ETSI Conformance Tests on Digital Interfaces

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1 ETSI Conformance Tests on Digital Interfaces Application Note 57 Test Solutions with the PSM-139 Selective Level Test Set Wandel & Goltermann Communications Test Solutions

2 Contents ETSI Conformance Tests on Terminal Equipment Interfaces of Digital Lines Convenient measurements of physical characteristics of digital interfaces Page 4 Measuring the return loss of digital input and output ports Page 4 Measuring common-mode rejection (LCL and OSB) on digital input and output ports Page 5 Measuring impedance towards ground on input and output ports Page 6 Measuring immunity against commonmode interference Page 7 Test equipment recommendation Page 7 Imprint Author: Peter Ziemann, Dept. VM Wandel & Goltermann GmbH Elektronische Meûtechnik MuÈ hleweg 5 D Eningen u.a. Germany Subject to change without notice Order no: E 12.97/D1/57 Printed in Germany 2

ETSI Conformance Tests on Digital Interfaces Application Note 57 The growing volume of data traffic from commercial and private users represents an ongoing challenge for network operators.

3 ETSI Conformance Tests on Digital Interfaces Application Note 57 The growing volume of data traffic from commercial and private users represents an ongoing challenge for network operators. Besides public providers, utility companies, railroad operators and even communities are now offering network facilities, resulting in increased competition in the communications market. To ensure the basic conditions necessary for trouble-free data communications, appropriate standards have been defined by national and international bodies. Apart from the definitions of the protocols used, the electrical specifications of the interfaces are also very important. Among other things, the European Telecommunications Standardization Institute ETSI has produced standards (see table 1) covering important measurements on the interfaces used in ISDN, business telecommunications and the open network provision (ONP). Standard Contents ETS ISDN (PRA) user network interface ETS TM 2048 kbit/s interface ETS ONP 2048 kbit/s, network interface ETS ONP 2048 kbit/s, TE interface ETS ONP 64 kbit/s, network interface ETS ONP 64 kbit/s, TE interface FTZ 1 TR805 Interface tests to G.703, G.704 ITU-T I.431 ISDN (PRA) user network interface Table 1 The interface parameters are verified using defined analog test signals which simulate real operating conditions. Real in this sense means that the interface levels correspond to the intended operating levels. Table 2 shows the measurement parameters and the corresponding test voltages. At first glance, the specified voltages do not seem particularly high. However, taking the low impedances and unavoidable insertion losses of measuring bridges into account, power levels of more than +20 dbm may result. The test equipment used must also be able to transmit and receive using various source and sink impedance values. Test Impedance towards ground Tolerable longitudinal voltage Return loss Table 2 Test voltage 2 V rms 2 V rms 1 V peak /3 V peak The PSM-139 Selective Level Measuring Set combines with the PSV-39 Level Amplifier (20 db) and various WG bridges to form a standardscompliant test solution. The LevelPRO control and evaluation software includes predefined instrument setups and tolerance masks for convenient testing. Over the following pages, the main analog measurement techniques for conformance tests are described (see table 3). IRL ORL LCL OSB ITG TLV Input Return Loss Output Return Loss Longitudinal Conversion Loss Output Signal Balance Impedance Towards Ground Tolerable Longitudinal Voltage ETS ETS ETS ETS ETS ETS FTZ 1TR805 ITU-T I.431 Table 3 1

Convenient test procedures The LevelPRO control and evaluation software enables extremely efficient tests and measurements. All of the necessary instrument setups and tolerance masks are predefined.

