NF FRA5014, Instruction Manual

Page: 47 / 107

4.2 Servo System Measurements
FRA5014 4-5
◼
Expansion of measurement voltage range
The
FRA5014
’s built-in oscillator has 42 Vpk of dielectric insulation voltage relative to the case.
This means that in a circuit where the potential is about

40 V, signals can be injected directly.
By contrast, the analyzer’s input voltage range is limited to

10 V. If the signal exceeds this
range, direct measurements become impossible. Even in these cases, measurements can be
enabled by attenuating the signal or by shifting the analyzer’s ground potential to an external
circuit. When attenuating the signal, the signal’s attenuation and its frequency response must
be corrected.
Figure 4-2. Examples of Expanded Measurement Voltage Range
Signal input
FRA
CH1, CH2, CH3
or CH4 input

10 V max
0 to 20 V
10 V
For the shift power, use a battery (etc.) that has a
low internal resistance.
When there is a large gap among signal levels, a
shift power supply should be set up for each
channel to prevent interference between channels.
The original voltage should not be applied
anywhere that exceeds the FRA5014’s
nondestructive maximum input voltage of ±24 V.
Shift power
4.7 k ohm
4.7 k ohm
Signal input
FRA
CH1, CH2, CH3
or CH4 input
R
1
R
2

10V

20 V
220 pF
Coaxial cable, 1m
Characteristic
impedance, 50
Ω
The amount of error can be reduced by setting the signal source
impedance adequately low and having a parallel value of R1 and
R2 that is no more than a few kΩ.
To change the attenuation rate, set C1 R1

R2
C2. Here, C2
= cable’s capacitance + FRA’s input capacitance. If the
connecting cable is a 50
Ω coaxial cable (1 m), then C2 =
approximately 220 pF.
As the parallel value of R1 and R2 increases, so does the
frequency, and the error rate becomes greater. When using an
attentuator, evaluate its overall precision carefully.
C
1