From the picture, I'd guess the spurs are due to excessive line
frequency ripple from your 50 Hz mains (full-wave rectified = 100 Hz
spurs) - it could be internal to the reference source or other gear in
the system, or ground loops. Check the specs for residual line noise on
the various equipment (including the SA itself) - it seems it should be
much better than what's showing. If the overall setup specs should show
much better, then look for internal problems like bad filter capacitors
etc in power supplies, or external ground loops and cable shielding
failures. If your 1.5 GHz PLL mentioned is home-made, you may want to
look at that too, and ensure that its power supply is clean enough. If
you find and fix such failures, you may not need to change to a new
reference. Good luck.
Ed
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