4 Convenient test procedures The LevelPRO control and evaluation software enables extremely efficient tests and measurements. All of the necessary instrument setups and tolerance masks are predefined. Points 1 to 3 illustrate a typical test procedure Once the test instruments are connected, select the desired measurement type. This activates suitable parameter settings and ETSI limit masks. 2 Click on Measure to start the test procedure. If necessary, the user is guided through the normalization routine. The software then performs a limits comparison and saves the test result. 3 Results can be printed in graphical or tabular format. Measuring the return loss of digital input and output ports When transmitting electrical signals, the electrical interfaces must be suitably matched. For low-reflection (and thus low-loss) transmission of signals, the input and output impedances must agree. Likewise for the characteristic impedance of the lines. As a general rule, instead of measuring or specifying an absolute impedance value, the return loss is used, which is a measure of the deviation from the system impedance. In all of the standards listed above and the test setups considered hereafter, the system impedance is always 120 Ω. IRL Input Return Loss ORL Output Return Loss IRL ETS ETS ETS FTZ 1TR 805 Stimulus 3 V p The same test setup is used for measurements on the interface input (IRL) and the interface output (ORL). However, when measuring the output return loss, a pseudorandom bit sequence (PRBS ) should be fed to the interface to simulate real operating conditions. Due to the high level of the stimulus signal from the test bridge and the narrow bandwidth of the PSM-139, the results are never undesirably influenced by the pseudorandom bit sequence. IRL ETS ETS Stimulus 1 V p FSTART= 51 khz (10 khz) FSTOP= 3072 khz (384 khz) BANDW= 100 Hz SWEEP TIME= 10 s (3 s) PSM-139 TX= -7.0 db (-16.6 db) PSV-39 OUT= +13 db (+3.4 db) STIMULUS= 3 V p (1 V p) ( ) = Values as per ETS Parameters 2 Test setup ORL ETS FTZ 1TR 805 Stimulus 3 V p

5 LCL Longitudinal Conversion Loss OSB Output Signal Balance Measuring common-mode suppression on digital input and output ports One significant benefit of balanced communications systems is that interference equally affects both wires of a wire pair. Since the useful signal is derived from the voltage difference between the wires (transverse voltage), interference does not degrade the data. However, in practice the balance is less than ideal. By measuring the common-mode suppression (LCL, OSB), we can tell whether inputs and outputs meet the stated requirements. LCL measurement In ETSI standards ETS and ETS , the LCL measurement technique described in ITU-T O.9 is used on inputs and outputs. A generator feeds a sinusoidal longitudinal voltage to a balance measuring bridge, and a selective receiver measures the resulting transverse voltage. The LCL is obtained from the logarithmic ratio of the two voltage values. If we are using levels in db, then the result is even easier to obtain with p L-p T. With the LevelPRO software, the result is displayed over the entire frequency range following a single measurement run. To simulate real operating conditions, the interface ports must be activated during the measurement. Test method as per ITU-T O.9 and O.121 LCL ETS ETS OSB ETS ITU-T I.431 FSTART= 10 khz FSTOP= 256 khz BANDW= 100 Hz SWEEP TIME= 3 s PSM-139 TX= 0 db TX connected with SDZ-30 Jack1 RX connected with SDZ-30 Jack2 Input/ output Z= 75 Ω Parameters for LCL FSTART= MHz FSTOP= MHz FSTEP= MHz BANDW= 3.1 khz PSM-139 TX= OFF RX connected with SDZ-30 Jack2 Input impedance Z= 75 Ω Parameters for OSB Test setup OSB measurement In the standards ETS and ITU-T I.431, only the balance of output ports is determined. The OSB measurement technique also described in ITU-T O.9 is used. The OSB technique differs from the LCL technique described above in that no external generator is required. The digital output signal of the interface is used as the test signal. This signal is fed to the test bridge and the OSB is determined based on the resulting longitudinal voltage. Since the interface cannot supply a sinusoidal test signal, a digital signal is preferred with a discrete frequency spectrum that is as constant as possible. With HDB3-coded signals, this is achieved by sending an allones signal (AIS, Alarm Indication Signal). The spectrum exhibits maxima at 1024 khz + n x 2048 khz. Using LevelPRO in FREQUENCY STEPPING mode, we can measure at exactly these frequencies. To determine the OSB, the level difference p T-p L must be determined. The difference trace display function (Trace A-B) is useful for this purpose. It is possible to switch between the two levels (longitudinal and transverse voltage) using a button on the SDZ-30 bridge. 3

Measuring impedance towards ground on input and output ports ITG Impedance Towards Ground Although the useful signal on a balanced wire pair is not referred to ground potential, only limited

