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:
Frequency source
Network Analyzer (either a scalar network analyzer or
a vector network analyzer)
Detector with calibration source.
Reflection bridge
Co-axial Short
Cable under test (this could be any device under test)
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.
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.