Circuit for suppressing noise influences in the evaluation of signal states on transmission lines

A circuit arrangement is provided for suppressing periodic or non-periodic noise voltage components which are superimposed as an undesirable influence on the DC voltage signal to be evaluated on DC fed, two-conductor transmission lines, particularly for extension circuits in telephone systems which are fed in the subscriber circuit of the exchange location via symmetrical branches and evaluated with respect to the subscriber's loop state changes. By coupling to suitable circuit points of the subscriber circuit, the superimposed noise signal is separated by a differential amplifier and fed to a second differential amplifier. The original loop signals, superimposed with noise, is also fed to an input of the second differential amplifier so that a noise-free useful signal indicating the operational state arises at the output of the second differential amplifier.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a circuit arrangement for suppressing 
periodic or non-periodic noise voltage components superimposed by means of 
undesired influence on a DC signal to be evaluated as a line status signal 
in DC fed, two-lead transmission lines, particularly for the extension 
circuits in telephone systems fed via symmetrical branches in the 
subscriber circuit of the switching exchange and to be evaluated with 
respect to the changes in the status of the subscriber's loop. 
2. Description of the Prior Art 
For purposes of monitoring, it is necessary to determine the operating 
state of transmission lines. This is particularly true of extension 
circuits which carry a maximally allowable discharge current in the idle 
state and a minimum working current in the working state. The respective 
DC current value can be determined in the supply circuit assigned to each 
subscriber extension, since changes of potential which may be evaluated 
occur in this supply circuit given changes of operational status. 
Noise signals can be superimposed on the useful signals to be evaluated, 
which noise signals, for example, arise due to inductive or capacitive 
coupling. These periodic or nonperiodic noise voltage components can 
influence the evaluation device in such a manner that erroneous evaluation 
results occur. Additional measures must be provided in order to obtain 
sufficient security against erroneous response. 
By employing screening or, respectively, filter arrangements, in addition 
to a relatively high space requirement, there also occurs the disadvantage 
that such components can only be designed for a specific frequency or, 
respectively, for a specific frequency range. Furthermore, a complete 
elimination of the superimposed noise voltage components is not possible 
with such components. 
SUMMARY OF THE INVENTION 
The object of the present invention is to eliminate the superimposed 
disturbances mentioned above with a simple, frequency-independent 
structure and to derive a noise-free DC useful signal of proper polarity 
for the purpose of further evaluation. 
The above object is achieved in an arrangement of the type generally 
mentioned above, according to the present invention, in that, for the 
rejection of the noise components, one input of a differential amplifier, 
preferably formed of an integrated operational amplifier, is coupled to 
the junction of two resistors forming a high-resistance shunt arm between 
the supply points of the subscriber circuit and the other input is coupled 
to one pole of the feed source which is uninfluenced by line status. In 
addition, a suitable input DC voltage amplitude, identical in terms of 
value, is formed for the DC voltage values which are tapped at these 
coupling points, the input DC voltage amplitude being formed by means of a 
fixed divider arrangement connected to each input. Also, the output signal 
of the differential amplifier, exclusively representing the rejected noise 
component, is supplied to the one input of a further, identical 
differential amplifier, whose other input is coupled to a circuit point 
respectively directly carrying the original DC voltage signal superimposed 
with a noise voltage, so tht the noise-free useful signal characterizing 
the line signal state can be tapped at its output. 
One then proceeds from a principle which is well known in another context, 
namely to undertake a separation of the noise components and, for purposes 
of composition, to directly place these in relationship to the information 
signal which still contains such noise components. Since the AC resistance 
of the feed source, for example the office battery, is negligibly small, a 
reference potential occurs which is free of the noise voltage components 
coupled in via the line. The other reference values superimposed with the 
noise voltage is tapped from the junction of the two resistors forming the 
high-resistance shunt arm which respect to the supply points. Such great 
resistance values can be selected for these resistors that a disruptive 
influencing of the overall circuit does not occur. Given identical 
resistance values, a DC voltage arises at the junction and corresponds in 
terms of value to half the supply voltage and exhibits the same noise 
superposition as the voltage, for example, which can be directly tapped at 
the feed resistance and corresponds in terms of DC voltage to the useful 
signal. 
Independently of the status of the line, therefore, connection points for 
the separation of the noise components are selected in a differential 
amplifier, which connection points exhibit a rigidly predetermined DC 
voltage level. By so doing, they can be brought to a conforming DC voltage 
value in a simple manner for the drive of the differential amplifier. 
According to a further development of the invention, the coupling of the 
one input of the further differential amplifier supplying the adjacent 
useful signal in an undisturbed form as its output signal occurs to the 
circuit point feeding the original DC voltage signal superimposed with a 
noise voltage upon interconnection of an operational amplifier operated as 
a voltage follower. Accordingly, identical component units are employed in 
the total evaluation circuit as active units. 
