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 6-19 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 ρ = 0.01 directivity in dB = −20 log (ρ). 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 ρ=0.03, the
overall directivity becomes ρ=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 6-19, 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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