[SI-LIST] Re: 1 Gbps on 3.5 mil etch a hazard?
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 22 Oct 2010 16:25:16 -0700
Andrew,
Isn't that problem resolved by using "sufficient" pull-down resistors
at the source end of the link? I've always tried to have 3mA or so
going through these resistors at the VoL condition and have never
noticed any significant driver non-linearity. (On the other hand,
colleagues who blindly used 330 ohms or some such were often
perplexed by output waveforms that did not have full swing and which
exhibited a very slow high-to-low transition time.)
My simulation technology for this exercise was an Excel spreadsheet
which took into account VoL, VoH, any back-termination resistors, the
termination resistor(s) and Vtt (if any.)
Regards,
Paul Levin
Consultant
_______________
On 10/22/2010 1:32 PM, Andrew Ingraham wrote:
>> I like to hear about the non linearity etc because I need ammunition to get
>> my employer to spring for the full simulator.
> I can't tell you exactly how it would affect your simulation results...
>
> But consider this. ECL outputs tend to have a very low output
> impedance (around 7 ohms) only as long as the emitter follower remains
> on. But if one were to pull the output pin up by only a few tens of
> millivolts, the transistor would turn off and that 7 ohms goes to
> infinity ... well, very large. It won't stay like that continuously,
> except for a few special ECL parts that were designed to 'drive' below
> Vtt, thus allowing their outputs to go open-circuit when low. But it
> could happen on reflections. And then a simplified linear model
> doesn't show what happens at all.
>
> Why can't you use the ECL driver's SPICE model? Is it because it
> would exceed the capabilities of PSpice's demo version? There are
> other free SPICE programs out there (including LTspice) that are not
> crippled. I'm not suggesting they will solve your simulation problems
> thus making commercial software unnecessary; but LTspice (for example)
> is far more capable than the PSpice demo.
>
> But I think you need to pay to get a simulator that understands IBIS.
>
> Regards,
> Andy
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