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

From: Louis Brown
Subject: Re: [Discuss-gnuradio] GR, USRP, and GPIB measurements
Date: Sun, 03 Aug 2014 11:19:22 -0500

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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