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

[madengr]

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
dBFS.

Thanks,
Lou
KD4HSO


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
> 
> 
> 
> 
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