HP 8753E manual Reflection Tracking Em

-

		ERF F r e q u e n c y		T r a c k i n g
0		L
	11 EDF		,E SF
'11M				11 '1lA

Figure 6-36. Reflection Tracking Em

pg653d

These three errors are mathematically related to the actual data, &A, and measured data, &M, by the following equation:

(SllA&W)

“~4 = EDF + (1 - EsF$~A)

If the value of these three “E” errors and the measured test device response were known for each frequency, the above equation could be solved for S11A to obtain the actual test device response. Because each of these errors changes with frequency, their values must be known at each test frequency. These values are found by measuring the system at the measurement plane using three independent standards whose !&A is known at all frequencies

The first standard applied is a “perfect load,” which makes &A = 0 and essentially measures directivity (see Figure 6-37). “Perfect load” implies a reflectionless termination at the measurement plane. All incident energy is absorbed. With S 11~ = 0 the equation can be solved for Enr, the directivity term. In practice, of course, the “perfect load” is difficult to achieve, although very good broadband loads are available in the HP 87533 compatible calibration kits

•

1’ 50n S,lA= 0

0I i3

s (O)(ERF)

11M = EDF+l-E,,o

Figure 6-37. “Perfect Load” ‘Ikrmination

pg654d

Since the measured value for directivity is the vector sum of the actual directivity plus the actual reflection coefficient of the “perfect load,” any reflection from the termination represents an error. System effective directivity becomes the actual reflection coel3cient of the near “perfect load” (see Figure 6-38). In general, any termination having a return loss value greater than the uncorrected system directivity reduces reflection measurement uncertainty.

Application and Operation Concepts 6-63