Manuals:
https://www.manualslib.com/manual/1649195/Bruel-And-Kjaer-4220.html
Instructions and applications:
https://www.rosenkranz-elektronik.com/userUpload/pdf/bk_4220_datasheet.pdf
https://www.opweb.de/italiano/societa/Br%C3%BCel_and_Kj%C3%A6r/4220
