, thus a combined average power at -6 dBFS, and peak instantaneous
> power at -3 dBFS."
> Can you explain this a little more. How did you map 0.707 amplitude to -3
> dBFS?
> You say it s half power of full scale DAC so what is the full scale DAC
> power?
> For a USRP2 +WBX I found the following measures on spectrum Analyser,
> while
> sending a single tone with UHD_SIGGEN.py
> Ampl   Gain  Attenuator        Pout on spec Analyser     Actual power
> (Pout
> +60)
> 0.707    25           60                       -42.48 dBm            17.52
> dBm
> 0.707    20           60                      -48.53 dBm             11.47
> dBm
> 0.707    15           60                       -56.94 dBm            3.06
> dBm
> 0.707    10           60                       -64.75 dBm            -4.75
> dBm
> 0.707     5            60                       -74.36 dBm
> -14.36 dBm
> however I don't know how to relate it in terms of DAC power scale.
> Similarly for two tone you have said "The two tone TX test consists of
> 0.304 amplitude tones with 25 kHz spacing, offset 62.5 kHz from the center
> frequency. Each tone amplitude is -9 dBFS, thus a combined average power
> at
> -6 dBFS, and peak instantaneous power at -3 dBFS."
> I get the relation like : 1/sqrt(2) = 0.707 ==> -3 dBFS
>                                  1/(2*sqrt(2)) = 0.304 ==> -3 dbFS + 20log
> (1/2) == -3 dBFS - 6.02 = -6 dBFS
> Is my understanding correct. Could you clarify the calculation of average
> power as -6 dBFS, and  instantaneous power as -3 dBFS too?
> look forward to your response.
> Thanks
> Gayathri
> Discuss-gnuradio mailing list

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