RE: PAM-5, what are your BERs ?
Not to throw water on an oil fire, by why does the transceiver have to be
CMOS? If my memory serves me correctly, most of the SERDES devices used in
GE were BiCMOS or GaAs until recently.
At 01:15 PM 2/29/00 -0600, Vivek Telang wrote:
>Good points. Let me see if I can respond to them.
>You're right about the fact that equalization has its limits. These limits
>are well understood, and given the channel and noise characteristics, one
>can easily determine the number of levels that can be supported at a desired
>BER. My point was that this limit is far greater than the one that the open
>eye requirement would lead you to believe.
>The high-pass filter equalizer approach that you describe is not as optimal
>as a minimum-mean-squared-error (mmse) equalizer. It is important to realize
>that the narrow pulse does not simply get attenuated, but is dispersed over
>time. A good mmse equalizer processes numerous samples of the dispersed
>pulse and reconstructs the symbol while trying to minimize the noise energy.
>DFEs are particularly good at doing this.
>Your points about the linear amplifier (bandwidth, non-linearity) are
>correct; however, they are not an issue if the equalization is done digitally.
>I can hear you saying, well, if DSP can do all that, what's the catch? The
>catch is that to do any DSP at all, you need an A/D converter up front. This
>A/D needs to run at the symbol rate at least. Also, the A/D adds
>quantization noise that needs to be low enough so as not to be the
>performance-limiting factor. For Oscar's 10G DSP proposal, this means a
>5Gsps A/D with 6 effective bits of resolution. No mean task in CMOS. Also,
>all the DSP needs to run at 5GHz.
>So in summary, I would reiterate my two comments:
>1) A closed eye does not necessarily preclude low BERs.
>2) DSP techniques which would be required to operate under "closed eye"
>conditions are going to be a challenge to implement at 10G bit rates.
>* Vivek Telang
>* Cicada Semiconductor Inc.
>* 901 MoPac Expressway South
>* Building One, Suite 540
>* Austin, Texas 78746
>* 512-327-3500 x114 voice
>* 512-327-3550 fax
>From: NetWorthTK@xxxxxxx [SMTP:NetWorthTK@xxxxxxx]
>Sent: Tuesday, February 29, 2000 12:15 AM
>To: vivek@xxxxxxxxxxxxxxx; stds-802-3-hssg@xxxxxxxx
>Subject: Re: PAM-5, what are your BERs ?
>>From theoretical point of view, you reasoning makes some points. However,
>from the real implementation point of view, it is not quite true. Before
>starting analyzing the frequency response, just ask a question: "If we can
>simply keep equalizing the receiving signals to bring them back to the
>looking-alike to the original, transmitting signal, why we bother all those
>bandwidth issues? There must be some limitations to the equalization
>The eye closure is caused by the insufficient bandwidth of a receiving path;
>as a result, the narrow pulse (higher frequency pulse) is much more
>attenuated than the wider pulse (lower frequency pulse). We can cascade a
>high-pass frequency response equalizer to suppers the amplitude of a wide
>pulse, and keep the amplitude of the narrow pulse remain unchanged (but not
>amplified) to open the eye. However, if the amplitude of a narrow pulse is
>already too small to meet the minimum S/N requirement, the equalizer is
>useless. Theoretically, a linear amplifier can be added to bring the signal
>amplitude up to meet the minimum S/N requirement. The linear amplifier will
>need a BW larger than the transmitting signal rise time. Furthermore, any
>deficiencies in the linearity, will add both timing and amplitude distortion
>to the received data. The additional distortion is not included in the
>jitter specification; as a result, the link will cause higher BER.
>Especially in a high data rate link, a linear amplifier may cause more errors
>than the expected benefit. In practice, it is impractical to add a linear
>The right way is to keep eye open at the receiver input.
>NetWorth Technologies, Inc.