Teledyne LeCroy QPHY-ENET - Manual (Page 42)

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QPHY-ENET Operator’s Manual Rev G
100Base-TX Jitter Test
Jitter in 100Base-TX signals is defined as the time interval error between an ideal clock at the 125 MHz symbol
rate and the measured timing of the rising and falling edges of the positive and negative pulses. The reference
clock is recovered from the data signal under test using a numerical PLL computed from the threshold crossings
of the signal under test. The jitter test is performed on a scrambled IDLE stream. The IDLE stream is generated
by a 100Base-TX interface when it is terminated with no Link Partner.
1. Apply power to the Device Under Test. An MLT3 signal trace should be displayed on the oscilloscope
screen at this point.
2. Touch the 100Base-TX button under “Select Standard.”
3. Select Jitter in the “Select Test” field.
4. Check Differential Data on 2 Channels.
5. Enter C2 in the Source for + Data field and C3 in the Source for - Data field.
6. Touch the Set Up and Start Test button to begin testing.
7. P1 indicates the total jitter (peak-to-peak), and the number of passing sweeps (acquisitions) out of the
total.
8. The nominal bit rate is shown in the Base Frequency field. Click the Find Frequency field if the bit rate
is significantly different from 125 MHz or if the jitter number is excessively large.
9. The Average Result checkbox, when checked, averages the measured value of p-p jitter over the
number of acquisitions selected in the Avg Weight field.
Note: The results continue to be tested against the specified limit on each acquisition. The average is a moving window: for each acquisition
up to the specified weight, the average includes one additional measurement. After the weight count is reached, the oldest value is dropped
from the average and the latest measurement is added in.
Figure 31. 100Base-TX Jitter Test