1000BASE-T/100BASE-TX/10BASE-T Physical Layer Compliance Tests Manual
Intel Confidential
43
100Base-TX Transmitter Return Loss
Note:
It is important to try to keep the test setup as close as possible to its original position to achieve the
best calibration.
8. Test the calibration by connecting the 50
Ω
load and confirming the results are close to the
theoretical value of 9.54 dB. Also verify that the open and short loads produce the expected
results.
9. From the 100 BASE-TX PHY Configuration Tests menu in gigconf.exe, select the TX Return
Loss: 9.1.5 test.
The resulting display should be similar to
.
Figure 11-2. Transmitter Return Loss
10. Set the marker to the worst case return loss between 2 MHz and 30 MHz. Record the
amplitude (dB of return loss).
11. Repeat step 10 from 30 MHz to 60 MHz and from 60 MHz to 80 MHz.
12. Verify the measurements meet specifications.
Calculating Theoretical Return Loss
To solve for theoretical return loss given known impedances:
Z
transmitter
Balanced output impedance of the balun (for example, 100
Ω
).
Z
load
The impedance of any load connected to the output of the balun.
RL_in_dB
For any load on the 100
Ω
output of the balun, this simplifies to:
RL_in_dB
For a 50
Ω
load with the 100
Ω
output of the balun, this becomes:
RL_in_dB
which simplifies to:
RL_in_dB = 20log
10
3 = 20 * 0.477 = 9.54 dB
20
Z
transmitter
Z
load
+
Z
transmitter
Z
load
–
--------------------------------------------------
log
=
20
100
Ω
Z
load
+
100
Ω
Z
load
–
------------------------------------
log
=
20
100
Ω
50
Ω
+
100
Ω
50
Ω
–
----------------------------------
log
=
