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Co-axial Cable Insertion and Return Loss Measurement


Contents:

  1. What is return loss
  2. Equipment required to measure return-loss of a co-axial cable
  3. Measurement of Co-axial cable losses
    1. Insertion loss measurement
    2. Return loss measurement
  4. The relation between VSWR and Return Loss(Conversion formulas)
  5. Glossary of terms
  6. References

1. What is Return Loss:

Return loss is a measure of VSWR (Voltage Standing Wave Ratio), expressed in decibels (db). The return-loss is caused due to impedance mismatch between two or more circuits. For a simple cable assembly, there will be a mismatch where the connector is mated with the cable. There may be an impedance mismatch caused by nick or cuts in a cable. At microwave frequencies, the material properties as well as the dimensions of the cable or connector plays important role in determining the impedance match or mismatch. A high value of return-loss denotes better quality of the system under test (or device under test). For example, a cable with a return loss of 21 db is better than another similar cable with a return loss of 14 db, and so on.

2. Equipment required to measure return-loss of a co-axial cable:

A co-axial cable is chosen to measure the return-loss for study purpose. Typically, for a device or a system, return-loss is measured at the input or at the output. The following equipment are used to measure the return loss of a co-axial cable at microwave frequencies:

    1. Frequency source
    2. Network Analyzer (either a scalar network analyzer or a vector network analyzer)
    3. Detector with calibration source.
    4. Reflection bridge
    5. Co-axial Short
    6. Cable under test (this could be any device under test)
    7. A 10dB attenuator (optional, but recommended).

3. The Measurement of Co-axial cable losses:

The measurement process consists of calibrating the test set-up for insertion and return-loss. If you have dual channel network analyzer, both insertion and return losses can be measured simultaneously. You can also measure insertion and return losses separately as is done here.

3.a Insertion loss Measurement:

Step 1. Set the sweep source to the required frequency range. Make sure that the output of the sweep source is within the desired amplitude limits, otherwise, it may saturate the detector head and any measurements taken would not be accurate. You may use an attenuator at the output of the sweep source to mitigate any problem that may arise due to mismatch between the cable under test and the sweep source. It is recommended to use a 10 dB attenuator for this purpose.

For example, you can set the values as below:

Sweep frequency (measurement frequency): 100MHz 2.3 GHz

Sweep power: 12 dBm

Note: Also, make sure that you are measuring same impedance. For example, if the cable is 50Ohm, and the Sweep generator output is 75 Ohm, you need to use a 50 to 75 Ohm impedance matching device.

Step 2: Calibrate the test system by connecting as shown in the figure 1, bypassing the cable under test. Calibration is nothing but setting a reference line taking all stray measurement errors into consideration.

Step 3: Now you have done the equipment calibration, connect the cable as shown in the figure 2, without disturbing any other parameters such as sweep power output or the attenuator value. The trace in the network analyzer display now shows the Insertion loss of the cable against the frequency.

3.b Return loss Measurement:

Step 1. The sweep source is already set during insertion loss measurement. You may use an attenuator at the output of the sweep source to mitigate any problem that may arise due to mismatch between the cable under test and the sweep source. It is recommended to use a 10 dB attenuator for this purpose.

For example, you can set the values as below:

Sweep frequency (measurement frequency): 100MHz 2.3 GHz

Sweep power: 12 dBm

Note: Also, make sure that you are measuring same impedance. For example, if the cable is 50Ohm, and the Sweep generator output is 75 Ohm, you need to use a 50 to 75 Ohm impedance matching device.

Step 2: Calibrate the test system by connecting as shown in the figure 3, bypassing the cable under test. Calibration is nothing but setting a reference line taking all stray measurement errors into consideration. You need to short the bridge port as shown in the figure. For better accuracy, Open/Short method is can be used. In Open/Short method, you calibrate for the system by using both an Open and a Short instead of only a Short used in this example.

Step 3: Now you have done the equipment calibration, connect the cable as shown in the figure 4, without disturbing any other parameters such as sweep power output or the attenuator value. The end of the cable needs to be terminated with a 50 Ohm termation.

The Graph:

The trace in the network analyzer display in figure 5 shows the Insertion Loss and Return Loss of the cable against the frequency. Note that the Insertion Loss is typically low in the desired band of frequencies, and the Return Loss is high. Typically, Insertion loss will of a fraction of a db (for co-axial cable) and the Return loss is 10 dB or more.

4. The relation between VSWR and Return Loss

5. Glossary of Terms:  

Attenuation
Loss of signal in transmission through a rf unit such as filter.cable, usually referring to signal amplitude or signal power. Normally measured in decibels (dB).


Insertion Loss

1. Insertion Loss (dB) is defined as the drop in power as a signal enters an RF component. This value not only includes the reflected inconming signal, but also the attenuation of the component.

Insertion Loss (dB) = 10 * LOG10(Output Power/Incident Power)

Return Loss

1. Return Loss (dB) is defined as a ratio of the incoming signal to the same reflected signal as it enters a component. Return Loss (dB) = 10 * LOG10(Reflected Power/Incident Power)
2. The ratio in dB of maximum power sent down a transmission line to the power returned toward the source, Also equal to 20 times the log of the reciprocal of the reflection coefficient.

Ripple
Generally referring to the wavelike variations in the amplitude response of a rf component.  Ripple is usually measured in dB.

Time Delay
The amount of time it takes for certain signals to pass through a rf unit such as a cable or filter.

VSWR
Voltage Standing Wave Ratio simply put is the ratio of the maximum to the minimum voltage of a standing wave (which is the instantaneous sum of incident and reflected waves). Ideally,  100% of the incoming signal should pass through the component without any reflection, in which case, there would be no standing wave. A 2:1 VSWR (or mismatch) means that 12% of the incoming signal was reflected.

6. References: 

RF Cables Measurement Tools 

http://www.wa1mba.org/rfconn.htm

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