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Introduction
A common question from SynQor customers is, “Why do I see large high frequency noise spikes on my oscillo-
scope when testing the output ripple of a SynQor converter?” Customers often worry that these large spikes will
cause their digital circuitry to malfunction, either because of absolute voltage deviation or because of noise cou-
pling to adjacent circuitry. Fortunately this noise spike is largely a measurement artifact and does not reflect the
actual voltage deviation between the power planes. With some simple test set-up changes, a much more accu-
rate measurement is possible.
Measuring Output Voltage Ripple
Many SynQor customers evaluate our dc/dc converters using the SynQor Evaluation Board. Other customers
prefer to make electrical characteristic measurements using their own evaluation boards or test them directly on
prototype or pre-production load boards. Regardless of the method of evaluation, we have found that in many
cases when measuring the output voltage ripple, the converter under test can exhibit large high frequency noise
spikes on the oscilloscope.
It is widely recognized that large noise spikes can be caused by magnetic coupling through a scope probe
ground loop. This source of noise can be eliminated by using a BNC cable, since the area of this ground loop
is greatly reduced. It is not widely recognized that if the BNC cable is unterminated, then spikes will be creat-
ed at the scope end of the cable, with amplitudes larger than those present on the circuit side of the cable.
A BNC cable is a transmission line with a 50 ohm impedance. At the circuit end of the cable, there is a very
small amount of high frequency differential mode noise. This high frequency noise resonates and reflects from
one end of the cable to the other. The high Q of the cable actually makes the peak noise higher than the actu-
al noise source. To eliminate reflections, the transmission line must be terminated either with a 50 ohm resistor
from signal to ground at the scope (referred to as parallel or end termination), or with a 50 ohm resistor in series
with the signal on the eval board end (referred to as source termination). A diagram showing proper evalua-
tion test set-up and termination is shown in Figure 6 on page 3.
An astute reader may ask whether the addition of a 50 ohm resistor in itself will filter out a presupposed noise
spike. Figure 4 (page 2) shows that when a 50 ohm resistor is placed in circuit, but at the wrong end of the
cable, that the noise spike is exactly the same amplitude as with no 50 ohm resistor.
Examining the Data
There are many other mechanisms that could theoretically cause this noise spike, such as near-field magnetic
coupling or common-mode noise. To demonstrate that transmission line integrity is the primary effect, SynQor
experimented by collecting the data shown on page 2. All of the scope plots are at a 500 MHz bandwidth limit
and have a scale of 20mV per division.
SynQor -
Advancing the Power Curve
•
888-567-9596
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www.synqor.com
Page
1
Output Voltage Ripple
Measurements
Application Note 01-08-01 Rev. 01
Summary:
This application note explains why some customers occasionally
see the existence of high frequency noise spikes when measuring
the output voltage ripple on a SynQor converter. The paper details
how to solve this measurement issue with the proper test set-up and
termination methods.
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