Come back to 100Base-TX - possible?

Come back to 100Base-TX - possible?
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Subject Author Date
Come back to 100Base-TX - possible? Michelot 03-31-2008
Posted by Michelot on March 31, 2008, 7:29 am
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Bonjour Rich,

In june 2001, that is yesterday, you wrote that, about 100Base-TX:

"The scrambling prevents IDLEs (and any other repetitive data pattern)
from concentrating all of the transmitted energy into a narrow
frequency band, and hence violating EMI requirements. (Without
scrambling, the MLT-3 encoded IDLE would appear as a continuous, 31.25
MHz continuous-wave signal.)"

I agree with that statement.

In the same conditions, with the scrambling, we have now 125 MHz /
2047 = 61 kHz instead of 31.25 MHz continuous-wave signal.

The value 2047 come from the (2^11 - 1) period with the 11 registers
PRBS.

Perhaps the crosstalk can be acceptable, and the energy in this
frequency (NEXT or FEXT) would be low.

Thanks if you have a comment.
Best regards,
Michelot

Pure Networks
Posted by glen herrmannsfeldt on March 31, 2008, 2:43 pm
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Michelot wrote:

> "The scrambling prevents IDLEs (and any other repetitive data pattern)
> from concentrating all of the transmitted energy into a narrow
> frequency band, and hence violating EMI requirements. (Without
> scrambling, the MLT-3 encoded IDLE would appear as a continuous, 31.25
> MHz continuous-wave signal.)"

> I agree with that statement.

> In the same conditions, with the scrambling, we have now 125 MHz /
> 2047 = 61 kHz instead of 31.25 MHz continuous-wave signal.

If it is periodic at 61kHz you want the Fourier transform (or Fourier
series) of the signal to see how it is distributed. Energy will be
at multiples of 61kHz. With a good scrambler it will be distributed
somewhat evenly among the harmonics, or decreasing with higher frequencies.

> The value 2047 come from the (2^11 - 1) period with the
> 11 registers PRBS.

> Perhaps the crosstalk can be acceptable, and the energy in this
> frequency (NEXT or FEXT) would be low.

This is also true. Both inductive and capacitive crosstalk
decrease with decreasing frequency. There will still be energy
up to 62.5MHz, assuming the appropriate filter to keep higher
harmonics out.

-- glen


Posted by Michelot on March 31, 2008, 4:03 pm
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Bonsoir Glen,

> If it is periodic at 61kHz...

Yes, it is

> With a good scrambler it will be distributed
> somewhat evenly among the harmonics, or decreasing with higher frequencies.

For 100Base-TX, the scrambler is well defined in 802.3.

100Base-TX: 11 registers
1000Base-T: 33 registers
10GBase-R: 58 registers

> ...Both inductive and capacitive crosstalk
> decrease with decreasing frequency.

You're right, we don't have to forget it.


About the fact of the "good scrambler", I don't know if we can
construct a PRBS with any generator polynomial (e.g. beyond the 11th
degree).

Best regards,
Michelot

Posted by glen herrmannsfeldt on April 1, 2008, 1:45 am
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Michelot wrote:

>>If it is periodic at 61kHz...

> Yes, it is

>>With a good scrambler it will be distributed
>>somewhat evenly among the harmonics, or decreasing with higher frequencies.

> For 100Base-TX, the scrambler is well defined in 802.3.

> 100Base-TX: 11 registers
> 1000Base-T: 33 registers
> 10GBase-R: 58 registers

So all you need is the output of the scrambler with
idle as input. 2047 different voltages, and calculate
the Fourier series from that. That will give the amplitude
and phase of the harmonics from 61kHz to 62.5MHz in 61kHz steps.

-- glen


Posted by Michelot on April 2, 2008, 6:31 am
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Bonjour Glen,

> So all you need is the output of the scrambler with
> idle as input.

It's the main reason why the scrambling exists: to avoid high
frequencies.

> ...2047 different voltages, and calculate
> the Fourier series from that. =A0That will give the amplitude
> and phase of the harmonics from 61kHz to 62.5MHz in 61kHz steps.

Ok, thanks for these comments. We avoid this high energetic frequency
(or energetic frequency band centred around 31.25 MHz).

Best regards,
Michelot

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