Spanish PDF, 474 KB, titled CDMA and FSK and numbered 5, apparently a chapter in a communications course. It explains CDMA, code division multiple access, and FSK, frequency shift keying, rather than giving a schematic for a particular device. Theory like this helps technicians understand how radio sections in phones and modems encode and share channels, though board-level RF faults still call for the specific schematic. An identical copy of this document exists under the same name and size in the index.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @canalesquematicos. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.