Connection Considerations; Adapters - Agilent Technologies 8719D User Manual

Connection Considerations

Adapters

adapter needs to have low SWR or mismatch, low loss, and high repeatability.
In a reflection measurement, the directivity of a system is a measure of the error introduced
by
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 p-O.01 (directivity in dR = -20 log p). 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 p=O.O3, the
overall directivity becomes p=O.O4 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 dR. The last
example shown in Figure 6-97 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.
Total = Ad + ,-,
Figure 6-97. Adapter Considerations
device. It typically includes any signal that is detected at the