You can find the receiver threshold specifications in our most excellent PTP 820 specifications sheets, which are here: http://www.cambiumnetworks.com/spec-sheets/ptp820_specs
The receiver threshold specifications are also listed in the PTP 820S Technical Description, PTP 820C Technical Description, and PTP 820G Technical Description, which are here: https://support.cambiumnetworks.com/files/ptp820/
I hope this helps!
Dave