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Measuring High Voltages using the
U8903A Audio Analyzer
Application Note
This note shows how the U8903A-109 BNC
accessory kit can be used to make an
attenuator that will extend the input voltage
range of the U8903A to 300 vRMS.
02 | Keysight | Measuring High Voltages using the U8903A Audio Analyzer - Application Note
The U8903A Audio Analyzer can measure a maximum of 140 vRMS on the
unbalanced BNC connection. This is more than adequate for most applications but
some situations do need to extend beyond this limit.
This note shows how the U8903A-109 BNC accessory kit can be used to make an
attenuator that will extend the input voltage range of the U8903A to 300 vRMS. An
example is shown in Figure 1.
Attenuator
Attenuation can be achieved in many ways depending on the requirements. For this
application there are two main requirements. The first is that that attenuator must
have a high input impedance otherwise power dissipation and voltage drop will
be too high. The second requirement is that the attenuator must have low output
impedance so that it does not from a low pass filter with the input capacitance of
the U8903A.
The `L' type attenuator is best suited to this application and the schematic is shown
in Figure 2.
Input
100 kOhms
3.3 kOhms 100 kOhms 100 pF
Attenuator U8903A
Figure 2.
The requirement for high input impedance and low output impedance mean that
the attenuator will naturally be of high value, the values shown result in attenuation
value of a little over 30 dB.
Component Choice
The 100 k resistor will be dissipating 0.85 W with a 300 vRMS input. A 1 W metal
film 1% tolerance device is the best choice here. For the 3.3 k resistor power is not
such an issue but physical size is important. A 0.125 W metal film 1% tolerance
device is most suitable. The 100 k resistor in this particular example was from
Vishay Dale, model number CCF60. The 3.3 k was from Multicomp, model
number MF12.
03 | Keysight | Measuring High Voltages using the U8903A Audio Analyzer - Application Note
U8903-83401 BNC Accessory
The U8903A-109 BNC accessory kit is used to house the attenuator components
as shown in Figure 3. Application note 5991-1466EN should be referred to for
assembly instructions. Given the high voltage application care should be taken
when positioning and soldering the resistors. Some heat shrink tubing may be
beneficial on the ground lead of the 3.3 k resistor.
Figure 3.
Measurement Compensation
With the attenuator connected to the input of the U8903A and 300 vRMS applied
to the attenuator there will be around 9.5 v at the instrument input. This places the
instrument in the 12.8 v range. The instrument should be fixed in this range via the
input range menu.
With the range set we need to compensate for the attenuator value, this can be
done with the aid of the external gain function under the input settings of the
analyzer as shown in Figure 4. The instrument will now compensate for the external
attenuation and display the level at the input of the attenuator. A 300 vRMS
measurement is demonstrated in Figure 5.
Figure 4. Figure 5.
Summary
This note shows how the U8903A-109 accessory kit can be used to make a simple
attenuator that will extend the measurement range of the U8903A audio analyz-
er up to 300 vRMS. Measurement compensation using the instruments UI is also
demonstrated.
04 | Keysight | Measuring High Voltages using the U8903A Audio Analyzer - Application Note
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