Wavetek 3000B Data Sheet

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Today, a power supply must deliver performance and reliability, and
equally important, compliance with national and regional power quality
standards (such as the IEEE 519-1992 specification in the United
States). The engineer must characterize power levels, output purity,
and harmonic feedback into the power line and be prepared to meas­ure high-frequency switching device outputs, noise levels, power char­acteristics, and more.

The oscilloscope has become a favored tool for power supply design
and troubleshooting because it easily meets these diverse require­ments. The TDS3000B oscilloscope:

Measures voltage, current, and power with equal ease
Measures floating voltages safely
Provides harmonic measurements
Makes the user’s job easy and safe

The TDS3000B Digital Phosphor
Oscilloscope Advantage

The TDS3000B Series digital phosphor oscilloscopes (DPOs) equipped
with the Fast Fourier Transform Application Module (TDS3FFT) has the
bandwidth, waveform capture rate, and advanced analysis capability to
meet today’s power measurement, design, troubleshooting, and testing
requirements. In addition, they are compatible with a wide range of
probe types, including passive (voltage), active FET, current, differen­tial, and high-voltage probes. This latter type of probe provides a safe
means of connecting to ungrounded (floating) circuits. The built-in
TekProbe® interface simplifies power measurement by providing cor­rectly-scaled readings automatically, in proper units (volts, mil­liamperes, etc.).

TDS3000B DPO
Solves Today’s
Power Measurement
Problems

Due to the industry-wide shift to switching power supplies, the definition of “power measurements” has
changed, and with it, the number of design applications that require such measurements.

Meets Power Supply Design, Troubleshooting, and Testing Needs

Application Note

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Harmonic measurement and display

Power harmonic measurements require a tool that can display the sig­nal’s various spectral components. The TDS3000B’s harmonic display
reveals the relative magnitude of the harmonics to the fundamental fre­quency. The optional Fast Fourier Transform Application Module (FFT)
equips the TDS3000B oscilloscopes for this type of reading. It provides
advanced analysis features that convert a time-domain acquisition into
a true harmonic display, which is essential for harmonic measurements
on 50 and 60 Hz lines.

Bandwidth to handle the fastest frequencies

Switching transistor and IGBT circuits, noise, and transients all make
demands on an oscilloscope’s ability to capture power supply signals
accurately and reliably. Switching frequencies are increasing with each
new generation of power supplies. Viewing these signals calls for
ample bandwidth. TDS3000B DPO models range in bandwidth from
100 MHz to 500 MHz, sufficient for even the fastest power supply
switching frequencies.

DPO dependably captures and displays
transients

Along with bandwidth, an oscilloscope’s waveform capture rate – the
number of times per second the instrument can trigger, compile the
waveform, draw a fresh display, and re-arm for the next trigger –
determines how reliably transients are acquired. The TDS3000B instru­ments offer a waveform capture rate that far surpasses digital storage
oscilloscopes, which means that transients are much more likely to be
acquired. It is easy to distinguish an occasional transient from the
characteristics of the background waveform because the DPO’s intensi­ty-graded digital phosphor display highlights the most frequently­occurring areas of the signal.

Measuring Instantaneous Power
with the TDS3000B

Characterizing the instantaneous power dissipation in switching tran­sistors is part of almost every power supply design project. It’s key to
choosing a component (such as the power MOSFET in Figure 1) that is

both cost-effective and reliable under the stresses of worst-case oper­ation. The procedure involves making a floating measurement simulta­neously with a current measurement. The TDS3000B’s TekProbe inter­face is compatible with the P5205 High-Voltage Differential Probe and
the TCP202 Current Probe; other current probes are available for high­er current measurements. This pairing provides exceptionally accurate
results.

The high-voltage differential probe is necessary because the voltage of
interest (V

ds

on the MOSFET circuit) is across the drain-to-source ter­minals of the transistor, and neither is grounded. The TDS3000B, like
most oscilloscopes, is not designed to measure floating signals directly.
A differential probe is required for making safe floating measurements
with the TDS3000B. The P5205 accepts the ungrounded signal and
delivers a single-ended, grounded signal to the scope input.

Before making power measurements, it may be necessary to equalize
the delay between the voltage and current probes using a procedure
known as “deskewing.” The P5205 and the TCP202 are inherently
matched to within ±2 ns, minimizing delay errors, but other probe
combinations will need to be deskewed. This step is of critical impor­tance, since a small offset in the timing of the voltage and current
traces can cause a large error in the instantaneous power reading.

