Instrumentation & Measurement Magazine 25-5 - 56

Fig. 4. (a) Test setup for pre-silicon validation of server platform; (b) Voltage waveform captured in infinite persistence mode.
be done using the VRTT. The worst-case minimum and maximum
voltages can be captured to determine if there are any
violations of system level specifications. A typical transient
measurement procedure follows generating the transient load
current profile and applying it to the system using the VRTT
software and capturing the measurements using the tool.
When a motherboard has two sockets, such as the one shown
in Fig. 4a, it is important to perform the VRTT test measurements
on both sockets, since the PDN characteristics might be
different.
A Bode plot can also be taken during pre-silicon validation
to determine the stability of the VR voltage control loop.
For this measurement, the control loop is 'broken' by adding
a small resistor in the feedback loop of the VR. A small
disturbance signal is injected across the resistor, and the corresponding
transfer function (Vout
/Vin
) is measured and Bode
plots are produced. This can be achieved by using a vector
network analyzer (VNA). The application of the disturbance
signal across the resistor is by an injection transformer to avoid
affecting the loop. The gain and phase plots can be examined
to determine the stability of the VR. An impedance measurement
of the system under test (VR and motherboard) can also
be taken. This involves the use of a current injector and VNA.
A current is injected (Iin
) at the output of the system (VR sense
56
point). One channel of the VNA is connected to the output of
the current injector and another channel is connected to the
output of the system to measure the voltage (Vimp
measurement, Vimp / Iin, can be calculated across frequency to
obtain the impedance.
). A gain
One of the most important aspects of validation for correlation
purposes is capturing the transient response waveform in
infinite persistence mode on a digital oscilloscope. In infinite
persistence mode, multiple waveforms are superimposed over
each other when the oscilloscope triggers each time. The waveforms
'persists' on the oscilloscope display until it is cleared.
Infinite persistence mode helps acquire lots of data to find a
tolerance band (maximum and minimum) of the transient
voltage waveforms under a certain load condition. The range
in which the transient voltage waveforms lies when doing
infinite persistence can help determine when models are accurately
correlated. An example of a digital oscilloscope capture
of a transient voltage waveform in infinite persistence mode is
shown in Fig. 4b.
In addition to ac and transient measurements, pre-silicon
validation can include the VR efficiency, dc load regulation,
and ripple measurements. The peak-to-peak voltage ripple is
measured via an oscilloscope when a static minimum current
and maximum current load condition is applied to the system.
IEEE Instrumentation & Measurement Magazine
August 2022

Instrumentation & Measurement Magazine 25-5

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-5

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Instrumentation & Measurement Magazine 25-5 - Cover2
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Instrumentation & Measurement Magazine 25-5 - Cover3
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