Forums

equalizing CAT6A twisted pair

Started by John Larkin October 27, 2011
We've been experimenting with pushing differential drive logic signals
over CAT6A ethernet-type cable. We'd like to move packets of data in
the form of a burst clock on one pair and data on two other pairs.
Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at
around 80 meters. The eye diagrams are marginal as-is, but it looks
like some fairly simple transmit-end pre-emphasis might make things a
lot better.

http://www.panoramio.com/photo/61238884


Has anybody done stuff like this? It's not very amenable to
simulation, because the Spice lossy txline model doesn't include skin
effect.

John

"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message 
news:36uja79vhhfmr28ab67hlle8f3bd7p721l@4ax.com...
> > We've been experimenting with pushing differential drive logic signals > over CAT6A ethernet-type cable. We'd like to move packets of data in > the form of a burst clock on one pair and data on two other pairs. > Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at > around 80 meters. The eye diagrams are marginal as-is, but it looks > like some fairly simple transmit-end pre-emphasis might make things a > lot better. > > http://www.panoramio.com/photo/61238884 > > > Has anybody done stuff like this? It's not very amenable to > simulation, because the Spice lossy txline model doesn't include skin > effect. > > John >
My first approach would be to get the eye more open just by doing simple pre/de-emphasis on the send side. The linear receive equalizers that the FPGA people use in their serdes would be easy to do, too. You'd probably never need to implement a decision feedback type of system at your data rates and cable lengths, but those DFE's really do help at 10Gbps. I'm about to find out how they work at 25Gbps. Bob -- == All google group posts are automatically deleted due to spam ==
On 10/27/2011 5:43 PM, John Larkin wrote:
> > We've been experimenting with pushing differential drive logic signals > over CAT6A ethernet-type cable. We'd like to move packets of data in > the form of a burst clock on one pair and data on two other pairs. > Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at > around 80 meters. The eye diagrams are marginal as-is, but it looks > like some fairly simple transmit-end pre-emphasis might make things a > lot better. > > http://www.panoramio.com/photo/61238884 > > > Has anybody done stuff like this? It's not very amenable to > simulation, because the Spice lossy txline model doesn't include skin > effect. > > John >
In the early 80s, there were plenty of papers on twisted pair equalization. My recollection was they synthesized a response that was the square root of frequency. I think they had models for the cable in the papers as well. Generally you want a matched filter (i.e. both ends), but for a wire channel (i.e. not likely to have additive noise), I suppose putting all the pre-emphasis on the transmit side is fine.
On 28/10/2011 01:43, John Larkin wrote:
> > We've been experimenting with pushing differential drive logic signals > over CAT6A ethernet-type cable. We'd like to move packets of data in > the form of a burst clock on one pair and data on two other pairs. > Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at > around 80 meters. The eye diagrams are marginal as-is, but it looks > like some fairly simple transmit-end pre-emphasis might make things a > lot better. > > http://www.panoramio.com/photo/61238884 > > > Has anybody done stuff like this? It's not very amenable to > simulation, because the Spice lossy txline model doesn't include skin > effect.
Wouldn't Manchester encoding be preferable and more reliable? It will be a bit tedious otherwise having to tweak the pre-emphasis depending on how long a cable is attached the to device. -- Regards, Martin Brown
On 10/27/2011 5:43 PM, John Larkin wrote:
> over CAT6A ethernet-type cable. We'd like to move packets of data in > the form of a burst clock on one pair and data on two other pairs. > Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at > around 80 meters. The eye diagrams are marginal as-is, but it looks > like some fairly simple transmit-end pre-emphasis might make things a > lot better. > > http://www.panoramio.com/photo/61238884 > > > Has anybody done stuff like this? It's not very amenable to
Hi, What about using a gigabit ethernet transceiver with a built in microcontroller? cheers, Jamie
On 28 Okt., 02:43, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> We've been experimenting with pushing differential drive logic signals > over CAT6A ethernet-type cable. We'd like to move packets of data in > the form of a burst clock on one pair and data on two other pairs. > Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at > around 80 meters. The eye diagrams are marginal as-is, but it looks > like some fairly simple transmit-end pre-emphasis might make things a > lot better. > > http://www.panoramio.com/photo/61238884 > > Has anybody done stuff like this? It's not very amenable to > simulation, because the Spice lossy txline model doesn't include skin > effect. > > John
seen this? http://www.analog.com/library/analogDialogue/archives/43-01/cat5_cable_driver.html -Lasse
On Thu, 27 Oct 2011 17:43:47 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

