Subscriber circuit

Details Subscriber circuit Subscriber circuit

1998-04-28

2001-10-30

Tran, Sinh (Department: 2743)

Telephonic communications

Subscriber line or transmission line interface

C379S395010

Reexamination Certificate

active

06310953

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to subscriber circuits and, more particularly, to a subscriber circuit which can be used both as a subscriber circuit for an analog system and a subscriber circuit for a digital system.
2. Description of the Related Art
In a digital telephone system, a basic-rate subscriber circuit transmitting a digital signal (according to the 2B1Q system or the ping-pong system) is used. In an analog telephone system, a subscriber circuit for transmitting a sound signal is used. Hereinafter, a subscriber circuit for a digital subscriber system will be referred to as a digital subscriber circuit, and a subscriber circuit for an analog subscriber system will be referred to as an analog subscriber circuit,
In the past, an analog subscriber circuit and a digital subscriber circuit are implemented by different circuits since, the digital system carries two times as much information as the analog system and is also different from the analog system in a signal symbol and a signal voltage used in the system.
FIG. 1
is a table comparing the analog subscriber circuit, the digital subscriber circuit for the 2B1Q system and the digital subscriber circuit for the ping-pong system, with respect to a signal symbol, a signal speed, a signal amplitude and termination.
For example, the signal speed of the analog system is 300-3400 Hz, the signal speed of the 2B1Q system is 160kbps and the signal speed of the ping-pong system is 320 kbps. The signal amplitude of the analog system is 1.55V
OP
(V
OP
indicates a voltage between 0 and a peak value). The signal amplitude of the 2B1Q system is 2.5V
OP
and the signal amplitude of the ping-pong system is 6V
OP
. The termination resistance of the analog system is 600 &OHgr;, the termination resistance of the 2B1Q system is 135 &OHgr; and the termination resistance of the ping-pong system is 110 &OHgr;.
Accordingly, when a subscriber wants a change from an analog system to a digital system (2B1Q or ping-pong), it is necessary to manually substitute a new printed circuit board (PCB) for an existing one.
For example, if a system is installed in a location away from an exchange, a maintenance person must be called in to exchange PCBs. Thus, it is time-consuming and uneconomical to shift from one type of system to another.
Accordingly, it is demanded that both the analog facilities and the digital facilities are implemented by the same PCB.
FIG. 2
shows a construction of an analog subscriber circuit according to the related art.
Referring to
FIG. 2
, a codec
1
performs conversion of a sound signal. The codec
1
is connected between a subscriber circuit and a highway (HW). A data input terminal (D-IN) and a data output terminal (D-OUT) are connected to the highway.
A digital sound signal is converted into an analog signal by the codec
1
and output from an analog output terminal (A-OUT). A received signal is fed to an analog input terminal (A-IN). A feedback circuit
2
is connected between the analog input terminal A-IN and the analog output terminal A-OUT. The feedback circuit
2
is formed by an internal termination resistance
2
a.
Amplifiers
3
and
4
receive an analog output of the codec
1
so as to amplify the same (the amplifier
4
is an inversion amplifier). A bias voltage supply unit
5
receives outputs of the amplifiers
3
and
4
so as to provide a bias voltage. The bias voltage supply unit
5
is formed by a bias circuit
5
a
. A power supply voltage V
BB
(−48V) is supplied to the bias circuit
5
a.
A signal output unit
6
receives an output of the bias voltage supply unit
5
and outputs a sound signal. The signal output unit
6
consists of amplifiers AP
1
and AP
2
constituting a buffer amplifier. Operation amplifiers are used to construct these amplifiers AP
1
and AP
2
. A phase compensation circuit
7
performs phase compensation for these amplifiers and is implemented by a capacitor C
1
.
An output of the amplifier AP
1
is connected to line A via an external termination resistance R, and an output of the amplifier AP
2
is connected to line B via an external termination resistance R. A telephone set (not shown) is connected between line A and line B. A signal reception unit
8
receives the sound signal from line A and line B and is formed by a differential amplifier AP
3
. The differential amplifier AP
3
is implemented by an operational amplifier, and the output of the signal reception unit AP
3
is connected to the analog input terminal A-IN of the codec
1
described above.
A description will be given of an operation of the circuit constructed above.
The analog subscriber circuit of
FIG. 2
superimposes an analog signal of 300 Hz-3400 Hz and a maximum level of 3.17 dBmO (zero-relative level) to an bias voltage generally having a level of −48 V, as a differential signal, and outputs the superimposed signal to the subscriber lines (line A, line B). The zero-relative level indicates a level with respect to a reference level of 0.
For example, in the case of an impedance line of 600 &OHgr;, a voltage level for supplying lmW to the impedance line of OdBmO=600 &OHgr; is 0.775 (V).
A signal for transmission is output from the analog output terminal (A-OUT) of the codec
1
, amplified by the amplifiers
3
and
4
, and superimposed on the bias voltage by the bias voltage supply unit
5
. Each output of the bias circuit
5
a
is fed to line A and line B of the subscriber as a differential analog signal from the signal output amplifiers AP
1
and AP
2
.
The received signal is input to the differential amplifier AP
3
of the signal reception unit
8
, connected to the internal termination resistance
2
a
, and input to the analog input terminal (A-IN) of the codec
1
. The received signal is fed back via the feedback circuit
2
to the signal output unit, so that termination of the two lines is effected.
In this way, in an analog subscriber circuit, a feedback system involving many amplifiers is produced so that the termination is effected accordingly. The phase compensation circuit
7
is provided for each of the operational amplifiers AP
1
and AP
2
so as to prevent oscillation and provide proper phase compensation.
FIG. 3
shows a construction of an operational amplifier. Because feedback is employed in the analog subscriber circuit, the phase compensation circuit
7
is provided so as to prevent oscillation.
FIG. 4
is a graph showing a gain-frequency characteristic of the operational amplifier used in a subscriber circuit. The gain (dB) is vertically plotted and the frequency (Hz) is horizontally plotted. The graph of
FIG. 4
is called a Bode diagram.
Curve f
1
indicates an open loop characteristic of the amplifier, wherein phase compensation is not performed. The characteristic as indicated by curve f
1
is used in the ping-pong system. Referring to curve f
1
, stages are produced at 6 dB/oct and 12 dB/oct. Inclination at 12 dB/oct; indicates that the output phase is 180° displaced with respect to the input so that a negative feedback ends; up as a positive feedback. If the feedback circuit remains unmodified, oscillation results.
Curve f
2
indicates a frequency characteristic, wherein phase compensation is performed. The characteristic as indicated by curve f
2
is used in the 2B1Q system. Curve f
3
indicates a frequency characteristic, wherein sufficient phase compensation is performed so that oscillation is not caused in the circuit even when feedback is performed. The characteristic as indicated by curve f
3
is used in the analog system. Inclination of curve f
3
is 6 dB/oct.
Referring to
FIG. 2
, the external termination resistance R coupled to the subscriber line is provided as a protection resistance of, for example, 50 &OHgr;. For example, when the termination of 600 &OHgr; is to be implemented, the feedback circuit provides the remaining 500 &OHgr; (=600−50×2).
FIG. 5
shows a construction of a conventional subscriber circuit for the 2B1Q system. In
FIGS. 2 and 5
, like numerals desig

Affiliated with

Also associated with

Rating

Subscriber circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Subscriber circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Subscriber circuit will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2578560