6 Measuring impedance towards ground on input and output ports ITG Impedance Towards Ground Although the useful signal on a balanced wire pair is not referred to ground potential, only limited separation can be achieved between the two signal lines and ground due to parasitic capacitances. Without ground coupling, the floating inputs and outputs would be subject to uncontrollably high commonmode voltages due to interference. Accordingly, the coupling must be realized with as little frequency dependency as possible in the interface. The impedance towards ground should not be too low, however, since we do not want the useful signal to produce high leakage currents towards ground. The ITG-30 test bridge is used to determine the impedance towards ground. This bridge was specially developed for this application and yields very precise results in the important impedance range around 1kΩ. FSTART= FSTOP= BANDW= SWEEP TIME= PSM-139 TX= PSV-39 OUT= Input/ output Stimulus= Parameters 50 Hz 1 MHz 25 Hz 10 s db +8.2 db Z= 75 Ω 2 V rms Test setup In conjunction with the PSV-39 Level Amplifier, the PSM-139 Selective Level Measuring Set generates a sinusoidal voltage of 2 V rms referred to ground which is fed to the balanced test object. The test bridge determines the leakage current and uses a test impedance to generate a voltage proportional to the impedance towards ground. This voltage is measured, converted by the software into the corresponding impedance value and displayed as an impedance curve vs. frequency. ITG ETS ETS ETS ETS ETS FTZ 1TR 805 ITU-T I.431 As was the case with the return loss and balance measurements, normalization of the test setup is very straightforward. With the IMB-30 Impedance Bridge, a 1 kω reference impedance is actually built-in. Normalization data are stored by the software, meaning you do not have to renormalize as long as the test parameters stay the same. The control software monitors these conditions. The menu-driven program assists you during the normalization process and is configured to handle the main bridges available from Wandel & Goltermann. Limits 4

TLV Tolerable Longitudinal Voltage Immunity to common-mode interference In an ideal balanced system, coherent interference cannot negatively affect the original signal.

Accordingly, it is important to check the tolerable longitudinal voltage or immunity to common-mode interference when characterizing interface input ports.

7 TLV Tolerable Longitudinal Voltage Immunity to common-mode interference In an ideal balanced system, coherent interference cannot negatively affect the original signal. However, such interference does produce longitudinal voltages towards ground that can cause bit errors on digital inputs. Accordingly, it is important to check the tolerable longitudinal voltage or immunity to common-mode interference when characterizing interface input ports. Parameters : FSTART FSTOP BANDW SWEEP TIME PSM-139 TX PSV-39 OUT Input/output Longitudinal voltage = 10 khz = 30 MHz = 3.1 khz = 100 s = ±11.8 db = +8.2 db Z = 50 W = 2 V rms (+/±1 %) BER-Tester Test object Test equipment e. g. WG PA-20, WG PF-30 T-Balancing Network Test loop Test setup for common-mode interference measurements V rms Longitudinal voltage Frequency TLV ETS ETS ETS ETS FTZ 1TR 805 ITU-T I.431 PSM-139 and PSV-39 generate a test signal, which is coupled into the balanced line as a longitudinal voltage using a T-Balancing network. The PSM-139 displays the common-mode level for verification purposes. Proper matching is required to meet the demanding requirements for frequency response (Z = 50 W). Accordingly, a coaxial cable with a characteristic impedance of 50 W must be used. While the test signal is swept over the frequency range, a BER test is performed. No bit errors should occur over one minute. The standard test method used here is CRC-4 (cyclic redundancy check) from ITU-T G.704. Test equipment : PSM-139 Level Measuring Set (32 MHz) BN 2203/17 PSV-39 Level Amplifier BN 2249/01 LevelPRO (Control and Evaluation Software) BN 2203/93.01 PA-20 PCM Performance Analyzer BN 4525/50 or PF-30 Bit Error Analyzer BN 4526/50 SDZ-30 Signal Balance Ratio Bridge BN 2234/01 Reference Element LCL = 0 BN 2234/01.01 RFZ-30 Return Loss Bridge BN 2234/10 ITG-30 Impedance Bridge BN 2234/15 TBN-30 T-Balancing Network BN 2234/25 7

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