The 1979 vintage RF spectrum analyzer HP4195A  sn2904J01587 shipped out  for repair today to http://www.avalontest.com

It has a 25 MHZ oscillation when you go  below 150 MHZ in your sweep....atm1 with the larger amplitude shows this 25 MHZ

Atm2 is displaying  full sweep-sign scans from 1 to 500 MHZ.....here one can clearly see the three segment of the scan:

1, large amplitude 25 MHZ oscillation dominating the spectrum up to 150 MHZ

2, the mid section from 150 MHZ  to 300 MHZ with medium size amplitude is normal

3, from 300 MHZ to 500 MHZ the amplitude is decreasing.......showing the disadvantage of using a 300 MHZ oscilloscope