[SI-LIST] Re: AC Coupled Signals
- From: "Hassan O. Ali" <hassan@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 24 Sep 2007 18:29:12 -0400
As usual, simple questions receive more responses in this forum. Here is=
mine from my experience (non-proprietary, I must add):
If the cap doesn't introduce any substantial impedance discontinuities t=
henlocation doesn't matter. The cap becomes just like any segment of the l=
ine. If they do cause impedance discontinuities, it's better to be placed c=
lose to the receiver (RX) so that the reflected signals die out along the l=
ine before reaching the transmitter (TX).
Another advantage: close to the RX the incident signal is weak= (of low
amplitude, having been attenuated by the line loss) and hence the =
reflectedsignal will also be weak (being a fraction of a weak incident sig=
nal). Close to the TX, the signal is strong, and hence the reflected signal=
amplitude is higher for the same impedance mismatch conditions. Depending =
upon the type of mismatch, the reflected signal adds or subtracts to/from t=
he incident signal and may potentially aggravate crosstalk effects. Those c=
rosstalk effects would normally be worse if the cap is placed close to the =
TX than close to the RX.
Best regards.
Hassan.
On Mon , steve weir sent:
Edi, I assure you tha= t we have proven that it does matter and there is
considerable gain to be had by doing it optimally. The flip side of the coin
is that if it is done suboptimally results can range from OK to
pretty bad. There are definitely certain arrangements that cause
problems. I would love to go into the details of why this is but that
information is all proprietary.
Regards,
Steve.
Edi Fraiman wrote:
>Hi,
>
>I think that AC cap location is does no= t matter. You should be more
>aware that capacitance body might chang= e the characteristic impedance of
>your transmission line. In this case yo= u should adjust transmission line
>width.
>Actually is best described by Dr. Howar= d Johnson's "Potholes"
analogy.=3D20
>
>
>Edi
>
>
>---Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx [si-list-bounce@xxxxxxxxxxxxx[1]]
>On Behalf Of npatel@xxxxxxxxxx[2] >Sent: Monday, September 24, 2007 12:51 =
PM
>To: si-list@xxxxxxxxxxxxx[3]
>Subject: [SI-LIST] AC Coupled Signals
>
>Hi all,
>In case of AC coupled signals does anyo= ne know of an optimum placement
>for the caps? I mean should they be pla= ced near the source, receiver,
>middle of the transmission line?
>How much difference does it make in the= opening of the eye?
>The signals are differential CML runnin= g at 3.0Gbps
>
>Thanks,
>Nikhil=3D20
>
>
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