52
QPHY-ENET Operator’s Manual Rev G
1000Base-T Transmitter Distortion - Mode 4 with Disturbing Signal
The distortion test measures the error in the signal under test relative to an ideal waveform generated by a
mathematical model of the PAM-5 coded signal. TF-ENET-B section D, which contains the disturbing signal, is
used for this test. The disturbing signal is set to a frequency of 20.833 MHz (125 MHz / 6) at a peak-to-peak
amplitude of 2.7 V at the input to the DUT. This is set using the Fixture Calibration procedure. The test requires
that the absolute maximum peak error be less than 10 mV.
The fixture used for this test allows the DUT to be subjected to the 2.7 V disturbing sine wave, but allows very
little of the disturbing sine wave to reach the oscilloscope. This allows the distortion measurement to be made on
an 8 bit oscilloscope. Due to the presence of the remnant of the disturbing sine wave, plus any noise, averaging is
required to be able to read <10 mV distortion. Our implementation averages Ethernet symbols in the vertical
domain only so that low-frequency wandering of the clock between acquisitions does not degrade the distortion
measurements. It takes a substantial number of sweeps for the “distortion” result to fall to below 10 mV and
remain stable; expect to average at least 100 sweeps. Clear sweeps clears this average. For automated test
support, the software sets a variable, which is available via automation, indicating when it believes the result
value, should be read. The variable is:
App.Ethernet.Dstr.GBM4ProducedGoodResult
This variable becomes set when the result is <8 mV after the first sweep, or is <9 mV after more than 30 sweeps
(this is the usual good case), or after 150 sweeps.
WARNING: The fixture requires the disturbing sine wave. Each side (0 degrees and 180 degrees) must
come from a 50 Ω source. If the disturbing sine wave is not present the BNC inputs must be terminated
with 50 Ω. Otherwise, reflections of the DUT’s signal from the un-terminated connections increase
distortion.
1. Calibrate data path (see the
X
Signal Path Calibration Procedure
X
topic on page
X
74
X
).
2. Calibrate Disturber sine wave (see the
X
Disturber Calibration Procedure
X
topic on page
X
77
X
).
3. Connect AWG to J3 and J4.
4. Connect DUT to J68.
5. Install jumpers on J17/J19, J99/J100, J91/J92 and select Pair for your test (refer to
X
Table 5. Section D test
pair jumper configurations
X
).
6. Transmit signal with disturbing sine wave should look like the picture shown in
X
Figure 41
X
.
7. Set the Device Under Test to transmit the mode 4 waveform.
8. Adjust the Volts/div control in the Vertical menu to maximize the signal plus disturber on the oscilloscope
screen.
9. Select 1000Base-T under “Select Standard”.
10. Select Mode 4 Distortion in the “Select Test” field.
11. Enter the channels to which the SMA cables are connected: C2 for the Source for + Data field and C3 for the
Source for - Data field.
12. Click the Set Up and Start Test button to begin the test.
13. The initial distortion values may be above 30 mV but quickly drops as the averaging process proceeds. The
measurement reaches a minimum value and then varies around this level.
(28 pages)
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