The most common and most likely answer to this crossword clue is the 10 letter word HETERODYNE
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Buddy explains!

The straight part
We know that "mixed signal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mixed signal = heterodyne.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does HETERODYNE mean?
HETERODYNE noun- The beat so produced
- Either the sum or difference of the two oscillations
- To produce heterodyne interference in a radio
- To change the frequency of a signal by such a process
- Of two oscillations: having two slightly different frequencies such that, when combined, they produce a beat