Agilent Technologies 4294A Specifications download pdf (Page 41)

3-2. Using test cables at high frequencies

The 4TP configuration is the best solution for wide-range impedance measurement. However, in

basic 4TP measurement, its cable length is limited by the measurement frequency because the length

of the cable must be much shorter than the wavelength. The following equation gives a typical

guideline for determining the limitation:

F × L ≤ 15

Where: F is the measurement frequency (MHz)

L is the cable length (m)

When the cable length is 1 m, the maximum frequency limit will be approximately 15 MHz. If the

cable length or frequency exceeds this limit, the auto balancing bridge may not balance. Generally,

4TP measurement instruments have this cable length limitation, but there are some exceptional

instruments, such as 4294A, which permit using test cables at frequencies over the limits. These

instruments obviate the limitation by having measurement circuit impedance matched to the char-

acteristic impedance of specified test cables at high frequencies. (Note that practical cable length

limit due to increase in measurement error still exists.)

Cable length compensation is also necessary for higher frequency impedance measurement (typical-

ly above 100 kHz). In the higher frequency region, transmission characteristics of the cable will

cause an error and it depends on the type of cable used. As a recommendation, use the standard

Agilent test cables. Agilent’s impedance measurement instruments can compensate for known

transmission characteristics of the cables. A detailed discussion on the error and how to compen-

sate for it is provided in Section 4.

3-3. Test fixtures

The test fixture plays an important role in impedance measurement in both mechanically and elec-

trically. The quality of the fixture determines the limit of the total measurement quality. Discussion

on how to choose or fabricate a fixture follows.

3-3-1. Agilent supplied test fixtures

Agilent Technologies supplies various types of test fixtures depending on the type of device being

tested. To choose the most suitable test fixture for the DUT, consider not only the physical layout of

the contacts but also the usable frequency range, residual parameters (usable impedance range),

and the allowable DC voltage that can be applied.

The contact terminals of the test fixtures (DUT connection) can be either 2-terminal or 4-terminal

that are suited to different applications. The DUT connection configuration and suitable application

of Agilent’s test fixtures are summarized in Table 3-1.

Note: The terminal configuration in this case is described for the contact terminals only, not the whole circuit in the test

fixture.

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Agilent Technologies 4294A Specifications Page 41