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In Reply to: RE: Interesting the 1st comment posted by jkeny on July 17, 2015 at 01:48:05
"Yes but the gotcha in all of this is that ethernet transformers do not provide isolation from RF noise - the most likely cause of audible issues when it gets into audio equipment."
Of course they limit energy transfer and filter frequencies in all bands. It's not like these Ethernet transformers are no impedance. They'll have a response tuned to Ethernet frequencies which is likely to be somewhat hot in the RF bands (as compared to 60Hz for example) but due to tiny cores and coil wire gauges are severely crippled in their ability to transmit appreciable power even in the bands they are tuned for. This is not even to mention the core losses, leakage inductance coupling energy to god knows where, I^2*R losses etc.
Ethernet transformers are just not a very effective power conversion device and when properly used can filter noise significantly.
Though, it is correct to say they are imperfect.
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Well I believe the problem with all differential signaling is common mode noise due to imbalance in the two or of phase signals. Don't think Ethernet transformers attenuate much if any of this CM noise?
In general transformers are designed to reject common mode signals while passing differential mode signaling, though nothing is perfect. Based on the construction and limitations set forth by mother nature, there is no such thing as a perfect transformer. Most common mode noise will be blocked by even crappy pulse type transformers as far as I know.
A problem with Ethernet circuitry CAN be common mode noise, sure. Diff signaling in itself only increases signal to noise ratio compared with the diff noise. However, the impedance "balancing" typically used in this type of circuitry allows for some amount of common mode noise rejection. Together they work to limit the negative affects of noise of both types.
The nature of the sum of the noise (common mode noise component plus differential mode noise component = sum of noise problem) problem will depend on the noise source and the victim circuits ability to reject it. In my experience the types of power supplies and high speed digital circuitry used in typical PC's can cause significant common mode and/or differential mode problems if not treated with care. Especially with low cost power supplies the diff mode and common mode filtering often employed to reduce these emissions and there are eliminated for cost reasons.
Poorly implemented circuits will be more susceptible. Awesomely implemented circuits will be less susceptible but not to the point of zero susceptibility, that would be impossible.
Depending on the source of the noise there is no guarantee you wont have differential mode noise problems in Ethernet circuitry nor is there any guarantee you wont have common mode problems...it all depends.
I might be wrong here but aren't ethernet transformers only good for differential noise issues? To deal with common mode noise requires a CM choke - yes it's a transformer but in a different topology/configurstion to the topology of ethernet transformers.
Ethernet pulse transformers are are just transformers tuned to comply to appropriate Ethernet hardware requirements, transfer Ethernet signaling power at the appropriately low power levels required for the job with required signal integrity. Like other transformers they are designed to respond to differential signals and reject common mode signals.
In transformers current is passed through a primary winding, magnetic flux in the core couples energy to the secondary winding to accomplish power transfer for whatever reason. The idea is to couple power from one winding to the other. Most of the time transformers have a different winding ratio on the primary and secondary to condition the transferred energy in some way.
Common mode chokes used properly ideally transfer no power from one winding to the next. They are used to link equal but opposing flux from the common mode noise current noise flowing through both windings in order that they destructively interfere. Common mode chokes I've seen are 1:1 winding ratios.
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