Kishan Jainandunsing
(April 2006)

In this column we explain the COM Express specification in a series of tutorials. Last month we gave a high-level overview of the specification. This month we take a closer look at insertion loss budget specifications for the high-speed serial differential signals when designing carrier boards.

Part IV : Carrier Board Insertion Loss Budgets

Introduction

When designing COM Express carrier boards, extra attention must be given to insertion loss budgets for the high-speed serial differential interfaces, such as PCI Express, Serial ATA, USB 2.0 and Gb Ethernet. The COM Express bases insertion loss budget calculations on commonly used FR4 PCB material and uses a rule-of-thumb signal attenuation value of 0.28dB/inch/GHz. The current release of the spec does not include insertion loss budget specifications for PCI Express Generation 2, SATA Generation 3 and 10 Gb Ethernet.

PCI Express Insertion Loss Budget Specifications

Figure 1 shows the situation for the case of a PCI Express add-on card socket on the carrier board. Table 1 provides the maximum allowable attenuation for each portion of the signal paths in Figure 1.

Figure 1. PCI Express insertion losses for the add-on card case

Segment	Loss (dB)	Notes
Module	4.90	Includes the module's PCB traces (max 5.15 inches), coupling caps and mezzanine connector
Carrier Board	4.40	9 inches @ 0.28db/inch/GHz
Slot	1.25	As defined in "PCI Express Card Elecromechanical Spec Rev 1.1.
Add-on Card	2.65	As defined in "PCI Express Card Elecromechanical Spec Rev 1.1. Excludes coupling caps.
Coupling Caps	1.19	As defined in "PCI Express Card Elecromechanical Spec Rev 1.1.
Total	13.20

Table 1. PCI Express insertion loss budgets for the add-on card case

Figure 2 shows the situation for the case of a PCI Express device on the carrier board. Table 2 provides the maximum allowable attenuation for each portion of the signal paths in Figure 2.

Figure 1. PCI Express insertion losses for the carrier board device case

Segment	Loss (dB)	Notes
Module	4.90	Includes the module's PCB traces (max 5.15 inches), coupling caps and mezzanine connector
Carrier Board	8.30	15.85 inches @ 0.28db/inch/GHz
Coupling Caps	1.19	As defined in "PCI Express Card Elecromechanical Spec Rev 1.1.
Total	13.20

Table 2. PCI Express insertion loss budgets for the carrier board device case

Serial ATA Insertion Loss Budget Specifications

Figure 3 shows the situation for the case of a SATA connector on the carrier board. Table 3 provides the maximum allowable attenuation for each portion of the signal paths in Figure 3 for SATA generation 1, whereas Table 4 provides loss budgets for SATA generation 2.

Figure 3. SATA insertion losses on the carrier board

Segment	Loss (dB)	Notes
Module	1.91	Includes the module's PCB traces (max 3.0 inches), coupling caps and mezzanine connector
Carrier Board	3.07	7.2 inches @ 0.28db/inch/GHz
Cable	6.00	Cable and cable connector allowance
Total	10.98

Table 3. SATA Gen 1 insertion loss budgets for the carrier board

Segment	Loss (dB)	Notes
Module	2.46	Includes the module's PCB traces (max 2.0 inches), coupling caps and mezzanine connector
Carrier Board	2.52	3.0 inches @ 0.28db/inch/GHz
Cable	6.00	Cable and cable connector allowance
Total	10.98

Table 4. SATA Gen 2 insertion loss budgets for the carrier board

USB 2.0 Insertion Loss Budget Specifications

Figure 4 shows the situation for the case of a USB connector on the carrier board. Table 5 provides the maximum allowable attenuation for each portion of the signal paths in Figure 4.

Figure 4. USB 2.0 insertion losses for the carrier board

Segment	Loss (dB)	Notes
Module	0.72	Includes the module's PCB traces (max 6.0 inches) and mezzanine connector
Carrier Board	2.68	14.0 inches @ 0.28db/inch/GHz
Cable	6.00	Cable and cable connector allowance
Total	10.98

Table 5. USB 2.0 insertion loss budgets for the carrier board

Ethernet Insertion Loss Budget Specifications

Figure 5 shows the situation for the case of a 1Gb Ethernet connector on the carrier board. Table 6 provides the maximum allowable attenuation for each portion of the signal paths in Figure 5.

Figure 4. USB 2.0 insertion losses for the carrier board

Segment	Loss (dB)	Notes
Module	0.10	Includes the module's PCB traces (max 3.0 inches) and mezzanine connector
Carrier Board	0.15	5.0 inches @ 0.28db/inch/GHz
Cable	24.00	Cable and cable connector allowance
Total	24.25

Table 5. 1Gb Ethernet insertion loss budgets for the carrier board

End of Part IV