[SI-LIST] Re: USB and SATA Common-Mode Impedance - Q on importance
- From: David Instone <dave.instone@xxxxxxxxxx>
- To: Charles.Grasso@xxxxxxxxxxxx
- Date: Tue, 24 Oct 2006 10:36:54 +0100
Charles,
SATA spec Table 27 for Tx and Table 28 for Rx specify single ended
impedance for Gen1 and Common mode return loss for Gen2, both are
measured into the units connector. The following is a paste from the
latest rev 2.6 spec
> Serial ATA devices and hosts shall comply with the electrical
> specifications shown in Table 26,
> Table 27, Table 28, Table 29, Table 30, and Table 31. The transmitter
> consists of the driver,
> printed circuit board, and mated connector pair. The receiver consists
> of the receiver IC, printed
> circuit board, and mated connector pair.
> Unless otherwise stated, all specifications include the mated
> connector pair.
In practice, if the connector is close enough to the chip then the
connector and chip parasitics dominate.
You are correct that USB does not specify Zcm for the unit. 7.1.1.3 states
> The D+ and D- circuit board traces which run between a transceiver and
> its associated
> connector should also have a nominal differential impedance of 90 Î
7.1.6.2 (see first few para on page 144) allows almost unlimited
variations in impedance
> In the Exception Window (a sliding 1.4 ns window inside the Through
> Impedance time window), the differential
> impedance may exceed the Through limits. No single excursion, however,
> may exceed the Through limits for
> more than twice the TDR rise time (400 ps).
Again if your traces are short enough the connector and chip parasitics
will dominate.
That said, if I have the real estate (in 3D) I still try to match the
board Zcm to the cable Zcm 'cos "It's the right thing to do".
Dave Instone
Oxford Semiconductor Ltd
25 Milton Park
Abingdon
Oxon ox14 4ea
UK
www.oxsemi.com
+44 (0)1235 824963
Grasso, Charles wrote:
> Hi All,
>
>
> In reviewing the USB and SATA specs I noticed that the Zcm (Common-mode
> impedance) is defined for the respective cable assemblies. However the
> Zcm id NOT spec'd
> for the circuit board construction, which leads to the following
> questions that I
> hope some folks can help me with:
>
>
>
> 1. How important is the Zcm for USB and SATA?
> 2. Should the Zcm for the pwb be matched to the cables?
> 3. HDMI does not spec Zcm for cable or pwb. Is this an oversight?
>
>
>
>
>
> (With due deference to EricB - but I have to use the terminology from
> the standards!)
>
>
>
>
>
> Best Regards
> Charles Grasso
> Compliance Engineer
> Echostar Communications Corp.
> Tel: 303-706-5467
> Fax: 303-799-6222
> Cell: 303-204-2974
> Pager/Short Message: 3032042974@xxxxxxxxx
> <BLOCKED::mailto:3032042974@xxxxxxxxx>
> Email: charles.grasso@xxxxxxxxxxxx
> <BLOCKED::mailto:charles.grasso@xxxxxxxxxxxx>
>
>
>
>
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