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:
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