3 ms·

Thank you for correcting me. I misspoke and said ethernet when I was thinking modem. Many are rated from ~300Hz. And I did assume low signal levels in a similar

• klodolph 4y ago
  I think modem transformers might have a similar problem of limited bandwidth--they may be rated from 300 Hz, but they are also rated to 3.4 kHz or something similar. Modems were mass-manufactured, and transformers tend to be expensive, so you would expect any expensive part in a mass-manufactured product to be engineered to within a hair's breadth of the specification for modems, which have something like 300 Hz - 3.4 kHz frequency range. On the low end, you have core saturation. On the high end, you have parasitic inductance and capacitance, as well as loss from hysteresis and eddy currents. Solving these problems basically means making the transformer larger and more expensive. I did a quick search on Mouser, and found wideband (20 Hz - 20 kHz) audio transformers at around $6 in small quantities. Let's say that you get it for $3 in bulk. At that point, you're adding something like $20 or $30 to the retail cost of whatever it is you're selling. If you're okay with the telephone-quality of the simple modem transformers, then you might as well use the optoisolator--neither option sounds good, but the optoisolator is cheaper. You know, as long as you have a power supply on both sides--which is not a given.