Specifications

5-12. Test signal voltage enhancement
When measuring the impedance of test signal level dependent devices, such as liquid crystals, induc-
tors and high value ceramic capacitors, it is necessary to vary the test signal voltage. Many of the
auto balancing bridge instruments employ a test signal source whose output is variable typically
from 5 mV to 1V rms. Particularly, the 4284A precision LCR meter with the option 4284A-001 can
output a test signal voltage of up to 20 V rms and is the most suitable for this application.
In some cases, measurement needs exist for evaluating impedance characteristics at large test signal
voltages of interest. There are such cases that higher test voltages than the maximum oscillator out-
put of the instrument are required. For auto balancing bridge instruments, output voltage enhance-
ment is possible if the test signal is amplified as shown in Figure 5-38. A voltage divider is also
required so that the input voltage of the Hp terminal is the same as the output voltage of the Hc ter-
minal. The DUT’s impedance is a concern. Because the current flowing through the DUT is also
amplified and flows directly into the Rr circuit, it should not exceed the maximum allowable input
current of the Lc terminal. Typically, this is 10 mA. For example, when a 10 V rms signal is applied
to the DUT, the minimum measurable impedance is 10 V/10 mA = 1 kΩ. Also, it should be noted
that measured impedance is 1/A (gain of amplifier) of an actual DUT’s impedance. For example,
when a 10 pF capacitor is measured using ×10 amplifier, displayed value will be 100 pF.
Note: For RF I-V instrument, it is impossible to amplify the test signal because at the test port the signal source output is
not separate from the voltmeter and current meter inputs.
Figure 5-39 shows a measurement setup example to boost the test signal voltage by factor of 10
(A=10). The amplifier used in this application should have constant gain in the measurement fre-
quency range and output impedance less than 100 Ω. R3 and C2 in Figure 5-39 are used to compen-
sate for measured impedance value and its frequency response, respectively. To increase measure-
ment accuracy R3 and C2 need adjustment. This can be accomplished by comparing the measured
values with known values of a reference device. For better accuracy, perform the open/short/load
compensation at a test signal level below 1 V rms (not to cause an excessive current to flow in short
condition.) The required circuit constants of the divider are different depending on the input imped-
ance of the Hp terminal of the instrument.
Figure 5-38. Schematic diagram of test signal voltage enhancement circuit
5-28