TTMER3: Baseband data transmission on copper cable

Measurement tasks

Mandatory measurement tasks:

Parts written in italic are the recommendations by the leader of the measurement.

1. Examine the time function, spectrum and eye pattern of the PRBS signal which modell well the transmittable signals!
   • Use the pseudo-random signal generator model to the generation of signals.
   • For examining the time function it is worth to display a part of the signal in the "SINGLE" mode of the oscilloscope.
   • Use the FFT module of the oscilloscope to the spectrum analysis! Scanning of the frequency axis should be 2-3 times bigger, than the clock signal!
   • For the examination of the eye pattern we should filter the signal with the help of the low pass filter with a changeable boundary frequency! Low pass filter will modell the channel here. Take care of the termination of the filter. Filter was sized to resistances of 600 ohms. Examine the eye pattern of the A PRBS signal at:
     • f_a >> f₀ "uneconomical"
     • f_a > f₀ "less uneconomical"
     • f_a = f₀ "economical"
     • f_a = 0.8 x f₀ "distorted"
     • f_a = 0.5 x f₀ "small bandwidth"
     transmission, where f_a - is the cut-off frequency of the filter, f₀ - is the frequency of the clock signal.

     When you prepare the eye pattern, you should make the drawing onto each other at the AUTO STORE operation mode of the oscilloscope! Before this switch off the interpolation between the measurement results (VECTORS) under the DISPLAY button! If you won?t do this, we will get a blurred figure after stopping the recording (STOP button), because the program of the oscilloscope will take the results drawn onto each other to results obtained after each other.

   Examine the time function, spectrum and eye pattern of the pszeudoterner (duobinar, bipolar) signal coded to the line transmission!

   • Instructions of the previous point are valid here, too.
   • The examination of the eye pattern is enough to make in an ?economic? operation mode!

   • Describe the similarity and difference between signals at the time- and frequency domain!
   • Which signal would you use for transmission of information in the case of a channel of intercom type and with small upper boundary frequency?

2. After the model examinations let us examine a real tool. Examine the time function, spectrum and eye pattern of the line signal of a HDSL modem!
   • Insert between the two modems instead of the examinable copper pair a 20 dBs attenuator to separate the signals of the two directions! Don?t use a measuring head to the connection of the oscilloscope, it is useless now (there is no remote feeding).
     We do not have any triggers for the examination of the eye pattern. It is worth to start the oscilloscope from the peak of the received signal for the preparation of an acceptable figure.

3. Measure the amplitude characteristics of the copper cable at your disposal with the help of the measuring transmitter and measuring receiver!
   • Take care of the termination of the cable! Cable was designed for ~ 120 ohms of wave impedance!
   • Execute the measurement in a synchronized operation mode in the whole frequency domain of the measuring transmitter, with min. 10 frequency value.
   • Represent the measuring result graphically! With what kind of function can the measuring result be approached?
   • Till how large bit rate can a cable be used for digital signal transmission, if the attenuation can maximally be 6dBs at the half of the bit frequency? (It is required according to Recommendation ITU-T G.703.)
     Control this result with an eye pattern testing!
     Derive the clock signal of the pseudo-random generator from the measuring transmitter!

4. Determine the length of the copper cable at your disposal with TDR measurement!

Facultative measuring tasks:

1. Verify, that higher signal transmission rate can be obtained on the cable that in the previous measuring task! Assemble a HDSL modem connection with the usage of the tested copper pair. Check the proper operation of this connection with bit error rate measurement.

   Error rate should be measured in the operation mode of Siemens K4305 UNFRAMED.