Hi, folks, I recently did some comparison test with my USRP and USRP2 boxes and got slightly different results. Test setup - Signal source : HP signal generator (forgot the name of the equipment) - Daughter cards for USRP and USRP2 : BasicRX - Test configuration : I divided a 12.1MHz CW signal from the SigGen with a 3-dB divider and fed the outputs to both USRP and USRP2 boxes. Please find the two snapshots here. http://zoolu.co.kr/episodes/3 Both have 4MHz span and centered at 12.3MHz with fft size of 8196. Questions 1. It seems that the noise floor of USRP2 is about 10 to 12 dB lower than that of USRP while the USRP has better SNR. Which are considered relatively more important in general between a lower noise level (probably be able to detect weaker signals) and a better SNR when you plan to build a signal detect solution? 2. I can see that the USRP has a bit more flatten noise floor shape but USRP2 has less spurious signals. Are those because of different performances of different ADCs or actually the two signals fed to each boxes are different although I branched from a single source? 3. I can see a rather strong peak of 12.3MHz signal in the USRP. I guess this is out of the local oscilator signal from CORDIC in the FPGA. However, I don't see the signal in the USRP2. Is this a leakage from the FPGA to ADC for the USRP unit? If then, is there any good way to get rid of this? 4. I know that the relative signal level can be changed according to the numbers of FFT sizes. How do you guys calibrate the level? What I can think instantly is to measure a reference signal of know level (ie, 0 dBm) and put some codes to calibrate the signal level. Just wonder if there is any better method used in general. Best regards, Ilkyoung.
on 2009-01-13 16:51
on 2009-01-13 18:51
Hi, 1) The comparison is not fair with these decimation values. For USRP2, you used a decimation of 25. Using this odd decimation will disable the FIR filters of the USRP2 FPGA. 2) To make the comparison valid, I suggest for you to use decimation of 64 for USRP and decimation of 100 for USRP2. In this case, both of them will have a bandwidth of 1MHz. Re do you experiments and post the results again. 3) The ADC of USRP2 is 14Bit, while that of USRP is 12 Bit. This should give about 12 dB quantization noise improvement to USRP2 over USRP. Best Regards, Firas
on 2009-01-13 19:03
..... and Don't forget to reduce input signal level. I see distortion in your graphs. Use -20dBm as input signal level. Regards, Firas -- View this message in context: http://www.nabble.com/USRP-and-USRP2-FFT-result-co... Sent from the GnuRadio mailing list archive at Nabble.com.
on 2009-01-13 21:28
ILKYOUNG KWOUN wrote: > and fed the outputs to both USRP and USRP2 boxes. > relatively more important in general between a lower noise level > (probably be able to detect weaker signals) and a better SNR when you > plan to build a signal detect solution? > What you are seeing here is the effect of the PGA (programmable gain amplifier) on the USRP. In your usrp_fft plot you can see it is set to 10dB. The USRP2 does not have an equivalent to this PGA, so to make a better comparison, you should set the slider to 0dB manually, or use "-g 0" on the command line. Bringing the gain down will also lower the noise floor of the USRP1, but not down to the level of the USRP2. In general, the USRP2 will have a lower noise floor, and lower spurious due to the 14-bit ADC. Also, I think that the two python programs you are using will have the same number of points in the FFT, but you should make sure if you are trying to compare them. Remember, you can keep lowering the height of the apparent noise floor by using more points in your FFT. The real amount of noise will be the same, but you will be dividing it into more and more bins. 1024 is a good number to use, since it is comparable to the resolution of your screen. > 2. I can see that the USRP has a bit more flatten noise floor shape > but USRP2 has less spurious signals. Are those because of different > performances of different ADCs or actually the two signals fed to each > boxes are different although I branched from a single source? > The spurs you see are from a variety of causes. You didn't say what signal level you are feeding in to the boards, but about 9dBm will clip in the USRP2 and in the USRP1 with gain set to 0. Your gain is set to 10dB, so -1dB would be the clipping point. ADCs are typically spec'ed at "-1dBFS" which means a sinusoidal signal 1dB lower than full scale, so try 8dBm. Signal generators may only be accurate to .2 to .5 dB, so you may have to adjust in 0.1dB increments. Also, the non-flat noise floor in the USRP2 may be a real part of the signal you are looking at. It is very easy to see the phase noise of the signal generator you are using. I actually wasted a couple of days trying to figure out why I was seeing that much phase noise, until I realized it was the noise of the generator and not the USRP2. I had assumed that a high quality signal generator from Rohde and Schwarz would have better phase noise than the USRP2, but that actually isn't the case at low