Agilent Technologies 4294A Specifications download pdf (Page 57)

Figure 4-5. Open/short/load compensation

4-2-5. What should be used as the load?

The key point in open/short/load compensation is to select a load whose impedance value is accu-

rately known. The criteria is as follows.

Use a stable resistor or capacitor as the load device. The load device’s impedance value must be stable

under conditions of varying temperature, magnetic flux, and other component dependency factors.

So, avoid using inductors which are relatively sensitive to measurement conditions, for the load.

Use a load of same size and measure it in the same way as the DUT will be measured. As shown in Figure 4-6,

if the load is measured under different electrode conditions, its measured data will not effectively

compensate the residuals. It is a good idea to use one of the actual DUTs as a working standard. If

the load is a different type from the DUT (e.g. load is C and DUT is R), at least keep the same dis-

tance between the electrodes.

Use a load that is close in value to the DUT. Whatever the load value is, the load compensation is effec-

tive over the entire measurement range if the measurement circuit has a linear characteristic. In

practice, the circuit between the UNKNOWN terminals and the DUT may have a non-linear factor,

especially when an additional circuit includes a non-linear component such as an inductor, active

switch, amplifier, etc.. As shown in Figure 4-7, additional measurement error will be added when

the measured DUT value is far from the load value used for the compensation. So, the impedance

value of the load should be as close as possible to that of the DUT to be measured. If various imped-

ances are to be measured, select a load that is nearly the center value of the DUT’s impedance range.

In addition, the load value should not be near the open or short impedance. Otherwise, the load

compensation will not be effective and the result of the open/short/load compensation will be much

the same as (or even worse than) that of open/short compensation.

Use an accurately known load value. The impedance value of the load must be known before performing

the open/short/load compensation. To measure the load value, it is practical to use the same mea-

surement instrument, but under the best possible measurement conditions. Set the measurement

time, averaging and OSC level so that the instrument can measure the load with maximum accuracy.

Also, use a test fixture which mounts directly to the instrument. Figure 4-8 shows an example of

such a measurement.

4-5

1 2 ... 52 53 54 55 56 57 58 59 60 61 62 ... 125 126

No comments

Agilent Technologies 4294A Specifications Page 57