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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: Tue, 5 Aug 2014 13:03:05 -0400


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.


On Sun, Aug 3, 2014 at 12:19 PM, Louis Brown <address@hidden> wrote:

1).  It looks like the amplitude in UHD_SIGGEN_GUI is normalized to the peak amplitude of the combined two tones, thus it is safe to set to 1.0.  Notice if you switch to a single tone, the amplitude increases when viewed on the spectrum analyzer.  You must measure your output with a power meter or spectrum analyzer to get the power in dBm, then adjust the TX_gain to change the level.  That was really the premise behind my experiment, to figure out what is the output power for various gains and frequencies.  I beleive max limit on the WBX is 31.0 since it has two 5-bit attenuators.  I suppose I could have looked in the data sheet for the B200 transceiver chip.

2)  I just choose 60 dB as I had to use something when initialized the usrp in the code.  It was really just arbitrary.  No, with the WBX you will be limited to 0-31 dB for the TX gain setting as the B200 is different hardware than the WBX.  I actually have an N210+WBX also, but I have not gotten around to measuring it yet.

3) The one tone test uses a tone at 100 kHz.  The two tone test uses ones at 50 kHz and 75 kH, hence 25kHz tone spacing at 62.5 kHz offset.  All fall within the 250 kHz sampling rate.  It does not matter if the intermod products fall outside the sampling rate as those are generated after the (ideal) DAC.

4) You must always stay below DAC full scale (+/- 1 signal).  Even a sinusoid of amplitude 1.0 is not good since the DSP within the FPGA tend to periodically overflow; at least I’m seeing that behavior within the B200.  3 dB below full scale (1/sqrt(2)) is sort of an arbitrary choice I made.  The two tones have a 3 dB crest factor; the power crests up to 3 dB above average power.  Hence I wanted the power crest to still be 3 dB below full scale.  The digital modulations and multi-carrier can have much higher crest factors, thus you must back off to prevent DAC overflow.

5) 89 dB is the max setting for the B200.  I just choose the 70 dB as that provides 0 dBm output power at 100 MHz.

6) If the Tektronix analyzer has a serial or ethernet maybe you could control it through that.  I used Linux GPIB.  I planned on using PyVISA but the Linux GPIB already had a python module.  I may update it to use PyVISA, as VISA provides a hardware abstraction so you can use GPIB, serial, ethernet, etc.  without having to change the code.  You will need to create a new class within instruments.py for the tektronix analyzer, duplicating the methods of the 8566B.


On Aug 3, 2014, at 1:50 AM, Gayathri Ramasubramanian <address@hidden> wrote:

> Hello
>  It was interesting to see the work you had done as I am trying to do something similar but with USRPN210 -WBX daughter-board.
> I am quite new to GNU Radio and have no experience of programming in python and so generally use the inbuilt blocks for the flowgraphs.
> There are hardware availability limitations (GPIB not available) in my experiments and hence I am trying to do these 2 tests manually by increasing the signal input from a standard agilent signal source in steps of 10, then in steps of 1 to check the Receiver Side IMD3. I check the spectrum using UHD_FFT.grc. I have to do for TX side too.
>  Could you clarify if:
> 1. the UHD_SIGGEN / UHD_SIGGEN_GUI be used to generate the one tone and two tone signal with the parameters you have given. If yes, what should be the amplitude setting while executing that to generate the signal. UHD_SIGGEN help says max ampl is 1, but there is not idea of what it translated to in terms of dBm. Also there is not max limit given for gain (though the USRPN210 spec says that max gain is 30 dB in steps of 0.5).
> 2. Why have you first set the self.gain to 60 while generation the signal and then for sweep started it from 70 increasing in steps of 1 dB until 89 dB. Is this range good for most of the WBX and SBX RF daughter boards /USRPN210s , and in that case I can hopefully do the same in my test to find the point of compression.
> 3. Could you also clarify how you chose the sample rate to be 250e3 - my understanding is that since your carrier freq is 100e3 (one tone) and 75e3 (higher of the two tone), the sampling freq of 250e3 is greater than both  these carrier frequencies. fs >= 2fh or is it based on the bandwidth of the signal generated.
> 4.I want to get an understanding of the amplitude being set via for UHD_SIGGEN.py and the ain. I understand that for one tone you set the ampl = 1/sqrt (2) and for two tone each tone had an amplitude of 1/[sqrt(2)*2]
> 5. The loop is executed based on gain.list in your program, what is the reason behind chosing this range 70 to 89 dB? Or was the aim just to check the variation of IMD3 power with gain from 70 dB to -89 dB.
> 6 can you program be used with Tektronix spectrum analyser and without GPIB?
> I would be grateful if you could kindly clarify the above points. Usually the UHD_SIGEN is set to following while testing in my case:
> @ RF freq 400 Mhz, 900 Mhz and 1.8GHz
> baseband freq : 1e3
> ampl 1,
> gain 30,
> sample_rate 1e6
> Look forward to your response
> Thanks
> Gayathri
> On Sun, Aug 3, 2014 at 12:47 AM, madengr <address@hidden> wrote:
> There have been a few messages on both GR and USRP lists about choosing
> proper hardware gain settings.  I figured I'd write some code to sweep a
> B200 over frequency and gain, and generate some plots that are handy for
> choosing proper TX gain.  It also demonstrates controlling test equipment
> via Linux GPIB while controlling the GR flow, and plotting the results,
> which makes a handy piece of test gear on the bench.  I plan on doing the RX
> side next, where results must be pulled from the GR flow via messages or
> probes.
> https://github.com/madengr/usrp_rf_tests
> Thanks,
> Lou
> --
> View this message in context: http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727.html
> Sent from the GnuRadio mailing list archive at Nabble.com.
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> Discuss-gnuradio mailing list
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