[SI-LIST] Re: Frequency components of high speed serial channels
- From: "Orin Laney" <olaney@xxxxxxxxx>
- To: <mrose@xxxxxxxxxxxxxxxxx>, <orphanou@xxxxxxxxxxxx>, <Peter.Pupalaikis@xxxxxxxxxxxxxxxxxx>, <joel@xxxxxxxxxx>
- Date: Fri, 28 Feb 2014 15:08:18 -0800
The highest fundamental frequency you can generate with an NRZ signal
(output of your standard logic gate) amounts to transmitting alternating
ones and zeros. This frequency equals half the clock rate. Then, you need
at least the third harmonic of the 101010... square wave at the far end to
approximate the squareness. This peaks at 3/2 of the clock rate, so the
first spectral two lobes contain most of the signal energy. Adding more
lobes increases squareness, which is to say rise and fall times get shorter,
but this amounts to rapidly diminishing returns beyond the point where rise
and fall times are already satisfactory. So, the bandwidth of your signal
as generated is often rather wider than is actually required at the
receiving end.
The practical limit on bandwidth occurs when increasing it admits more
additional noise (thermal, crosstalk, EMI, et al) than additional signal.
Coding methods far more efficient than NRZ are used for noisy channels.
The spectrum has nulls at integer multiples of the clock rate, so placing
your filter cutoff at a null minimizes waveform distortion from roll-off
effects. Bessel filters are preferred for time domain waveforms, not that I
haven't seen designers use Butterworth because it's the only kind they
understand and know how to design. Then they wonder why the output rings.
Orin
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Rose, Michael
Sent: Friday, February 28, 2014 1:43 PM
To: orphanou@xxxxxxxxxxxx; Peter.Pupalaikis@xxxxxxxxxxxxxxxxxx;
joel@xxxxxxxxxx
Cc: SI-List
Subject: [SI-LIST] Re: Frequency components of high speed serial channels
Silly question...
If the RX serdes rolls off noise (may have pole at ~0.7 Nyquist), doesn't
that limit the bandwidth of interest on the channel?
Mike
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Antonis Orphanou
Sent: Thursday, February 27, 2014 1:08 PM
To: Peter.Pupalaikis@xxxxxxxxxxxxxxxxxx; joel@xxxxxxxxxx
Cc: SI-List
Subject: [SI-LIST] Re: Frequency components of high speed serial channels
[cid:image002.jpg@01CF33A3.D5410E10]
Just a visual of the spectral density for a 10Gbps waveform RZ; In theory
you can set a limit of the total power of the signal you want to include and
integrate the spectrum waveform (as shown above) to get the upper frequency
limit. As other members have indicated in their responses, 3~4x the Nyquist
at the rise times we all use should be a good starting point....if your
specs are super tight, use more bandwidth to resolve your waveform...
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Peter.Pupalaikis@xxxxxxxxxxxxxxxxxx
Sent: Wednesday, February 26, 2014 8:25 PM
To: joel@xxxxxxxxxx
Cc: SI-List
Subject: [SI-LIST] Re: Frequency components of high speed serial channels
Hi Joel:
My rule of thumb:
If the rise time is 30% of the UI or slower, look out to 2x the bitrate,
otherwise 3x.
I would not use fractions as the fundamental and harmonics you are referring
to are for a clock. The data signal forms "lobes" with nulls at multiples of
the bitrate.
Pete
Sent from my iPhone
>On Feb 26, 2014, at 6:45 PM, "Joel Brown"
<joel@xxxxxxxxxx<mailto:joel@xxxxxxxxxx>> wrote:
>
>When designing high speed serial channels such as PCIe Gen2 5 GBPS and SATA
>III 6 GBPS what frequency components are of interest? Do you only consider
>the fundamental (2.5 and 3.0 GHZ) or does the third harmonic also need to
>be present for good signal integrity? Most the parts I look at have have a
>few db loss at the fundamental but significant loss at the third harmonic.
>Thanks - Joel
>
>
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