SM200C noise at lower frequencies

I’m not sure what signal you’re looking at there, but it is possible that level of noise is actually on your signal. You can turn the preselector on and off below 640 MHz, and change your reference level by 10 dB. If the noise level changes considerably, it may be partially from intermodulation inside the SM200C. Otherwise, it is likely part of your input signal.

You will notice in the specifications that the DANL (noise floor) specification is 4 dB higher below 700 MHz vs. above 700 MHz. A 4-5 dB step in the noise floor is normal. This is due to a design decision to optimize for noise figure in the 700 MHz – 2700 MHz communications band, and to push for better linearity at low frequencies.