> >We've been experimenting with pushing differential drive logic signals >over CAT6A ethernet-type cable. We'd like to move packets of data in >the form of a burst clock on one pair and data on two other pairs. >Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at >around 80 meters. The eye diagrams are marginal as-is, but it looks >like some fairly simple transmit-end pre-emphasis might make things a >lot better. > >http://www.panoramio.com/photo/61238884 > > >Has anybody done stuff like this? It's not very amenable to >simulation, because the Spice lossy txline model doesn't include skin >effect. > >John
National Semi has some receivers with equalizers. Try DS25BR100, 101, 110.
On Oct 28, 8:24=A0am, Martin Brown <|||newspam...@nezumi.demon.co.uk>
wrote:
> On 28/10/2011 01:43, John Larkin wrote:
> > Has anybody done stuff like this? It's not very amenable to > > simulation, because the Spice lossy txline model doesn't include skin > > effect. > > Wouldn't Manchester encoding be preferable and more reliable? >
Yes, a very long time ago (1974) for transmitting digital television signals over multi-pair telephone wire. It is very important to ensure you do not have long runs of 1s or 0s in the data stream because doing so will make you vulnerable to frequency dependent dispersion. The propagation velocity in the twisted pair is slightly frequency dependent, so if the spectrum of the data is very different to that of the clock the data transitions will become time-shifted relative to the clock. This can then cause data-dependent glitches. Manchester coding is a good way of overcoming this problem. John
On Fri, 28 Oct 2011 08:24:19 +0100, Martin Brown
<|||newspam|||@nezumi.demon.co.uk> wrote:

>On 28/10/2011 01:43, John Larkin wrote: >> >> We've been experimenting with pushing differential drive logic signals >> over CAT6A ethernet-type cable. We'd like to move packets of data in >> the form of a burst clock on one pair and data on two other pairs. >> Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at >> around 80 meters. The eye diagrams are marginal as-is, but it looks >> like some fairly simple transmit-end pre-emphasis might make things a >> lot better. >> >> http://www.panoramio.com/photo/61238884 >> >> >> Has anybody done stuff like this? It's not very amenable to >> simulation, because the Spice lossy txline model doesn't include skin >> effect. > > >Wouldn't Manchester encoding be preferable and more reliable?
Possibly, but the data eye diagram still needs to be open. But the decision to use the burst clock and the two data lanes was made by our customer, and it's not worth a battle to change it. We have other fish to blacken.
> >It will be a bit tedious otherwise having to tweak the pre-emphasis >depending on how long a cable is attached the to device.
I had in mind that some small number of R-C networks could be switched in with low-resistance CMOS switches under software control, with the selection depending on the known cable length... no tweaking. The intersymbol distortion looks to be almost tolerable without equalization, so a direct shot should work, without adaptive equalization or tweaking. If we use three or four RC peaking networks, and maybe allow more than one to be switched on simultaneously, the solution space gets huge. http://www.panoramio.com/photo/61267150 http://www.panoramio.com/photo/61267153 John
On Oct 27, 5:43=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> We've been experimenting with pushing differential drive logic signals > over CAT6A ethernet-type cable. We'd like to move packets of data in > the form of a burst clock on one pair and data on two other pairs. > Speeds range from 100 MHz clock (200 Mbps) at 20 meters to 10 MHz at > around 80 meters. The eye diagrams are marginal as-is, but it looks > like some fairly simple transmit-end pre-emphasis might make things a > lot better. > > http://www.panoramio.com/photo/61238884 > > Has anybody done stuff like this? It's not very amenable to > simulation, because the Spice lossy txline model doesn't include skin > effect. > > John
There is a LaPlace function you can add to simulate skin effect. However, that really slows down your simulation times For expediency a 5 or more step parallel inductor ladder works well and is much faster, resultsusually close enough.