The XHDATA D-808 schematic and internal design are highly regarded by radio enthusiasts, though technical quality varies significantly between the "Original" and "New" hardware revisions. Core Design & Quality Overview
However, I can offer some general guidance on what you might be looking for:
The D-808 is built around the Silicon Labs Si4735-D60 DSP chip. It uses a "Low-IF" architecture, processing signals at 45kHz for AM and 128kHz for FM. Circuit Layout: Includes an RF front-end with mixers and filters.
RF Front End: The RF front end is designed with high-quality components to ensure good sensitivity and selectivity right from the input stage. The use of high-performance RF transformers, capacitors, and inductors contributes to the receiver's ability to handle a wide range of signals without significant degradation.
“Extra quality” is vague – schematics are either readable/accurate or not. High-resolution vector PDFs or clear photos of a traced PCB are rare for this radio.
The Current Reality: A Crowdsourced Patchwork
As of today, XHDATA does not publicly release service manuals. The device uses a proprietary blend of the Silicon Labs Si4735 DSP chip (for AM/FM/SW) paired with a separate MCU and analog front-end for SSB.