Minimizing Error When Using Adapters - Agilent Technologies 8753ET User Manual

Calibrating for Increased Measurement Accuracy

Minimizing Error When Using Adapters

Minimizing Error When Using Adapters
To minimize the error introduced when you add an adapter to a measurement system, the adapter needs to
have low SWR or mismatch, low loss, and high repeatability.
Figure 6-20 Adapter Considerations
In a reflection measurement, the directivity of a system is a measure of the error introduced by an imperfect
signal separation device. It typically includes any signal that is detected at the coupled port which has not
been reflected by the test device. This directivity error will add with the true reflected signal from the device,
causing an error in the measured data. Overall directivity is the limit to which a device's return loss or
reflection can be measured. Therefore, it is important to have good directivity to measure low reflection
devices.
For example, a coupler has a 7 mm connector and 40 dB directivity, which is equivalent to a reflection
coefficient of  = 0.01 directivity in dB = 20 log (. Suppose we want to connect to a device with an SMA
male connector. We need to adapt from 7 mm to SMA.
If we choose a precision 7 mm to SMA adapter with a SWR of 1.06, which has =0.03, the overall directivity
becomes =0.04 or 28 dB. However, if we use two adapters to do the same job, the reflection from each
adapter adds up to degrade the directivity to 17 dB. The last example, shown in Figure 6-20, uses three
adapters that shows an even worse directivity of 14 dB. It is clear that a low SWR is desirable to avoid
degrading the directivity of the system.
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