TDS3000B DPO Solves Today’s Power Measurement Problems

Application Note

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Figure 1. Connecting a differential probe to a power MOSFET.

Connect to CH2

Connect to CH1

The TDS3000B has a deskew memory that stores the delay differen­tials between probes. To deskew a pair of probes, drive both with the
same pulse and use cursors to measure the time difference. Then
enter this information into the

deskew memory

(accessible via the
Vertical MENU). This procedure stores the delay value for the pair of
probes being used.

With the probes deskewed, connect them as shown in Figure 1 (a
power MOSFET circuit of the type found in switching power supplies).

The

Autoset

function on the TDS3000B can be used to set up the initial
waveform display. Autoset automatically adjusts the range and scaling
to bring the waveforms into view. The TDS3000B’s color LCD displays
the voltage, current, and power waveforms in different colors, eliminat­ing a common source of confusion. Thanks to the TekProbe interface,
the numeric readout and scaling of all three waveforms is accurate –
no interpretation is required.

The power waveform is simply a point-by-point multiplication of the
voltage and current waveforms (CH1 x CH2). The TDS3000B’s wave­form

MATH

button gives you access to math operations that use the
two waveforms as variables. Figure 2 shows the result. The voltage,
current, and power waveforms are displayed in the correct units. The
maximum instantaneous voltage, current, and power can be displayed
with the Max measurement function accessed by pressing the

MEA-

SURE

button.

Troubleshooting Power Supplies

Digitizing oscilloscopes are common in the power measurement field,
but the TDS3000B’s digital phosphor acquisition technology makes a
big difference when troubleshooting, especially when identifying exces­sive modulation effects in a switching power supply.

The TDS3000B has a waveform capture rate that is fifty times higher
than that of a typical digital storage oscilloscope (DSO). This provides
two advantages when investigating modulation effects. First, the
TDS3000B is active much more of the time, and less time is spent pro­cessing waveforms for display. Thus the scope has hundreds of times
more opportunities to capture the modulation. Secondly, the digital
phosphor display makes it easier to see the modulated waveforms in
real time. The TDS3000B’s display intensifies the areas where the sig­nal trace crosses most frequently, much like an analog scope. The
modulation is dimmer than the main waveform that repeats
continuously.

Viewing modulation effects with the TDS3000B is extremely simple.
Figure 3 shows the modulated signal controlling the output of a cur­rent-mode control loop on a power supply. Modulation is important in a
feedback system to control the loop. However, too much modulation
can cause the loop to become unstable. Notice that the waveform is
dimmer in regions where the modulation is less frequent.

TDS3000B DPO Solves Today’s Power Measurement Problems

Application Note

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Figure 2. Power Measurement Screen shows volts, amps, and power waveform
in correct units.

Figure 3. Modulation effects on a power supply control loop.

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Capturing transients with the TDS3000B is also simple. Its

Edge

Trigger

function gives you all the flexibility you need to set up the
slope, level, coupling, and trigger delay. If the power supply under test
is already integrated into a system, it may be desirable to trigger on
the “problem” signal elsewhere in the system and monitor a test point
on the power supply to see if there is a transient that occurs at the
same time.

Of course, the power supply’s DC output needs to be clean and free of
transients also. The TDS3000B’s

ROLL mode

, combined with the

Peak

Detect

feature, is the best tool for viewing aberrations on slow signals

or DC levels.

ROLL mode

simply scrolls the trace slowly from right to
left, much like a strip chart recorder. It produces a clear, bright trace at
very slow sweep speeds.

Peak Detect

lets the oscilloscope capture
glitches as narrow as 1 ns, even at slow sweep speeds. Combining the
two features produces a steady, legible trace that immediately reveals
transients.

Measuring Line Harmonics

Measuring line harmonics is a critical task for today’s designs.
Switching power supplies tend to generate odd-order harmonics, which
can find their way back into the power grid. The effect is cumulative;
as more and more switching supplies are connected to the grid (for
example, as an office adds more desktop computers), the total per­centage of harmonic distortion returned to the grid can rise. Since this
distortion causes heat buildup in the cabling and transformers of the
power grid, it’s necessary to minimize harmonics. Regulatory standards
such as IEC1000-3-2 are in place to oversee power quality.

