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 > > > > > Discuss-gnuradio mailing list > Discuss-gnuradio@ > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio -- View this message in context: http://gnuradio.4.n7.nabble.com/GR-USRP-and-GPIB-measurements-tp49727p49821.html Sent from the GnuRadio mailing list archive at Nabble.com. _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
