Hey David, Thanks very much, your inputs be a great help!
Cheers -Michael On Thu, Sep 20, 2012 at 12:00 PM, David Bengtson <david.bengt...@gmail.com>wrote: > Comment's interspersed below. > > On Sun, Sep 16, 2012 at 8:51 AM, Michael Hill <moth...@gmail.com> wrote: > >> Hi guys, >> >> Thanks for the responses! Again, let me point out i am quite new to alot >> of the IIP3 concepts and still have my areas of confusion regarding the >> board's operation, so if I say anything weird or counter-intuitive please >> correct me! >> >> To start with some info from Dave's queries: >> >> 1- Thats right.. these are the sorts of things i'm looking to >> characterise. I'm not sure if this would be considered some fault with my >> USRP/daughterboard or within the expected variance in behaviour for the >> daughterboards :( >> > > so a starting point would be to do a single tone measurement, sweeping the > gain. That would let you make sure that the gain is relativly linear and > figure out the relationship between the gain steps and output power. You > can also see how bad the DAC spurs are with the transmitter, something > that's important. to understand. > >> >> 2. An example shot is attached. I'm using a real Spec Amp. Thats right >> regarding the tones.. I noticed that too. However one thing that might be >> confusing about this test is that i'm not directly increasing the power by >> 1dB at a time (in which case I'd expect the 2F1-F2 and 2F2-F1 tones to >> increase by 3dB..) im changing the output power by varying the gain.. >> (rather then power directly). So that seemingly would impact the IIP3 >> values..(e.g. im gathering this would explain why my 2F1-F2 and 2F2-F1 >> tones aren't increasing by a 3:1 ratio..) >> >> In general, the most accurate IP3 measurements are when the IP3 tones > are only 3 to 6 dB above the noise floor. Doing this lets you be assured > that you're not measuring anything else, (IP5 etc). averaging is also a > good idea, as that reduces noise in the measurement. You're measuring with > a 100 Hz RBW/VBW, you might be better served by increasing that to 1 or 3 > kHz and trace averaging. I'd also reduce the output power until the IP3 > tones are 6 dB above the noise floor, and use that single measurement to > calculate your OIP3. As your output power increases, Higher order > intermodulation products will start to come up at the same frequency and > give you a bad measurement, something that's no longer OIP3. > > > > >> 3. Will that affect my measurements for the 2F1-F2 and 2F2-F1 tones >> though for this calculation? >> > > Yes, IP5/IP7 tones can start showing up at higher output levels, close to > compression. If you widen up the bandwidth of the measurement to go out to > catch the IP5/7 tones, I bet you'll see them with your current measurement > setup. > >> >> Ben >> >> 1. (Re: OIP3) thanks thats a good point.. o_0 I should adjust that. >> >> 2. (Re: Useful) But knowing the OIP3 will surely still give me an >> indication of when my transmitter will start showing non-linearities... >> won't it? I'm not planning on using a USRP as a receiver... >> > > OIP3 measurement is a reasonable transmitter quality metric. There are > others that are system specific you could use, but it's all measuring the > same thing. > > >> >> 3. Re: Clipping. I expect your right, i've used an amplitude of A = 0.25 >> to avoid the DAC from saturating / causing clipping.. so ideally.. (if i'm >> right) any non-linear behaviour will be due to the OIP3 rather then the DAC. >> > > it's worth making sure of that by measuring the DAC output directly, if > possible. > > > > > >> >> >> >> On Fri, Sep 14, 2012 at 3:19 AM, Ben Hilburn <b...@ettus.com> wrote: >> >>> Michael - >>> >>> Hm, I don't understand what you are attempting to measure. Measuring the >>> input IP3 (IIP3) for a transmitter doesn't mean anything. All that matters >>> is output IP3 (OIP3). The e-mail you linked to is Marcus explaining how to >>> measure the IIP3 through the receive-only port of a daughterboard, using >>> the same daughterboard as a signal source. You are attempting to measure >>> the IIP3 of a transmitter only - whatever data you collect won't tell you >>> anything meaningful. >>> >>> David - >>> >>> Gain levels are approximate, and it is also possible that at the upper >>> amplitudes the ADCs are clipping anyway. >>> >>> Cheers, >>> Ben >>> >>> >>> On Wed, Sep 12, 2012 at 7:56 PM, David Bengtson < >>> david.bengt...