  This paper describes specification and early design of back end signal
processing subsystems for the Black Hole Explorer (BHEX) Very Long Baseline
Interferometry (VLBI) space telescope. The "back end" consists of two
subsystems. First, the block downconverter (BDC) is a heterodyne system that
performs a frequency translation of the analog signal from IF to baseband and
amplifies and filters it for digitization. Second, the digital back end (DBE)
samples the analog signal with an analog-to-digital converters (ADC) and
digitally processes the data stream formatting them to the VLBI "VDIF" standard
and converting to Ethernet packets for 100 gigabit-per-second (Gb/s) Ethernet
transport to the optical downlink system. Both the BDC and the DBE for BHEX
support eight channels of 4.096 GHz bandwidth each, for a total processed
bandwidth of 32.768 GHz. The BHEX back end benefits from mature terrestrial
back end heritage, described in some detail. The BHEX back end itself is in the
early stages of design, with requirements, interface specifications, and
component trade studies well advanced. The aim is to build a prototype using
terrestrial grade parts which are available in functionally identical space
grade equivalents, and to use this prototype to advance the back end Technology
Readiness Level (TRL) preparing for a Small Explorer (SMEX) proposal in 2025.
