# unexplainable aliases in spectrogram

My scope has 200 MHz bandwidth and can sample at 4GS/s

I'm sampling a 138 MHz RF signal at 500 M Sa/s storing 14 M points.

When I plot the spectogram in matlab I get the following spectrogram(Data,12000,8000,[],fs,'yaxis')

Finally here is the frequency spectrum

While the signal does show up at 138 MHz, there appear to be aliases at 50 MHz, 190 Mhz and complex conjugate aliases at 86 MHz, 172MHz and 224 MHz. Does anybody know why this could happen?

edit:

• Welcome to DSP.SE! Can you update the second image? All I'm seeing is mostly white with some blue bumps right down the bottom. A better scale for the $y$ axis would improve the display. – Peter K. May 11 '16 at 11:48
• that's because of the DC compoment; I'll remove that from the display and update the image – Thomas May 11 '16 at 12:55
• In addition to changing the scale, maybe try using a logarithmic y axis in the second plot. – MBaz May 11 '16 at 12:56
• What is the characteristics of this signal? Is it FM modulated signal? – Nir Regev May 11 '16 at 20:39
• maybe it's not aliasing what you see and they are the replicas of the signal spectrum? please, put better images – Behind The Sciences May 12 '16 at 6:11

Looking at the spectrogram, the prominent artifacts go up or down in frequency synchronously to the 138 MHz signal but have larger bandwidths. That is an indication that they are its harmonics, due to a nonlinearity somewhere in the system. With the sampling frequency of 500 MHz some of the positive and negative harmonics alias to the following (bold) frequencies that match with the prominent artifact bands:

• -138 MHz * 2 = -276 MHz. -276 MHz + 500 MHz = 224 MHz
• -138 MHz * 3 = -414 MHz. -414 MHz + 500 MHz = 86 MHz
• 138 MHz * 4 = 552 MHz. 552 MHz - 500 MHz = 52 MHz

The apparent bandwidths and complex conjugacy seem to match those calculations.

You did not mention what equipment you are using, but it looks like there is no proper antialias filter.

The less prominent artifact bands can be explained similarly:

• 138 MHz * 5 = 690 MHz. 690 MHz - 500 MHz = 190 MHz
• -138 MHz * 6 = -828 MHz. -828 MHz + 2 * 500 MHz = 172 MHz
• I agree with Olli (+1). The system you are looking at is highly nonlinear... especially to generate even order harmonics. – Peter K. May 12 '16 at 16:48
• Yes I was wondering if it might be due to a slew rate limitation in the oscilloscope input amplifier, but the distortion is too severe for just that. Maybe a bad contact somewhere acting as a semiconductor, dunno. – Olli Niemitalo May 12 '16 at 17:04
• Something like that would explain it. – Peter K. May 12 '16 at 17:06
• @Olli Niemitalo great analysis. I totally agree with the nonlinearity verdict :) – Nir Regev May 12 '16 at 21:59
• I'm using a Rigol DS4024. Thnqks for your answer! – Thomas May 14 '16 at 9:34