jon_sree@world.std.com (Jon Sreekanth) (05/07/91)
Posted to comp.dcom.telecom, sci.electronics 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
tomb@hplsla.HP.COM (Tom Bruhns) (05/08/91)
jon_sree@world.std.com (Jon Sreekanth) writes: >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 ^^^^^^^^ This is the key. No real transformer (or other component) is 'ideal'. >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 ^^^^^^^^ No, it's the (inductance+resistance) of the winding connected to the line. You can make this fairly high, but not infinity, and ... >line should see an AC short; if the secondary is connected to a ^^^^^^^^ No, it sees nominally the leakage inductance plus winding resistances (as reflected through the turns ratio for the secondary) --- which combined with the above says you want to make the coupling tight (but may want very low inter-winding capacitance for isolation), and the inductance high enough to allow an open on the secondary to reflect as a moderately high impedance, but not so high that the leakage inductance makes a shorted secondary reflect as too high an impedance -- and not so much wire that the resistance kills the performance. So the 600:600 is _nominal_, and you can generally use the transformer for other impedances (e.g., 300:300 or 1000:1000), but it's been optimized for 600:600 (as you might have guessed, over a particular frequency range, as well), and don't expect it to work well as 8:8!