4.0 Test Setups Using Service Monitors
4.3 Making RL Measurements-cont.
This figure is a picture of an actual screen of an HP8920 analyzer running a return loss curve. The return loss is around 10 dB so the 2 dB/div scale is used to give better resolution. Since the trace is normalized, 0 dB return loss is at the top of the screen.
This measurement was set up as described in calibration section 4.1.
The transmission line was connected to the DUT port. The far end was left open, causing all of the power to be reflected. Since this line is not a perfect 50 ohms, the trace has peaks and valleys in it.
To determine the insertion loss, take the reading at one peak (use marker 4). This reading is 10.5. Then take the reading of an adjacent valley. Let's use marker 5 which gives 10.9 dB. Use the following formula to find the average:
As discussed previously, the cable loss is 1/2 the return loss, so use the following to calculate line loss:
The loss of the line is calculated to be 5.35 dB. This line, a piece of RG-214/U that is 75 feet long, was measured with an HP8753C. This measured the loss at 5.39 dB @ 400 Mhz. The calculated loss of RG-214/U is 4.12 dB
maximum. Some of the additional loss may be due to connectors and the fact that this cable is 20 years old and has been kicked around quite a bit.
Line loss measurements
The setup and reading is the same as in the line loss measurements above. As a matter of fact, we can use the same curve from figure 4.3.1.
Refer to chapter 3.4 for more details on how to process the data from this measurement.
First, find the average frequency difference at the valleys. Use the valleys since they are slightly more pronounced than the peaks.
Length=.66(9.836 108 fps/9.0 10<SUP>6</SUP>)=72.13 feet
This is the same 75 foot cable used in the loss measurement. The error than is 72.13/75 or 3.83% short of the actual value. This is a reasonably close measurement using this type of equipment.