Re: [SI-LIST] : High Speed Backplane Connector Recommendations

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From: Bruce W. Marler (BMarler@WooshCom.com)
Date: Mon Jan 17 2000 - 07:45:56 PST


Raymond,
Do you know the subject wording on the thread on this forum where "D.C.
complained
about the output signal edge jitter caused by the wide common mode variation
at the differential inputs."?

I would like to go back and read that thread.
Bruce

----- Original Message -----
From: <Raymond.Leung@qsa.idt.com>
To: <si-list@silab.eng.sun.com>
Sent: Sunday, January 16, 2000 5:21 PM
Subject: Re: [SI-LIST] : High Speed Backplane Connector Recommendations

> While I have no comment about the coplannar waveguide as
> well as the orginal connector discussions, I would like to say
> something about the LVDS receiver. As mentioned before, the
> CMRR can never be ideal in high freq. D.C. has already complained
> in this forum about the output signal edge jitter caused by the wide
> common mode variation at the differential inputs, and I have a
> similar headache. Supressing the high freq. ac common mode
> is definitely a merit for reliable data recovery.
>
> Regards,
> Raymond
>
>
> ---------------------- Forwarded by Raymond Leung/QSA/AU on 17/01/2000
10:03
> ---------------------------
>
>
> "Bruce W. Marler" <BMarler@WooshCom.com> on 16/01/2000 03:47:35
>
> Please respond to si-list@silab.eng.sun.com
>
> To: si-list@silab.eng.sun.com
> cc: (bcc: Raymond Leung/QSA/AU)
>
> Subject: Re: [SI-LIST] : High Speed Backplane Connector Recommendations
>
>
>
> Ron,
>
> Yes, I understand that there is no necessity but I am wondering what
advantages
> there might be?
> Bruce
> ----- Original Message -----
> From: Ron Miller
> To: si-list@silab.eng.sun.com
> Sent: Friday, January 14, 2000 5:40 PM
> Subject: Re: [SI-LIST] : High Speed Backplane Connector Recommendations
>
>
> Hi Bruce
> I agree that there is no necessity to go to coplanar. I was just
answering
> his question
> about the possibility of doing so.
>
> Ron
>
> "Bruce W. Marler" wrote:
>
> Ron, This seems reasonable, especially given the high common mode
range of
> LVDS. I think of the high common mode tolerance of LVDS and the common
chassis
> ground as getting the signal in-the-ball-park so that the receiver input
stage
> is biased correctly, then the differential natura of the receiver can do
its
> thing by discriminating the differential signal. However, I think we
always need
> to remember that the CMRR of any device decreases extremely quickly at
high
> frequency and so we need to make sure that high frequency junk is kept off
the
> common mode as much as possible. What then would be the advantage of a
coplanar
> waveguide? Bruce
> ----- Original Message -----
> From:Ron Miller
> To: si-list@silab.eng.sun.com
> Sent: Friday, January 14, 2000 11:34 AM
> Subject: Re: [SI-LIST] : High Speed Backplane Connector
Recommendations
> Steve
> I disagreee with your conclusion. See below.
>
> sweir wrote:
>
> Bruce,
> No, both the EMI and the SI will be worse. To understand this,
think
> about
> how each of the LVDS drivers develops its signal:
>
> The LVDS "high" line sources from a current source in the driver,
while
> the
> "low" line sinks to the driver ground. The driver current source
has a
> limited common mode range, as does the sink. At the far end we
place a
> 100
> ohm resistor between the lines. So, ok current flow in one
direction or
> another creates a relative voltage difference across the resistor.
BUT,
> the common mode voltage is just the average voltage of the two.
Our
> receiver must be able to:
>
> 1) Survive the common mode voltage, and
>
> Yes, but anyone with an ounce of sense will still use the same
supply
> voltages
> so there should be no problem
>
>
> 2) Register the intended results
> For a demonstration of 1) you can destroy a perfectly good PC and
> printer
> by lifting the grounds on both and plugging each into extension
cords at
> opposite ends of your house, and then connecting the PC to the
> printer. When your refrigerator turns on and off watch as one or
the
> other
> lets all the smoke out.
>
> Again I say this is a fools scenario.
> Incidentally, all equipment uses the green wire for chassis/safety
ground
> and the
> green is connected at the power box, circuit breaker panel to the
low side
> of the power.
> Because of inductances it is possible to get an AC difference up to
a
> couple volts due to
> unequal loads on the AC phases at a wall socket. DC is not
effected.
>
> Within a system as in a PCB as was the question set up, the ground
> voltages are very
> close to equal at the receiver and driver.
>
>
> This is one of the reasons that box to box transports use
transformers
> or
> optics. If you operate within a box, it is generally much more
> economical
> to deal with a common ground.
>
> :Yes these work well to eliminate any offset where long cables are
used.
>
> Conclusion: within a chassis all grounds should be connected
together
> keeping
> any common mode voltages low. Then coplanar waveguide works nicely
> without
> a ground plane.
> Ron Miller
>
>
>
> Regards,
>
> Steve.
>
>
>
>
>
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