Cermetek Microelectronics, Inc.
CH1788 Surface Mount PLCC 2400bps Modem
2006 Cermetek Microelectronics, Inc.
Page 13
Document No. 607-0018 Revision A6 (01/06)
Figure 3.
PSTN Line Interface for all CH1788 Products.
1. Current Limiting PSTN Protection Line
Device.
Current 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 be
included. Refer to Cermetek Application Note #
126, Supplemental PSTN Line Protection, for
more details. Refer to Cermetek Application Note
# 130, Summary of Recommended Suppliers, for
a list of suppliers and manufactures part numbers.
A resettable/repairable fuse (rated at 0.15 amps)
is preferred because it resets automatically upon
removal of the current flow. Non-resettable
devices are also acceptable. 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.
2. PSTN Current Limiting Resistors.
Low value (10
Ω
) 1/2 watt resistors may be
required (identified as R1 and R2 in dashed Box
2), depending on end product application
environment, to enhance performance by reducing
susceptibility to PSTN current surges that may
occur before the OVERVOLTAGE PROTECTION
device (identified as D1 in dashed Box 3)
becomes fully activated.
Refer to Cermetek Application Note # 130,
Summary of Recommended Supplies, for a list of
suppliers.
3. Over Voltage and Lightning Protection.
To maintain conveyed FCC Part 68 approval for all
CH1788 Products, the required surge protection
must be supplied externally. It is required that a
Sidactor or equivalent fast acting device be used.
DO NOT USE AN MOV OR A TRANSORB.
The Sidactor or equivalent should be nominally
rated at between 270-300 Volts. In most
environments, 2 terminal surge suppressors are
adequate. For severe environments, use an
external 3 terminal device with an earth ground as
is shown.
4. EMI/RFI
Suppression.
EMI/RFI suppression is recommended. This is for
obtaining FCC part 15 A and/or B. It is also used
to suppress external EMI / RFI signals that can
affect modem operation, such as strong radio
signals, should the modem be near a source of
such signals. No external EMI/RFI noise
suppression circuitry is required to maintain
conveyed FCC Part 68 approval. Additional
suppression may be added as described below
without adversely affecting FCC Part 68 approval.
A. To provide adequate EMI/RFI suppression,
the capacitor/inductor network contained in
dashed Box 4 should be located as close to
the RJ-11C 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
0.005
µ
f. They must have a rating of 1.5KV
and typically are 0.001
µ
f ± 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 and the nature of the
EMI/ RFI. Consult an FCC part 15 consultant
if required.
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