According to a further development of the invention, the device coupled in 
the manner described to the circuit points can be employed for the 
evaluation of lines status of a plurality of lines. In this case, the 
coupling of one and the same device to the respective circuit points is 
then undertaken via a selection switching element which has the 
appropriate plurality of steps. By doing so, there occurs a total 
reduction of expense in use in the subscriber extensions for the 
suppression of the noise components, as is necessary in the evaluation of 
line status and undertaken in the device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawing, a subscriber location Tln can be coupled via an 
extension circuit Tltg via a subscriber connection circuit (not referenced 
in greater detail) to a switching installation of a switching location 
VST. Among other things, the subscriber connection circuit serves to 
transmit the voice signals in both directions and to form a terminating 
impedance for the line. In this exemplary embodiment, the line termination 
is formed by means of two windings w1 and w2 of a line transformer Ue. The 
ends of the two transformer windings which are not connected with 
respective conductors a or b are capacitively interconnected by a 
capacitor C. At the same time, these ends form the supply points for the 
DC feed of the subscriber location extending from the subscriber 
connection circuit. This DC supply occurs by way of the two feed resistors 
R1 and R2 which are connected to respective poles of a supply source, for 
example the exchange battery UB. In addition, the call feed is undertaken 
via the subscriber connection circuit. To this end, the source supplying 
the voice frequency signaling current is effectively connected by means of 
closing a switch s1 and the simultaneous opening of a switch s2 which 
bridges a pair of resistors R18 and R19 in the idle state, these resistors 
being connected in series with the resistor R2 and the transformer winding 
w2. By means of the transformer Ue, among other things, the two-wire 
system of the extension circuit is converted into a four-wire 
through-connection so that the transformer Ue functions as a hybrid 
coupler. 
For monitoring purposes, the state of the extension circuit should be 
determinable in the subscriber connection circuit which, among other 
things, can execute the functions indicated. This occurs by means of the 
evaluation of the potential relationships at a measuring point MP. 
Therefore, the voltage drop at the feed resistor R1 is evaluated. The DC 
voltage arising, for example, upon loop termination at the feed resistor 
R1 is superimposed with noise components which can be formed by means of 
inductive and capacitive couplings. Such a coupling of noise components on 
the two leads a and b of the extension circuit can, for example, be caused 
by means of a power line. It is necessary to suppress the influence of 
such noise components on the device, for example, the device A, which 
evaluates the operating status of the extension circuit so that a 
sufficient security against erroneous response is provided. The noise 
voltage components which are superimposed on the DC voltage at the 
measuring point MP occur in the same manner at a junction T1 of the two 
high-resistance shunt resistors R3 and R4. Accordingly, they coincide with 
the noise components at the measuring point MP with regard to magnitude, 
phase and frequency. If the same value of resistance is selected for the 
resistors R3 and R4, then the DC voltage arising at the junction T1 
corresponds in terms of value to half the voltage value of the supply 
source, which is represented, for example, by the exchange battery UB. The 
negative pole of the battery provides a reference point at which the DC 
voltage can be tapped disturbance-free, since the battery is to be viewed 
practically as a shorted circuit for the noise components which have been 
coupled onto the lines. Accordingly, there is the possibility of 
separating the noise components occurring at the junction T1, given a 
constant DC voltage component. This occurs with the aid of an operational 
amplifier OP1 which is connected as a differential amplifier by means of 
appropriate reverse feedback. The operational amplifier OP1 is wired in a 
known manner with a plurality of resistors R11-R14 for the formation of 
the differential amplifier. A pair of inputs E11 and E12 are formed via a 
pair of input resistors R11 and R13 and a driven via respective voltage 
dividers with the noise voltage-free DC voltage tapped from the one pole 
of the battery or, respectively, with the DC voltage superimposed with the 
noise voltage, by way of respective voltage dividers formed of a pair of 
resistors R7 and R8 and, respectively, a pair of resistors R9 and R10. The 
voltage dividers are selected in such a manner that the DC voltages 
arising at the divider point T3 or, respectively, the divider point T4 for 
the two inputs of the differential amplifier are identical in terms of 
value and are suitable for driving the differential amplifier. Given the 
precondition of input signals reduced by means of the two dividers and 
brought to the same DC voltage level, only the noise component arises at 
the output of the differential amplifier, the noise components being 
contained in the signal tapped at the junction T1. This exclusive noise 
component, separated in this manner, is supplied to an input E22 of a 
further differential amplifier. This differential amplifier is formed by 
an operational amplifier OP2 and a plurality of resistors R15-R17 
connected thereto in a well-known manner. The voltage tapped at the 
measuring point MP, via the voltage divider formed of the resistors R5 and 
R6, is supplied to another input E21 of this further differential 
amplifier via an operational amplifier OP3 which is connected as a voltage 
follower. Upon consideration of the respective divider ratios, therefore, 
a signal exclusively proportional to the noise component is supplied to 
the input E22 and a signal proportional to the useful signal consisting of 
the DC voltage and the superimposed noise component is supplied to the 
other input E21. At the output of the operational amplifier OP2 there thus 
arises a noise-free signal which is proportional to the DC voltage useful 
signal arising at the measuring point MP. Therefore, the operating status 
of the respective extension circuit defined by means of specific potential 
states at the measuring point MP can be registered by means of an 
evaluation unit A which is connected to the output of the operational 
amplifier OP2, i.e. with the noise-free signal arising at the output of 
the differential amplifier. 
If the coupling to the respective measuring point MP is undertaken via an 
appropriately driven selection switching element AS1 and AS2, then a 
multitude of extension circuits can be polled or monitored as to their 
operational state with a single component unit serving in the manner 
described for the suppression of noise components. Thereby, the proper 
allocation for the evaluation device A to the individual extension 
circuits must be assured on the basis of a respectively given address Ad. 
An integrated selector, for example, can be employed as the selection 
switching element AS1, AS2. 
Although we have described our invention by reference to particular 
illustrative embodiments thereof, many changes and modifications of the 
invention may become apparent to those skilled in the art without 
departing from the spirit and scope of the invention. We therefore intend 
to include within the patent warranted hereon all such changes and 
modifications as may reasonably and properly be included within the scope 
of our contribution to the art.