Table 18: Impedance requirements (Sheet 2 of 2)
Requirement | Units | 1.5 Gbps | 3.0 Gbps |
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Common mode impedanceb,e | ohm | 20 min/40 max | 20 min/40 max |
Transmitter source termination |
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Differential impedanceb | ohm | 60 min/115 max | 60 min/115 max |
Differential impedance imbalanceb,g | ohm | 5 | 5 |
Common mode impedanceb | ohm | 15 min/40 max | 15 min/40 max |
aAll times indicated for time domain reflectometer measurements are recorded times. Recorded times are twice the transit time of the time domain reflectometer signal.
bAll measurements are made through mated connector pairs.
cThe media impedance measurement identifies the impedance mismatches present in the media when terminated in its characteristic impedance. This measurement excludes mated connectors at both ends of the media, when present, but includes any intermediate connectors or splices. The mated connectors measurement applies only to the mated connector pair at each end, as applicable.
dWhere the media has an electrical length of > 4 ns the procedure detailed in
eThe receiver termination impedance specification applies to all receivers in a TxRx connection and covers all time points between the connector nearest the receiver, the receiver, and the transmission line terminator. This measurement shall be made from that connector.
fAt the time point corresponding to the connection of the receiver to the transmission line the input capacitance of the
receiver and its connection to the transmission line may cause the measured impedance to fall below the minimum impedances specified in this table. The area of the impedance dip (amplitude as ρ, the reflection coefficient, and duration in time) caused by this capacitance is the receiver termination time constant. The receiver time constant
shall not be greater than the values shown in this table. An approximate value for the receiver termination time constant is given by the product of the amplitude of the dip (as ρ) and its width (in ps) measured at the half amplitude point. The amplitude is defined as being the difference in the reflection coefficient between the reflection coefficient at the nominal impedance and the reflection coefficient at the minimum impedance point. The value of the receiver excess input capacitance is given by the following equation:
receiver termination time constant C =
(R0 RR)
where (R0 RR) is the parallel combination of the transmission line characteristic impedance and termination resistance at the receiver.
gThe difference in measured impedance to ground on the plus and minus terminals on the interconnect, transmitter or receiver, with a differential test signal applied to those terminals.
9.5.2.4Electrical TxRx connections
TxRx connections may be divided into TxRx connection segments. In a single TxRx connection individual TxRx connection segments may be formed from differing media and materials, including traces on printed wir- ing boards and optical fibers. This subclause applies only to TxRx connection segments that are formed from electrically conductive media.
Each electrical TxRx connection segment shall comply with the impedance requirements of table 18 for the media from which they are formed. An equalizer network, if present, shall be part of the TxRx connection.
TxRx connections that are composed entirely of electrically conducting media shall be applied only to homoge- nous ground applications (e.g., between devices within an enclosure or rack, or between enclosures intercon- nected by a common ground return or ground plane).
9.5.2.4.1Transmitter characteristics
The drive are D.C. coupled.
62 | Cheetah NS SAS Product Manual, Rev. B |