How to design a 1-500MHz sine wave level detector to -90dBm?
Thermal noise for a bandwidth of 500MHz at 50 ohms is about 20 \$\mu\$V at 20degC. You can double check this with this calculator but the long hand method is: -

\$ V_n = \sqrt{4\cdot K_B\cdot T\cdot R\cdot \Delta F}\$

Where Kb is Boltzmann's constant, T is temperature, R is 50 ohms and \$\Delta F\$ is 500MHz

Anyway 20uV and 50 ohms gives a power of 8 pico watts and this is -81dBm.

You won't get -90dBm without making the circuit tuned to reject noise in areas you are not interested in. Or maybe consider some form of refridgeration?