Connection Considerations

Adapters

lb 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.

Worst

Case

System

DiBtiVity

28d.B

17dEI

14 dE3

Leakage signals

 

Reflected

signal

 

 

* Coupler has 4OdE3 Directivity

44

3

;;

 

i

Adapter

 

\. _ _ _ _ _ + .

__----:* :-----_____- sc,-.. x . .....-.,

. . . . . . . . “,.I 1 D</pGi-j

7mm ---EmMale

%kd =PAd+Pd.

\

7mmtoSMA(f)

swR:1.05

7mmtoN(f)+N(m)toSMA(f)

swRz1.05 SWIM.25

7mmtoN(m)+N(f)toSMA(m)+SMAQto(f)

swR:1.05 swRz1.25swR:1;15

pg6237

Figure 6-99. 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 coeflkient of p=O.Ol (dlrectivity in dB = -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= 0.03, the overall directivity becomes p-O.04 or 28 dE% 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 6-99 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.

Application and Operation Concepts 6-169

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HP 8753E manual Connection Considerations, Adapters, Worst Case System DiBtiVity 28d.B 17dEI 14 dE3, Reflected Signal