Upvote:6
We don't have to look as far as Constantinople for the inspiration of the Aachen Palace Cathedral, or theorize about unknown trips to the east for Charlemagne. A building we know was visited by Charlemagne is considered to be the source of much of the structural design style:
The plan and decoration owe much to the sixth-century Basilica of San Vitale, Ravenna. Indeed, Charlemagne visited Ravenna three times, the first in 787. In that year he wrote to Pope Hadrian I and requested "mosaic, marbles, and other materials from floors and walls" in Rome and Ravenna, for his palace.
So we can associate directly the visit to Ravenna with the start of the construction project. No trip to Constantinople involved.The Basilica of San Vitale is also of Byzantine styling(emphasis mine):
The church is most famous for its wealth of Byzantine mosaics, the largest and best preserved outside of Constantinople.
Clearly, Charlemagne liked the Byzantine design, but he did not have to go all the way to Constantinople to observe it.