Re: [BP] Informative Channel
Hello, Charles,
I agree your "nits" points.
1. You are right for 4/pi (2dB) factor in a case of square wave at
TX output.
In actual case, for example there is 0.5pF stray capacitance
including TX output transistors, ESD protectors, etc.,
3rd harmonic will be -13dB referring to 5GHz sine wave, and the
factor between composite p-p vs sine wave is almost 1dB.
This is the reason why I ignored the factor previously. But I
agree that still 1dB difference exits.
2. I agree that RX equalization need to boost at least 5GHz.
4. Length of ..0101.. can be 31~32bit at PRBS31. In the case,
its spectra distributes approximate 250MHz width. I'm not sure
that it is wide enough to cover dip and peak of frequency ripple.
I understand 5dB is too match. please teach me how many dB is
reasonable.
I guess 0101010 (8bit length) spreads approximate 1GHz, and ok to
cover dip and peak.
I am still wondering BER at lower S/N case.
M. Sugawara
>sugawara,
>
> Good over all analysis, i will pick a few nits below, but i agree
>with the general overall results.
>
> In answer to your question, yes. If we want our silicon to work
>at 10Gb/s over realistic backplanes, we will have to make our receivers
>work at quite small levels, even with cross talk which will not be much
>smaller. This may mean a pre-amp, DC offset cancellation, etc. Early
>versions have high power requirements and command a premium price but
>most likely, if KR is successful, competitive forces will bring these
>down.
>
> Nits:
>
> 1. At the end of the channel the alternating 10 pattern will
> have its higher harmonics removed. The harmonics actually
> reduce the p-p amplitude so the remaining sine wave will be
> about 2dB (4/pi) higher than you predict.
> 2. Rx equalization is likely to enhance the alternating 10
> pattern some.
> 3. At the Nyquist rate, 5.15625GHz the equation 69.2 is down
> 26dB :-(.
> 4. Random data will not sustain an alternating 10 pattern very
> long, so there will be no spectral line to fall into a dip
> in the transfer characteristic. The finite length of the
> alternating region should spread the spectrum enough to
> allow it to be spread across a region which includes both
> dips and peaks.
> 5. The compliance test channel defined in annex 72A will be
> chosen by the silicon vendor to make his silicon look as
> good as possible while still staying below the spec line.
> You should be able to avoid ISI loss worse than the value
> shown as "typical", i certainly intend to make my test
> channel as close to the spec limit as possible.
>
> charles
>
>
>--
>|--------------------------------------------------------------------|
>| Charles Moore
>| Agilent Technologies
>| ASIC Products Division
>| charles_moore@agilent.com
>| (970) 288-4561
>|--------------------------------------------------------------------|
>
>> Hello, all,
>>
>> As silicon designer point of view, RX input level is very sensitive
>> issue. If it is too low, we have to install pre-amp, DC offset
>> chancellor, etc. and they consume more power and silicon area.
>>
>>
>> Let's try simplified consideration;
>> ..01010101.. signal somewhere PRBS consists of 5GHz sine wave
>> (and small 3rd harmonics...). Pre-emphasis should be set most strong
>> level, the other word it should be full swing 1Vppd typ at TX side.
>>
>> In a case of equation 69.2 per draft 0.91, -25.3dB at 5GHz,
>> ..01010101.. signal of 1Vppd TX makes 54mVppd or 27mVpp single-ended.
>>
>> However
>> 1. TX minimum amplitude is spec'ed 0.8Vppd, so any RX should receive
>> the signal from the TX (effective -2dB).
>> 2. Return loss max is 5dB at equation 72-2 per draft 0.91. The
>> mismatch makes 1~2dB signal level loss at each of TX-channel and
>> channel-RX. Let's assume total -3dB min.
>> 3. Frequency ripple may be allowed -5dB min at 5GHz in the discussion
>> on Jun 1.
>> Assumption of above 3 effects are -10dB min !!
>> Then the really worst case of eye height at RX input will be 18mVppd
>> or 9mVpp single-ended by ..01010101.. signal !!
>> It doesn't include crosstalks, noise and jitter.
>>
>> Should RX designers really consider such small level ??
>>
>>
>> Another values;
>> Figure 72A-2 in draft 0.91 shows -28.86dB "typical" ISI loss at
>> 5.156GHz. When we use the number, RX input will be 28.8mVppd or
>> 14.4mVpp single-ended from 1Vppd TX.
>> If we add above 3 effects, it will be 9.6mVppd or 4.8mVpp single-
>> ended.
>> If we consider maximum ISI loss rather than "typical",
>> Ooooom...
>>
>> Anyway these kind of small values of RX input may be smaller than
>> peaks of noise and crosstalk.
>>
>>
>> M. Sugawara
>> NEC Electronics America