Jon Sreekanth <jon_sree@world.std.com> (05/07/91)
This is probably a simple question : what is the meaning of a spec such as a transformer being 600 ohm : 600 ohm ? For example, some telecom transformers (phone line interface transformers) are spec'ed this way. My understanding is if a transformer is ideal, it reflects the secondary impedance to the primary. So, if a transformer primary is connected to the telephone line, and the secondary is left open circuited, the AC impedance that the telephone line sees is infinity, right? If the secondary is shorted, the telephone line should see an AC short; if the secondary is connected to a 600 ohm load, the telephone line should see 600 ohm. In summary, what does the magic 600:600 spec mean ? Any 1:1 transformer should be interchangeable, and line matching really means the secondary should be terminated into the proper impedance. What am I missing ? Thanks, Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com
meier@uunet.uu.net> (05/07/91)
In article <telecom11.339.13@eecs.nwu.edu> jon_sree@world.std.com (Jon Sreekanth) writes: > In summary, what does the magic 600:600 spec mean ? Any 1:1 > transformer should be interchangeable, and line matching really means > the secondary should be terminated into the proper impedance. You are essentially correct. However, for telephony, the reference impedance is usually 600 ohms (or sometimes 900 ohms), and so you want to easily know the matching impedance. Therefore, a 1200:600 transformer is the same as a 2:1 transformer, but the former figure more easily shows that it will match 1200 ohms to 600 ohms. Also, the dc resistance of telephony transformers is around 40 ohms, which is much less significant at 600 ohms as if it was used for 8 ohms. So, another reason for using 600:600 is to indicate that it is for high impedance applications. Don't try to use it for your stereo speakers. Rolf Meier Mitel Corporation