Abstract:
A measuring device for the interface of a transmission link with full-duplex transmission in two-wire common band operation includes a device for simulating the interface that allows the signal in the near-to-far direction on the transmission link to be separated from the return signal. To precisely perform jitter measurements on the interface of the transmission link during ongoing transmission using a conventional jitter measuring instrument, the measuring device has a differentiator followed by a comparator connected between a subtractor and the jitter measuring instrument.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to transmission link testing, and more particularly to a measuring device for the interface of a transmission link with full-duplex transmission in two-wire common band operation. 
     It is known (for example from U.S. Pat. No. 4,989,202) that an ISDN (Integrated Services Digital Network) transmission link has a U interface between the transmission line and a so called network. It is further known (cf. “Siemens-KMT-Report, No. 14, order no. E 80001-V331-W54, pages 6 and 7) to perform measurements on this U interface and to provide a corresponding measuring device for this purpose. This measuring device may be used to perform frequency-dependent measurements, functional tests as well as jitter measurements for example. An ISDN transmission link is a special form of a transmission link with full-duplex transmission in two-wire common band operation. 
     Furthermore, German patent specification DE 44 23 333 C1 discloses a measuring device for the interface of a transmission link with full-duplex transmission in two-wire common band operation. Connected to said interface are a large-scale integrated interface module, via a hybrid circuit and a transformer, on the one hand, and a line termination or a network termination as the test object on the other hand. The measuring device includes means simulating the arrangement of hybrid circuit, transformer and line or network termination, the input of said simulating means being connected to the transmission outputs of the interface module via a high-impedance differential amplifier and the output of said simulating means being connected to an input of a subtractor via a further high-impedance differential amplifier. A further input of the subtractor is connected to said interface via an additional high-impedance differential amplifier; the output of the subtractor communicates with a measuring system. 
     The invention is based on a measuring device of this type and it is the object of the invention to permit jitter measurements to be precisely performed on the interface of a transmission link with full-duplex transmission during the ongoing transmission using a conventional jitter measuring device. 
     BRIEF SUMMARY OF THE INVENTION 
     For accomplishing this object, the invention proposes a measuring device in which a differentiator followed by a comparator is coupled to the subtractor and a jitter measuring device as a measuring system is post-connected to the comparator. 
     The essential advantage of the measuring device according to the invention is that it also permits an unambiguous determination of phase jitter in the case of ISDN data signals with multivalid codes since the differentiator and the comparator post-connected to it only detect relevant data signals; such being characterized in that they feature the full signal level swing predetermined by the respective code whereas all other data signals are ignored during phase jitter measurement. 
     In order to realize a specially simple differentiator which may also be produced at low cost, it is considered advantageous to form said differentiator from a series connection of at least one capacitor, at least one inductor and at least one resistor. 
     The differentiated output of the differentiator may be transmitted to the comparator in a particularly simple and thus advantageous manner if a voltage dropping at the resistor is applied to the comparator. 
     Differentiators which are simple from a point of view of circuit engineering are bandpass filters for example for which reason it is considered advantageous if the differentiator is a bandpass filter. A further advantage of a bandpass filter is that low-frequency interference signals, e.g. caused by the interface, are suppressed by the bandpass filter so as not to affect the jitter measurements. 
     The invention is illustrated in more detail in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a view of an embodiment of the measuring device according to the invention, and 
     FIG. 2 is a detailed view of a part of the measuring device according to the invention as shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 first of all shows a transmission link  1  with an interface  2  which may be a U interface of an ISDN transmission link as a special form of a transmission link with full-duplex transmission in two-wire common band operation. Coupled to said U interface in a known manner is a large-scale integrated interface module  3 . In more detail, said large-scale integrated interface module  3  is connected to a hybrid circuit  5  via two transmission outputs  4 ; said hybrid circuit  5  is connected to said interface  2  via a post-connected transformer  6 , with a test object  7  being in turn connected to said interface  2 . This test object  7  may be a line termination or a network termination. 
     As is further shown in FIG. 1, a simulating means  8  is connected to the transmission outputs  4  of the large-scale integrated interface module  3  via a high-impedance differential amplifier  9 . Connected to the input of this simulating means  8  is a hybrid circuit  10 , the electrical characteristics of which are identical with those of the hybrid circuit  5  of the transmission link  1 . A transformer  11  post-connected to the identical hybrid circuit  10  is likewise identical with the transformer  6  of the transmission link  1 . Connected to the identical transformer  11  is a termination  12  whose electrical characteristics are identical with those of the test object  7 . The hybrid circuit  10 , the transformer  11  and the termination  12  form the simulating means  8 . Connected to the input of the identical termination  12  is a further high-impedance differential amplifier  13  the output of which is connected to an input  14  of a subtractor  15 . 
     A further input  16  of the subtractor  15  is connected to the interface  2  via an additional high-impedance differential amplifier  17 . Connected to an output  18  of the subtractor  15  is a differentiator  19  to which a comparator  25  is post-connected. The output of the comparator  25  in turn is connected to a jitter measuring device  26 . The differentiator  19 , the comparator  25  and the jitter measuring device  26  together form a jitter measuring system  28 . Said jitter measuring system  28 , said subtractor  15 , said simulating means  8  and said differential amplifiers  9 ,  13  and  17  together form a measuring device  29  which is connected to the interface  2  on the one hand and to the transmission outputs  4  of the interface module  3 , on the other hand. 
