Filter
Top Products
315889-002 53
Z(f) Constant Output Impedance Design
A.3 VTT Z(f) Measurement Method
An electronic load that has the capability to change the repetition rate up to 3 MHz of
the load step is needed. The Intel LGA771/775V2 VTT by Cascade Systems Design, will
meet this requirement. By monitoring the VTT current and voltage waveforms with an
oscilloscope capable of executing an FFT on these waveforms, the platform impedance
is found. A complete impedance profile is then generated by sweeping the input
waveform frequency across the range of interest. In order to automate the data
collection process, Intel has modified the VTT control software and a GPIB controlled
oscilloscope is used along with software supplied with the VTT.
These utilities allow the user to automatically display and collect the magnitude and
phase of the motherboard impedance in a Microsoft Excel* compatible data file. The
total time it takes to extract the impedance profile using this method is about 1-2
minutes. This technique is very useful in investigating and assuring MB performance
based on its stack up.
For more information on the measurement method and theory, see the paper
Microprocessor Platform Impedance Characterization using VTT Tools by K. Aygun, S.
Chickamenahalli, K. Eilert, M. Hill, K. Radhakrishnan and E. Stanford published at the
IEEE Applied Power Electronics Conference, 2005.
A.4 Results
As an example, Figure A-5 shows the test platform with 10 560 μF Al-Poly bulk
capacitors and 10 10
μF and 8 22 μF high frequency MLCC capacitors in the socket
cavity. Figure A-6 is the measured impedance profile of the board shown in Figure A-5
as capacitors are removed. The VID setting for this measurement was 1.35 V and load
Figure A-4. Time Domain Responses and Corresponding Fourier Spectra
of Voltage, Current and Impedance