Telephonic communications – Transmission line conditioning – Interference suppression
Reexamination Certificate
2000-07-14
2004-06-29
Barnie, Rexford (Department: 2643)
Telephonic communications
Transmission line conditioning
Interference suppression
C379S414000, C333S012000
Reexamination Certificate
active
06757386
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, generally, to circuits for reducing electromagnetic interference (EMI) in telecommunications equipment, and, in preferred embodiments, to apparatus, systems, and processes for efficiently reducing EMI that is conducted along telecommunications lines using Y capacitors.
2. Description of Related Art
“Noise,” defined herein as any type of undesirable signal that interferes with communication, is a problem increasingly important to telephony and other communication systems. In telephone lines, for example, the increased use of modems and facsimile machines to transmit data makes the reduction of noise an important aspect of telecommunication design.
One source of noise is electromagnetic interference (EMI). EMI is the undesired coupling of electromagnetic energy from an electromagnetic energy source to an affected circuit, system, or structure. EMI may be coupled from the source to the affected circuit, system, or structure through conduction along wires, by far field radiation from a transmitting antenna, through capacitive (electric field) coupling, and through inductive (magnetic field) coupling.
Telephones and other personal communications devices are often used in an environment that includes one or more nearby amplitude modulation (AM) broadcast radio stations or other types of wireless broadcasting systems having high power radio frequency transmitters and associated antenna systems, and are therefore subject to exposure to electromagnetic energy sufficient to interfere with or completely disable the operation of such a device. Interference to communications devices caused by such broadcasting systems results mainly from radiated radio frequency (RF) signals that are coupled into transmission lines and are then conducted as RF currents into communications devices connected to the affected transmission lines.
Because most present day communications devices employ active circuits (amplifiers, automatic gain control circuits, and the like) which provide certain advantages over older “passive” designs such as standalone telephones, they are more susceptible to interference caused by unwanted RF currents. This is due to the nature of the active circuitry, which has the undesired capability of readily demodulating audio from amplitude modulated RF carriers.
For example, AM interference with the desired operation of a telephone instrument occurs when the telephone's electronic circuits demodulate the audio signal component of amplitude modulated RF currents, amplify this unwanted audio signal component, and couple it to the telephone's receiver, thereby making it extremely difficult or effectively impossible to understand intended received speech. If the RF signal strength is high enough, the internal circuitry of the telephone instrument will be disabled, making it impossible to place a call.
This EMI problem is illustrated in
FIG. 1
, which shows a circuit equivalent of high powered, commercial, radio broadcast antenna transmitting RF signals into a two wire, metallic, telephone transmission line located in the vicinity of the antenna transmitter site. The unwanted AM broadcast signal is schematically represented by a noise source
10
having an associated noise source impedance
12
. The AM signal is injected through a pair of resistors
14
and
16
representing a balanced two-wire transmission line
18
consisting of tip and ring lines
20
and
22
in a telephone set
24
. The hardware of telephone set
24
typically includes a printed circuit board
26
, housed within an insulating (plastic) housing or case
28
. Mounted on printed circuit board
26
are one or more active electronic circuits, which are capacitively coupled to earth ground
30
by a naturally occurring mutual capacitance
32
. The undesired RF currents are conducted over the two wire (tip and ring) transmission line
20
in common mode fashion. The strength of the RF noise signal, which follows the dotted line path
34
(from the source
10
, through tip and ring transmission line
20
, the amplifier circuitry of printed circuit board
26
, and through mutual capacitance
34
to earth ground
30
) is often high enough to cause interference to telephone instruments located around the perimeter of an AM radio broadcast facility.
Regulatory safety administrations require compliance with telephone equipment specifications by requiring galvanic insulation between the tip and ring telephone lines and the board or system. This insulation can be realized by transformers, capacitors, optocouplers, or the like. Although various shielding/grounding schemes, such as those described in the U.S. Patents to Pesola et al., U.S. Pat. No. 5,271,056 and Bogese, U.S. Pat. No. 4,738,638, address the EMI problem in general, neither patent describes the above-referenced problem of unwanted demodulation by the telephone's electronic circuitry of AM broadcast signals as undesired common mode RF currents on the two wire (tip and ring) conductors. The Pesola et al. patent describes the use of a ground foil with a raised edge of frame plate for components of a radio telephone. The Bogese patent describes the replacement of one of the conductors of a telephone type modular jack with a ground strap having a wide surface for conducting high frequency EMI signals to ground, or a metallic connector cover provided for the purpose.
U.S. Pat. No. 5,642,416 to Hill et al. discloses an electromagnetic interference bypass filtering mechanism that suppresses RF noise currents conducted over the tip and ring leads of a telephone line-powered telephone instrument that may be produced by AM radio broadcast signals emanating in the vicinity of the telephone instrument. The filtering mechanism comprises a conductive material coated on the interior surface of the housing of the telephone instrument, so as to surround the printed circuit board containing the telephone circuitry of the instrument, coupled to earth ground. First and second capacitors are coupled between the tip and ring leads and the conductive material, and first and second inductors are coupled in series with the tip and ring conductors and connections of the tip and ring conductors to the printed circuit board containing the telephone circuitry. Each of the first and second capacitors has a value that is larger than the value of mutual capacitance between conductive traces on printed circuit board and the conductive coating on the interior of the telephone's case. The effective impedance to earth ground seen by common mode RF noise current signals on the tip and ring leads is therefore lower than that encountered in a path through the circuitry on the printed circuit board. Although this effectively increases the common mode current injected into the telephone, the lower impedance of the by-pass path through the first and second capacitors steers the common mode RF current around the printed circuit board, rather than through its active circuitry.
However, when selecting filtering components such as the capacitors in the Hill et al. patent, safety requirements must be satisfied. In Nordic countries, for example, if capacitors are used, the capacitors must be certified to the IEC 384-14-1 standard. These certified capacitors must also be designed to meet certain safety and insulation requirements as defined by safety standard IEC 60950. For example, certified capacitors are designed such that in case of failure or fault they will blow open rather than into a shorted state. Example certification bodies that certify capacitors include TUV, SEMKO, NEMKO, DEMKO or FEMKO. The IEC standard uses different Y designations, such as Y
1
, Y
2
, Y
3
and Y
4
, each representing a different voltage rating. Capacitors certified to the IEC standard are called “Y” capacitors.
Because these Y capacitors are certified, they must pass through more quality control steps, and thus they are generally more expensive. Another drawback to Y capacitors is that they are larger in size than regular high volt
D'Angelo Wilfrid C.
Latu Jean F.
Pater Daniel Jean
Barnie Rexford
Farjami & Farjami LLP
PC-TEL, Inc.
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