bud@uunet.uu.net> (05/08/91)
In article <telecom11.339.13@eecs.nwu.edu> 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 > 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. The RESISTANCE (ohms) of the wire in that transformer. The keyword in your question is "ideal". It is possible to make transformers that approach ideal. Unfortunately, at audio frequencies, this means lots of iron and copper. (Reminds me of the old joke about AE at Northlake: truckloads of coal, iron and copper ore in the back door, *tons* of step-by-step out the front.) For both cost and size purposes, compromises are made which result in some portion of the impedence seen at the terminals being the resistive components of the transformer. How significant this portion is is a function of the impedences being matched. The transformer you cited as 600:600 has perhaps two or three ohms of resistance in the windings of each side. This is acceptable at 600 ohms, but would cause serious problems if someone attempted to use it at 50 ohms. Bud Couch - ADC/Kentrox If my employer only knew .. standard BS applies