PC Board Layout Considerations

Chapter 1 - Introduction and Description

Safety: All creepages and clearances for the MT3334SMI have been designed to meet requirements of safety standards EN60950 and IEC950. The requirements are based on a working voltage of 250V. When the recommended DAA circuit interface is implemented in a third party design all creepage and clearance requirements must be strictly adhered to. The third party safety design must be evaluated by the appropriate national agency per the required specification.

User accessible areas: Based on where the third party design is to be marketed/sold or used, it may be necessary to provide an insulating cover over all TNV exposed areas. Consult with the recognized safety agency to determine the requirements.

Notice: Even if the recommended design considerations are followed, there are no guarantees that a particular system will comply with all the necessary regulatory requirements. It is imperative that specific designs be completely evaluated by a qualified/recognized agency.

Recommended Transceiver

Manufacturer – Analog Devices

Part # - ADM207EAR

This section discusses the FCC Part 68 and Industry Canada CS-03 Telecom compliance of the Multi-Tech Systems, Inc. Model MT5634SM/SMI socketed modem module.

This module was tested by the NVLAP accredited KTL Dallas Inc. laboratory and conforms to the above said standards. The modem module was tested in the MultiTech MTASR3-200, a 3-modem asynchronous router that uses three socket modems for dial-up telephone access. The developer’s guide (included on diskette) with the MT5634SM/SMI modem includes design specifications listing recommendations for the RJ-11 phone jack and fuse which are needed for FCC Part 68 compliance.

The MTASR3-200 tested has sockets for three modem modules each having traces of varying lengths out to the RJ-11 connectors. Trace lengths ranged from 1.97 to 3.15 inches. The trace widths were all 12 mil. Since three ports with varying trace lengths were tested, we concluded that as long as the customer’s printed circuit board characteristics are reasonably close to these parameters there should not be any issues relating to telecom compliance. In addition to a recommended external fuse, the modem module also has a Sidactor on board.

Multi-Tech believes that, provided the same components are used and parts placement is similar, Telecom compliance will be maintained for the system and no additional testing is required.

Calculating Z for Tip and Ring Traces

Trace Impedance - Side-by-Side Traces

120
Zo = Er	1n

p S

w + t

Where: Er = Relative permittivity of the PCB dielectric material. Typically 4 - 4.7

s= Spacing between traces - 12 mils (.012)

w= Trace width - 12 mils (.012)

t= Trace thickness - (.0022)

Zo		120
Zo	=	2.16	1n
Zo	=	54 W

.03769

.0142

Note: The trace inductance for the tested design varied from 61nH to 105nH.

MT3334SMI