frequencies like this. Once I got a better signal generator with the "low phase noise option" I saw it go down quite a bit. > 3. I can see a rather strong peak of 12.3MHz signal in the USRP. I > guess this is out of the local oscilator signal from CORDIC in the > FPGA. However, I don't see the signal in the USRP2. Is this a leakage > from the FPGA to ADC for the USRP unit? > If then, is there any good way to get rid of this? > Another cause of your spurs is probably harmonics from the signal generator aliasing into the passband. Remember, the BasicRX does not have any filtering, and the ADCs will respond to signals well past 400 MHz. Even a very good signal generator may have 2nd and 3rd harmonics which are only 50 dB down, and even higher order harmonics are measurable. Use a lowpass filter to get a purer tone. In testing the USRP2, I use a very high quality signal generator followed by a lowpass filter with more than 35 dB of harmonic rejection, and even then, the harmonics from the generator are still at a roughly comparable level to the USRP2s own harmonic performance. > 4. I know that the relative signal level can be changed according to > the numbers of FFT sizes. How do you guys calibrate the level? What I > can think instantly is to measure a reference signal of know level > (ie, 0 dBm) and put some codes to calibrate the signal level. Just > wonder if there is any better method used in general. > We automatically scale so that the height of the peak for the fft of a sinusoid (or any of the discrete spurs) should stay the same no matter how many points in the FFT. Anything which is wideband, like noise, will show up at reduced levels when you have more points. To calibrate to an absolute signal level, it is easiest to just do as you said and put in a known amplitude. You could also do your computations based on the gains of everything in the signal chain, but that is more work. Thank you for performing these tests. It is good to have many people measuring these things. Matt
on 2009-01-14 01:43
Thank you so much, guys. I will setup another experiment regarding your advices and post the result. Since I have to arange my friend's lab, I probably could do it on weekends. :-) Best regards, Ilkyoung.
on 2009-01-17 08:10
Guys, I managed another comparison test last night. http://zoolu.co.kr/episodes/6 *. Test tone signal quality I used a siggen as good as I could get. :-) According to the spectrum analyzer, it seemed that the siggen did not generate any noticeable harmonics(1st and 2nd pictures). Actually, I raised the power level up to 0 dBm when I tested last night. But took the snapshots with -34dBm level. *. Signal level I reduced the signal level down to -34dBm just to avoid any possible input level saturation. I believe it is low enough. :-) My questions 1. Still get spurs As you can see in 3rd and 4th pictures, I still have quite significant spurs in band. However, when I match the local oscillator frequency with that of test signal like picture #5, all spurs were gone away. So, I am still suspicious if those spurs came out of the LO leakage from the FPGA CORDIC. (I don't believe that happens in the digital domain. Probably, the digital switching noise spilled over to the analog part and it was fed to ADC input. That's my best guess.) Is there any comments, corrections, or suggestions? I probably have to figure out Kyle Pearson's work on rounding in FPGA that Frank mentioned before. 2. Automatic signal level scaling I made comparison between different FFT sizes and the peak level varied. As you can see, The Peak level was -11dB with FFT size of 8192 and -19dB with 1024 while the absolute signal power level was -34dBm. The differences are about 8 dB and it explains because the FFT size scale is 8(=2^3) times. Which part of the code does the automatic scaling? Ilkyoung.
on 2009-01-19 04:46
You are correct about the fft level not scaling with fft size. I remember putting this in some time ago, but it isn't there now, so I don't know what happened. In any case, I did some further analysis on the spurs. We are seeing 2 different things on the USRP1 and USRP2 The USRP1 definitely has a DC offset problem. I thought I had fixed this a long time ago, but I guess it never got put in there. It isn't hard to fix, but I'm busy with other things right now. I would be happy to accept a patch for the verilog code. On the USRP2, I believe that what you are seeing is perfectly reasonable. If you put in a full scale signal (about +8dBm), the peak shows up at about 31dB (on the arbitrary scale on the display, assuming you use 8192 points in the FFT). You are seeing the 12.1 MHz (DC) spur at about -60 dB, which is about -90 dB Full Scale. Since we are only sending 16 bit numbers over the bus, you can't expect to have better dynamic range than about 96dB. Matt
on 2009-01-19 06:01
ILKYOUNG KWOUN wrote: > Guys, > > I managed another comparison test last night. > http://zoolu.co.kr/episodes/6 > Just to follow up on this a little more, if you generate a complex sine wave in octave or Matlab, quantize it to 16 bits, and then plot the fft, you will see similar behavior. Matt