@gmail.com> wrote: >>> >>>> Let me preface this by saying I mostly lurk on this list, and haven't >>>> touched a USRP of any variety, so I'm unfamiliar with their gain lineup. >>>> However, looking at this data, a couple of things come to mind. >>>> >>>> 1-You're sweeping the gain by 35 dB, and the amplitude is moving by 28 >>>> dB, which seems to indicate to me that something strange is going on with >>>> the levels. >>>> >>>> 2-I graphed the levels of the tones, and they aren't moving in >>>> amplitude until the gain setting is 15 or so, which seems unusal to me. >>>> What does the spectrum analyzer display look like when you are doing this? >>>> Do you see visible tones at deltaF? What about at other frequencies? Are >>>> you using a real SA or the USRP? >>>> >>>> 3-The other thing to look at and worry about are IP5/IP7 tones, which >>>> can really be a problem when you're approaching P1dB on the output power. >>>> I'm suspicious that at higher gain settings, you're starting to see these >>>> tones show up. >>>> >>>> Here's the google spreadsheet I imported your data into. >>>> >>>> >>>> https://docs.google.com/spreadsheet/ccc?key=0AmZNNcsCPNTZdFRyWi1Ed2VHR29DRC1hSFY5V1AzV0E >>>> >>>> >>>> Dave >>>> On Mon, Sep 10, 2012 at 10:32 AM, Michael Hill <moth...@gmail.com>wrote: >>>> >>>>> Apologies that copy paste formatting was attrocious..! Data attached >>>>> in file. >>>>> >>>>> P.S. earlier I meant to say f1 = 4MHz and f2 = 5 MHz >>>>> In the attached data >>>>> >>>>> m1 = 2*f1-f2 >>>>> m2 = f1 >>>>> m3 = f2 >>>>> m4 = 2*f2-f1 >>>>> >>>>> thx! >>>>> >>>>> >>>>> On Mon, Sep 10, 2012 at 10:56 PM, Michael Hill <moth...@gmail.com>wrote: >>>>> >>>>>> >>>>>> Can anyone provide any insight into why i'm getting the results I am? >>>>>> The aim of this test was to measure the OIP3 during transmit, then >>>>>> determine the IIP3. >>>>>> >>>>>> *Conditions* >>>>>> I conducted two-tone tests using f1 = 4 MHz and f3 = 5 MHz. >>>>>> The LO was set to 2400, 2450, 2500, 4900, 5400 and 6000 MHz (I'm >>>>>> using the XCVR2450 with the USRP N210) >>>>>> The Amplitude variable has been set to 0.25, and the gain has been >>>>>> stepped from 0 to 35dB. >>>>>> >>>>>> The two tone test was essentially setup as the same in this link but >>>>>> with the the output measured by an oscilloscope, thus i'm sure i've done >>>>>> it >>>>>> right.. >>>>>> >>>>>> http://lists.gnu.org/archive/html/discuss-gnuradio/2011-10/msg00433.html >>>>>> >>>>>> >>>>>> *Measurements* >>>>>> I've measured the power amplitudes, also the gain has been calculated >>>>>> at each step (since the gain request may not actually get the specific >>>>>> gain). >>>>>> Measurements have been taken with an agilent spec amp at 300Hz. >>>>>> >>>>>> I've then calculated the OIP3 and subtracted the actual gain to get >>>>>> the IIP3. >>>>>> >>>>>> *Results* >>>>>> However, my data is a bit odd... An example has been provided in the >>>>>> table below. It seems that my intermodulation products are not increasing >>>>>> at 3dB for each 1dB.. >>>>>> What does this imply? >>>>>> >>>>>> Examples of my transmit IIP3 results are attached, these seem >>>>>> somewhat in the jagged ball park of the receiver results below for the >>>>>> WBX. >>>>>> http://code.ettus.com/redmine/ettus/documents/16 >>>>>> Not an apple to apple comparison I know.. but I don't have much to go >>>>>> on.. >>>>>> am I missing something basic (I am still.. in my opinion.. new) >>>>>> >>>>>> (Note: units below are MHz for frequencies, and dB for gain, dBm for >>>>>> the output power.) >>>>>> >>>>>> Carrier m1 m2 m3 m4 2400 3 4 5 6 2400 MHz 2403 >>>>>> 2404 2405 2406 Gain 0 -50.27 -11.88 -11.58 >>>>>> -49.31 1 -49.79 -11.05 -10.79 -48.74 2 -49.37 -10.37 -10.09 >>>>>> -49.06 3 -48.77 -9.62 -9.36 -47.62 4 -48.89 -8.93 -8.67 -47.66 >>>>>> 5 -48.16 -8.08 -7.79 -47.26 6 -48.5 -7.43 -7.16 -46.71 7 >>>>>> -47.9 -6.63 -6.36 -48.1 8 -47.52 -5.98 -5.7 -46.06 9 -46.65 >>>>>> -5.24 -4.98 -46 10 -48.13 -4.58 -4.36 -46.07 11 -46.95 -3.84 >>>>>> -3.63 -47.07 12 -47.6 -3.23 -3.02 -45.6 13 -47.14 -2.51 -2.32 >>>>>> -46.78 14 -47.74 -1.81 -1.62 -48.15 15 -47.98 -0.98 -0.78 >>>>>> -45.74 16 -47.06 -0.31 -0.14 -46.42 17 -47.58 0.48 0.67 -47.57 >>>>>> 18 -46.64 1.24 1.39 -47.44 19 -46.15 2 2.2 -46.19 20 -43.16 >>>>>> 2.87 3.04 -44.33 21 -40.62 3.71 3.86 -42.54 22 -39.47 4.48 4.61 >>>>>> -39.99 23 -36.41 5.24 5.36 -37.01 24 -35.01 6.03 6.16 -35.63 >>>>>> 25 -32.85 6.81 6.94 -33.63 26 -31.27 7.72 7.87 -31.36 27 >>>>>> -29.14 8.62 8.7 -30.24 28 -27.78 9.59 9.7 -27.9 29 -26.18 10.68 >>>>>> 10.82 -26.36 30 -24.77 12.03 12.14 -25.26 31 -25.29 12.04 12.15 >>>>>> -25.62 32 -18.65 13.93 14.03 -19.96 33 -10.29 15.26 15.33 >>>>>> -10.31 34 -9.91 15.25 15.33 -10.09 35 -4.42 16.22 16.3 -4.33 >>>>>> >>>>> >>>>> >>>>> _______________________________________________ >>>>> Discuss-gnuradio mailing list >>>>> Discuss-gnuradio@gnu.org >>>>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>>> >>>>> >>>> >>>> _______________________________________________ >>>> Discuss-gnuradio mailing list >>>> Discuss-gnuradio@gnu.org >>>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >>>> >>>> >>> >> >
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