# Curved noise floor and ugly unexplained peaks after FFT

It has been so long since I studied this at University, so I'll better ask here if I'm doing something really wrong here.

I'm using a USRP frontend for L1 band surveillance. I want a 20MHz bandwidth, but I found through testing that it's better to use a sampling frequency slightly higher than twice that bandwidth (I'm using 41MHz), so I can use a local oscillator to move the DC out of the spectrum. Probably there is some additional error of concept here, but this isn't the problem I'm facing now.

First thing to note: I'm using an FFT over a big chunk of data. Depending on the configuration this size may vary, but for example I'm currently using 8192 samples per FFT. Maybe this is too much and I should compose the whole FFT using many smaller ones, somehow?

In order to reduce noise, I'm performing 100 of these FFTs, summing their outputs and dividing by 100. Probably there's a better method, but so far ok.

After I apply the FFT (with FFTW library), I convert the output to dBW/Hz with the following formula (I'm assuming I have Vpeak):

10 * log10(psd_output / (2 * 50 Ohm)) - 60


Now, if I plot this vector, I get something like this:

The noise floor is getting some curvation (up to 5 dBs). I tried applying a hamming or a blackman-harris window to the FFT output with the hope or removing this effect, but it didn't work.

If I test with a jammer, I can see the peak, but I also see some other minor peaks which are unwanted (and that are not seen in an spectrum analyzer with the same configuration):

What am I missing? These should be simple operations, but I'm out of ideas at this point.

• I have not worked with radio signals before, but could it be that the Nyquist criterion is violated? It is important that the sampling rate is higher than the highest frequency in the signal, which is not necessarily equivalent to the signal's bandwidth. Doesn't the L1-band have a carrier frequency in the GHz range? You're probably doing the demodulation, but I thought I just ask. Feb 8 '18 at 9:31
• @applesoup most USRP devices come with a down/upmixer, and work in complex baseband with dual (IQ) ADC, so Nyquist reduces to once the signal bandwidth. Roman, which USRP exactly are you using? Only a few USRPs actually support sampling rates like 41 MHz, that's why I ask. Feb 8 '18 at 9:47
• @applesoup I might be wrong, but I think that it's not like that: my carrier is the central freq of the L1-band, where I want to sample 20MHz of the spectrum (thereof, that's why I use at least 20MHz of sampling freq at least). I use 41MHz of sampling freq, get 41MHz bandwidth and later cut the spectrum to my desired 20Mhz. Is that wrong somehow? Feb 8 '18 at 9:48
• @Marcus Müller I'm using the mini205i, isn't it ok for that sampling rate? I'm also using a rxgain of 40dB, in case the curve is due to that somehow Feb 8 '18 at 9:51
• @MarcusMüller I don't mind that I can't see GPS: the problems are (1) that curve over the noise floor (maybe because I ask the USRP for rx_bandwidth EQUAL to rx_rate?) and (2) the undesired peaks when I expected to see a flat noise floor, just like the one I see in a spectrum analyzer. Those peaks are too high to let me set a reasonable threshold to detect interferences, specially if they are in the upper zone of the 'curved spectrum'. See third example I just added. Does that seem normal? Feb 8 '18 at 10:22