3.8 Using the RF Substitution Method To Determine Insertion Loss and Gain
Often it is necessary to determine the insertion loss of one or more components in a test setup so that their effect can be removed from the final measurement results (e.g., see calibration section in TOI measurements, and section on importance of removing the loss between the DUT and noise source due to adapters and Bias T in noise figure measurements). One way of determining the insertion loss is to use method commonly used for making transmission measurements known as the RF substitution method . The underlying principle of this method is based on using a standard to set a reference level, then inserting the DUT and noting the difference between the new level and the reference level to obtain the gain or loss of the DUT.
- Set the RF signal source to the frequency and level you want. Choose a level that will be used in the application and ensure that it is within the rating of the DUT.
- Setup the measurement instrument (e.g., power meter) to display the frequency and level. Be sure to note its allowable range of input levels (e.g., power sensor) and make sure the instrument has been zeroed (calibrated).
- Measure the signal power using a power meter.
- Record the power meter reading as your reference.
- Insert the device to be tested between the power meter and the signal source at point A-A.
- The difference between the reference and the value in Step 5 is the attenuation or gain of the particular DUT.
- Repeat the test at different frequencies if your measurement is to cover a range of frequencies.
If a cable must also be used but not included as part of the overall insertion loss, the insertion loss of the cable alone will have to be determined and subtracted out from the total loss (see calibration section in TOI measurements).
In addition to attenuation or insertion loss measurements, the above procedure can apply to gain measurements. In gain measurements, attenuation can be added to the setup to bring the indicator back to its calibration setting before the active device was inserted. The gain is read off the attenuator or instrument.
The source used can be a signal generator, a frequency synthesizer, or a sweep generator in the CW mode. The basic requirements are that it can produce a CW signal at the proper frequency, has reasonable stability (does not drift significantly), and is capable of providing a sufficent power level to make the test meaningful. Keep in mind that the network analyzer has a synthesized source if you don't have a standalone signal generator.
Whenever you insert a component into a system, you should know what kind of reflection will be caused. Ensure that the components (e.g., attenuators) you use have low VSWR to minimize reflections and thereby enable your readings and overall setup to exhibit a high degree of accuracy. An optional lowpass filter will also improve accuracy.
The detectors and thermistor mounts used in microwave measurements are not frequency selective; that is, they pass and respond to frequencies over their frequency range. Most signal sources generate harmonics internally that can be mistaken very easily on the meters as part of the power level. One method to determine the size of the harmonics is to view the signal on a spectrum analyzer. A procedure for RF substitution using an oscilloscope as the meter is described in 
Note that a calibrated network analyzer can also be used to determine the insertion loss of the components.
 Thomas S. Laverghetta Modern Microwave Measurements and Techniques. Artech House, 1988
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