Application Note
Frequency Response Analysis Using the C8000 ATS
Date: August 14, 2007
Overview
The Venable model 3120 frequency response analyzer (FRA) may be seamlessly
integrated into the Chroma C8000 SMPS ATE using the PowerPro III (PPIII)
software platform. The FRA provides design engineers with essential capability for
making automated time domain and stability measurements using Chroma’s C8000
test systems. Specifically, the model 3120 is able to make measurements of gain,
phase and voltage versus frequency (.01Hz – 2.2MHz) as well as input and output
impedance of power supplies.
Venable Industries web site:
http://www.venable.biz
Model 3120 FRA manual:
http://www.venable.biz/Venable3120
Manual.pdf
Benefits
Integrating the FRA into the C8000 ATE provides the following benefits:
ƒ Automated frequency response test of power supplies including time domain
plot generation of gain/phase
ƒ Faster time-to-market for new power supply designs
Application
It is imperative that power supplies supporting sensitive communication systems do
not introduce parasitic signals into such systems. If so, these signals can produce
undesired results and interfere with and impair normal system operation. The use of
the FRA is beneficial to engineers attempting to develop power supply designs for
these sensitive applications. The diagram in figure 1 illustrates how the 3120 FRA
integrates with the C8000 automated test system.
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PC
Venable 3120
FRA
GPIB
Chroma
C8000 ATS
Load
CH1
Vout
Unit Under
Test
Source
CH2
Osc. Out
>>
Signal Injection
Transformer
(Bode Box)
<<
Vin
Figure 1
The bode box (Venable 200-002) is used couple the injection signal to the UUT.
This allows for isolation between the FRA and UUT.
Data Acquisition
Data acquisition and control are performed using a sequence of common and
device specific commands sent to the FRA via the GPIB interface (IEEE-488).
PowerPro III facilitates this by including common control test commands for
developers to utilize when creating test items. The PPIII Test Item Editor allows
for easy manipulation of recorded data for analysis purposes.
Commands are sent to the FRA as a string of up to 255 ASCI characters using the
WriteGPIBDev_CommandString test command within the test program. Each
command is separated by a semicolon. Related parameters precede each command
and are separated by a comma. Once the FRA processes the data string, the
commands are executed and the desired measurements are performed by the 3120.
The test program developer will then use the ReadGPIBDev_CommandString test
command to acquire measurement data from the FRA and store this data in an
array whose size can be defined within the test item.
Report Capabilities
PowerPro III includes a powerful reporting tool called Report Wizard which
facilitates the creation of detailed, custom reports which may be exported to a
Microsoft Word document for analysis. This allows for immediate examination of
test results in a user-friendly format.
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The array of data acquired from the FRA and stored in memory by the test
program can be applied to the correlation function in Report Wizard. Using this
function, the developer can create Bode magnitude and phase plots. The resulting
report now becomes beneficial in studying the frequency response of the UUT
output to determine if the gain and phase margin design goals have been met.
Figure 2
Bode magnitude and phase plots can be converted to display the proper logarithmic
scale after being exported to Word as shown in figure 2 above. The Word
document also provides for any additional report formatting.
Figure 3
Report Wizard offers other report functions to include header information, test
conditions and waveform capture to compare source input to UUT output. An
example of the waveform capture of an input signal and resulting output signal is
shown in figure 3 above.
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Additional Measurement Capabilities
Other measurements the C8000 ATS is capable of when integrated with the
Venable 3120 FRA are listed in the table below. Some measurements may involve
additional program development beyond what is provided by Chroma. Certain
measurements might also require purchasing accessories from Venable Industries to
use with the FRA.
Type
Feedback Loops
Any Part of a Loop
Any Amplifier
Octocouplers
Any Feedback System
•
•
•
•
•
•
•
•
•
•
•
Test or Measurement
Bode Plots or Transfer Function
Loop Gain & Phase Shift
Transfer Function
DC Gain
Open Loop Bandwidth
Phase Margin
Output Impedance
Current Transfer Ratios
AC Effects
Bandwidth
Input or Output Impedance
Technical Data
System Requirements
ƒ C8000 Base System with IEEE-488 PCI control board by National
Instruments
ƒ Venable 3120 frequency response analyzer
ƒ Microsoft Windows operating system: XP/2000/NT
ƒ PowerPro III software environment version 1.91 or higher
ƒ Tektronix Digital Signal Oscilloscope, TDS3012 or similar (figure 3)
7 Chrysler Irvine, CA 92618-2009 (949) 600-6400 Tel (949) 600-6401 Fax
WWW.Chromausa.com