     In FIG. 1, an arrow marked “U hin ” adjacent the interface  2  denotes the fact that a signal is applied to the interface  2  and thus also to the input of the additional high-impedance differential amplifier  17  which signal is transmitted by the large-scale integrated interface module  3  to the test object  7  via the hybrid circuit  5  and the transformer  6 . Furthermore, the signal U r{umlaut over (u)}ck  is applied to the interface  2 , which signal is emitted by the test object  7 . As a consequence, since the amplifier  17  has an amplification factor of  1 , both signals U hin  and U r{umlaut over (u)}ck  are present at the output of the additional differential amplifier  17  and are detected by the subtractor  15  via input  16 . However, only the signal U hin  output by the transmission outputs  4  of the large-scale integrated interface module  3  is applied to the input  14  of the subtractor  15  via the differential amplifier  9 , the identical hybrid circuit  10 , the identical transformer  11  and the further high-impedance amplifier  13 . Owing to subtraction, therefore, only the signal U r{umlaut over (u)}ck  appears at the output  18  of the subtractor  15 , which signal contains information from and about the test object  7  and can thus be examined in the jitter measuring system  28 , e.g. for phase jitter. As a consequence, the signal U hin  does not interfere with jitter measurement. 
     The signal U r{umlaut over (u)}ck  present at the output  18  of the subtractor  15  is transmitted to the differentiator  19  which may for example comprise a bandpass filter consisting of a capacitor, an inductor and a resistor. The differentiator  19  differentiates the signal U r{umlaut over (u)}ck  present at the input to form a differentiated signal U diff . The differentiated signal U diff  is passed on to the comparator  25 . The comparator  25  generates a voltage U komp  which will always have a value above zero when the voltage of the differentiated signal U diff  exceeds a voltage threshold value determined in the comparator  25 . The voltage U komp  is transmitted to the jitter measuring device  26  which may be of the type described in the document “Telekom-Praxis, Fernmelde-Praxis” (15-16, 15.08.90, ISSN 0015-0118, pages 676-691). The measuring device according to the invention also permits the unambiguous determination of phase jitter in ISDN data signals with multivalid codes since if the comparator  25  has a correspondingly high threshold value only those data signals will be detected which feature the full signal level swing. All other data signals will be ignored during phase jitter measurement. 
     FIG. 2 shows a portion of the arrangement according to the invention as illustrated in FIG.  1 . It is to be noted here that all elements of FIG. 2 which have already been explained with reference to FIG. 1 bear the same reference symbols as in FIG.  1 . The signal U r{umlaut over (u)}ck  is present at the differentiator  19  which is composed of a bandpass filter with a capacitor C, an inductor L and a resistor R. The voltage U diff  dropping at the resistor R is input to the comparator  25  which is formed by an operational amplifier  30  to which a threshold voltage U schwell  is input. At the output of said comparator  25  the voltage U komp  is formed which is transmitted to the jitter measuring device  26 . 
     The differentiated signal U diff  formed at the output of the differentiator  19  as a function of the signal U r{umlaut over (u)}ck  present at the input of the differentiator  19  has the following characteristics: 
     In the case of a signal transition of the signal U r{umlaut over (u)}ck  with a large signal level swing a high signal value is formed for the differentiated signal U diff  whereas in the case of a signal transition of the signal U r{umlaut over (u)}ck  with a small signal level swing only a small signal value is obtained for the differentiated signal U diff . 
     If the signal U r{umlaut over (u)}ck  is constant, the differentiated signal U diff  at the output of the differentiator  19  will have a zero voltage value. 
     As ensues from the above, the magnitude of the signal value of the differentiated signal U diff  is a direct function of the signal level swing of the signal U r{umlaut over (u)}ck  during the signal transition. The higher the signal level swing of the signal U r{umlaut over (u)}ck  is, the higher will be the signal value obtained for the differentiated signal U diff . The comparator  25 , i.e. the operational amplifier  30 , compares the differentiated signal U diff  with a threshold voltage U schwell  applied to the operational amplifier  30 . If the signal value of the differentiated signal U diff  exceeds the threshold voltage U schwell , then a voltage U komp  in excess of zero will be output by the output of the operational amplifier  30 . However, if the signal value of the differentiated signal U diff  is below the value of the threshold voltage U schwell , an output voltage value U komp  equal to zero will be obtained at the output of the comparator  25 . The threshold voltage U schwell  may for example be chosen such that the voltage U komp  will always have an output voltage value above zero when the signal U r{umlaut over (u)}ck  has a signal transition featuring the full signal level swing. The signal U komp  thus only contains the jitter information of the signal U r{umlaut over (u)}ck  with regard to the signal transitions featuring the full signal level swing. Consequently, the jitter measuring device  26  will only evaluate and measure the signal transitions of the signal U r{umlaut over (u)}ck  which feature the full signal level swing. For the sake of completeness, it is noted here that the comparator  25  may also be designed such that signal transitions having other signal level swings than the full one are used for measuring jitter. This is possible e.g. by the use of at least one further operational amplifier to which a further threshold voltage is applied.