# What cause the shape of frequency spectrum convex or concave?

I use gnuradio and fosphor to view the spectrum from USRP B210, default gain set to 34, output looks like concave as below:

Nothing changed just set gain to 35, output become flat, looks like convex:

What cause the shape of frequency spectrum convex or concave when I change gain from 34 to 35?

• Please focus on one question per post! Thank you :) Oct 5 at 18:52
• Asking more than one question in a question post might be OK if they are related so that a single answer (I mean a single explanation, not a single answer post) could cover them all. But many question posts have been closed because they needed more focus, because a single question post contained more than one question. This question post is under review, and I recommend: 1) edit this question post to delete one question, and 2) make a new question post for the deleted question. Oct 8 at 20:01
• I'm editing the question and removed question 2, "2.Whatever center frequency I set,0 frequency always has a peak.Why? Is that means my USRP B210 has something wrong?". Also upvoted. Oct 15 at 13:42

The edges of your spectrum is where the performance of the receiver is hitting the limit. there is a trade-off between flatness and total bandwidth. they could have made the bandwidth smaller to keep it nice and flat but figured, a little extra bandwidth might be nice to have. It's not like this is a spectrum analyzer.

The USRP SDR uses an downconverting IQ modulator before the ADCs. An error in quadrature phase or quadrature levels of the two IQ modulator clock signals or gain and delay balance of the IQ modulator(s) can result in DC artifacts in the spectrum of the IQ ADC data. The ADCs themselves can have a small DC offset in their sampling. Both of those artifacts will show up at 0 Hz in an FFT magnitude spectrum of the IQ data.