A simple sequence of settings will help you dig into the noise floor. With any Spectrum Analyser, setting up the (1) Span, (2) Attenuator, (3) Pre-amp and (4) Resolution Bandwidth settings - these are all important starting points to getting the best noise floor. Remember, safety first (no input or power RF nearby) and try adjusting these four settings to find out more about what to turn on, and off, to get the best from DSA815. (Like most other spectrum analysers, other settings e.g. Filter and Detector types can also affect your exact noise floor and can do so in non-linear or non-additive ways)
Remember, you don't always want, or NEED the lowest noise floor possible for your tests because you may compromise on breadth of SPAN or you may find yourself waiting for long SWEEP TIMES so take care to decide what is best for your test-methods.
As just ONE example of the many different ways of working with spectrum analysers, if you are working with EMC Pre-Compliance, it will usually be impractical to work at noise floors this low (for most of the time anyway!) because you may be hunting for noise changes over a wide range of frequencies or spans - at least initially....
BUT you do want them sufficiently low to see how your changes are improving your emissions!
Always consult your EMC expert about the noise floor required to get reliable test results in your test environment and for your EUT. In EMC work, for example when estimating the improvements you have made to your Golden Product (the unit unchanged from your first EMC lab visit), always try to work with a noise floor low enough that you can ignore it altogether.
THE GOLDEN RULE:
Using your Golden Product, make only like-with-like tests to judge your improvement in emissions - i.e. do both these sweeps on as identical test set-ups as you can:
(i) sweep your Golden Product with DSA815 (with negligible noise floor), then
(ii) sweep your IMPROVED product (with same negligible noise floor) and compare the emissions levels of both.
Keep switching between with different settings to cover your range of concern as necessary - if your noise floors are negligible, you may be closing in on expected improvements from your next precious EMC Lab visit!
The Golden Product approach, together with some wise EMC-experienced eyes, can be powerful -
...that way, you are effectively eliminating the ever-fickle noise floor from your sums.
THE ABOVE IS NOT TO BE USED AS ADVICE FOR YOUR PARTICULAR EMC TEST REQUIREMENT - GENERAL PRODUCT INFORMATION IS ALWAYS BEST SUPPLEMENTED WITH A PROVEN, WELL-DESIGNED METHODOLOGY FROM AN EXPERIENCED EMC PRACTITIONER.