When fitted with the TDS3FFT application module, the TDS3000B is an
excellent tool for harmonic analysis. An oscilloscope with FFT capability
is far more cost effective than buying a specialized harmonic analyzer,

and it permits you to use a familiar instrument to do yet another job.
The TDS3FFT module uses a Fast Fourier Transform (FFT) algorithm to
provide spectrum-analyzer-style display of signal-frequency compo­nents. It’s even possible, and often helpful, to display both the signal
waveform and its frequency-domain equivalent on the screen at the
same time.

The TDS3FFT module makes available an

FFT-specific menu

that sim-

plifies setup and measurement. The

FFT menu

resides under the

MATH

button menu

. The FFT can be performed on active signals or stored

waveforms that have been recalled.
The procedure is no more difficult than taking an ordinary waveform

measurement. Since the signal in this case is a repeating periodic
waveform (as opposed to a transient of some kind), it’s a simple matter
to trigger and display it. Pre-compliance testing for IEC61000-3-2 is
performed on the current waveform. At least five cycles should be dis­played to ensure good frequency resolution, and the vertical scale
must be set such that the full signal amplitude appears on-screen.

The TDS3000B’s user-configured parameters include the vertical scal­ing and the FFT window format. Rectangular, Hamming, Hanning, and
Blackman-Harris windows are available, each suited to a particular
type of signal. For a periodic repeating signal like the one in this
example, the Hamming window usually is best.

Vertical scaling for the FFT display may be either linear or logarithmic.
Linear scaling is more commonly used in power measurements.

TDS3000B DPO Solves Today’s Power Measurement Problems

Application Note

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Figure 4 shows the result of a harmonic analysis on the power supply’s
load current. You can use the TDS3000B’s cursors to measure either
the magnitude of individual frequency components, or their frequency.
The

ZOOM

function is also available to expand the FFT for closer
scrutiny. Using ZOOM does not affect the acquisition itself (trigger or
time base settings, etc.); only the display.

Proof of compliance with power-quality regulations is an important part
of many power supply design projects. The TDS3000B offers a full
range of storage and printing capabilities to aid in this effort. A dedi­cated

HARD COPY

button reproduces the screen image on the optional
plug-in printer, or on an inkjet or laser printer connected to the
TDS3000B’s standard parallel port. Similarly, the image can be stored
on a floppy disk in publishable formats including .BMP, .EPS, .TIF, and
more. These are compatible with leading word processing, page layout,
and office presentation programs.

Conclusion

Power measurements are no longer the province of dedicated tools
such as power meters and harmonic analyzers. The TDS3000B digital
phosphor oscilloscope, when equipped with the TDS3FFT application
module, is ready to handle voltage, current, power, and harmonic
measurements, and also serves as a versatile troubleshooting tool for
power supply design and troubleshooting.

TDS3000B DPO Solves Today’s Power Measurement Problems

Application Note

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Figure 4. TDS3000B shows the harmonic analysis of a 250 W class D power sup­ply.The cursor shows that the 3rd harmonic (at 180 Hz) exceeds the 850 mA limit
per IEC61000-3-2.

TDS3000B DPO Solves Today’s Power Measurement Problems

Application Note

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For other areas, contact: Tektronix,Inc. at 1 [phone]

Copyright © 2001, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and
foreign patents, issued and pending. Information in this publication supersedes that in all previously
published material. Specification and price change privileges reserved. TEKTRONIX and TEK
are registered trademarks of Tektronix, Inc. All other trade names referenced are the service marks,
trademarks or registered trademarks of their respective companies.

0301 TD/PG 3GW-12452-2

For Further Information

Tektronix maintains a comprehensive, constantly expanding collection of
application notes, technical briefs and other resources to help engineers
working on the cutting edge of technology.

Please visit “Resources For You” at

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The TDS3FFT Module adds FFT
measurement capabilities in dB or
linear RMS scales. A selection of
four FFT windows (Rectangular,
Hamming, Hanning, Blackman­Harris) are provided.

The P5205 Probe is a 200 MHz
active differential probe capable of
measuring fast risetimes of signals
in floating circuits.

The TCP202 DC Coupled
Current Probe is used for display-

ing and measuring current in elec­tronic circuits. It is ideal for power
supply and motor drive design and
device testing.

The TDS3PRT Plug-in Printer

provides instant portable documen­tation of your work.

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