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CH1786ET

CH1786 Family of Ultra Small 2400bps Modems

Unspecified 

Cermetek Microelectronics, Inc.

CH1786 Family of Ultra Small 2400bps Modems

Figure 7.

PSTN Line Interface for all CH1786 Products.

1. Currently Limiting PSTN Protection Line

Device.

Currently limiting devices are mandatory to meet

UL safety standards. To maintain conveyed FCC

Part 68 approval, the current limiting components

identified as F1 and F2 in dashed Box #1 must

also survive FCC Part 68 surge testing. Refer to

Cermetek Application Note #126, Supplemental

PSTN Line Protection, for more details. Refer to

Application Note # 130, Summary of Recommend

Suppliers, for a list of suppliers and associated

part numbers.

A. A Polyfuse (rated at 0.15 amps) is preferred

because it resets automatically upon removal

of the current flow. Non-resettable devices

are also acceptable. Refer to Application Note

#130 for a complete list of recommended

vendors and associated part numbers.

B. Resistors (10Ω carbon film or 1/8 watt

minimum) may be used in Canada, as Canada

has no requirements that PSTN equipment be

operational after a Type B surge test.

C. Although CSA CS-03 Part 1 (Canada) follows

the requirements of FCC Part 68 (USA),

Cermetek recommends contacting DOT

(Canada) and/or a certified independent lab to

verify compliance. For Canada, use either

10Ω resistors (carbon film or SMD parts 1/8

watt minimum) as described in paragraph B

above.

2. Over Voltage and Lightning Protection.

A. Surge Protection is provided by internal

circuitry contained within all CH1786 Products

except the CH1786NH (see Figure 7). No

additional external components are required to

maintain conveyed FCC Part 68 approval for

all CH1786 Products (excluding the

CH1786NH).

B. For CH1786NH, the required surge protection

(identified as D1 in dashed Box 2) must be

supplied externally. The purchaser must

include some form of surge protection as

described herein to maintain conveyed FCC

Part 68 approval.

C. In most environments, 2 terminal surge

suppressors are adequate. For severe

environments, use an external 3 terminal

device with an earth ground.

3. EMI/RFI Suppression.

No external EMI/RFI noise suppression circuitry is

required to maintain conveyed FCC Part 68

approval. However, additional suppression, if

required for other reasons, may be added as

described below in Sections 3A-3B without

adversely affecting FCC Part 68 approval.

A. To provide adequate EMI/RFI suppression,

the capacitor/inductor network contained in

dashed Box #3 should be located as close to

the RJ11 Jack as possible. Further, this

network should be provided with an excellent

ground path to the chassis.

B. Capacitors C1 and C2 should not exceed

0.005µf. They must have a rating of 1.5KV

and typically are 0.001µf ± 20%. Inductors L1

and L2 may be either individual inductors or a

dual inductor. Refer to Application Note #130

for a complete list of recommended vendors

and associated part numbers. For UL

applications, choose capacitors and inductors

that are UL 1950 listed. The actual values of

the components used may vary depending on

the end product design.

2003 Cermetek Microelectronics, Inc.

Page 13

Document No. 607-0004 Revision L1 (06/03)

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