Keithley 2016, 2015P, 2015 User Manual

AC Signal Analysis

Using Models 2015, 2015-P, and 2016 THD Multimeters
Quick Reference Guide

A GREATER MEASURE OF CONFIDENCE

WARRANTY

Keithley Instruments, Inc. warrants this product to be free from defects in material and workmanship for a
period of 3 years from date of shipment.

Keithley Instruments, Inc. warrants the following items for 90 days from the date of shipment: probes, cables,
rechargeable batteries, diskettes, and documentation.

During the warranty period, we will, at our option, either repair or replace any product that proves to be defective.
To exercise this warranty, write or call your local Keithley representative, or contact Keithley headquarters in

Cleveland, Ohio. You will be given prompt assistance and return instructions. Send the product, transportation
prepaid, to the indicated service facility. Repairs will be made and the product returned, transportation prepaid.
Repaired or replaced products are warranted for the balance of the original warranty period, or at least 90 days.

LIMITATION OF WARRANTY

This warranty does not apply to defects resulting from product modification without Keithley’s express written
consent, or misuse of any product or part. This warranty also does not apply to fuses, software, non-rechargeable
batteries, damage from battery leakage, or problems arising from normal wear or failure to follow instructions.

THIS WARRANTY IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED OR IMPLIED, INCLUDING
ANY IMPLIED WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. THE
REMEDIES PROVIDED HEREIN ARE BUYER’S SOLE AND EXCLUSIVE REMEDIES.

NEITHER KEITHLEY INSTRUMENTS, INC. NOR ANY OF ITS EMPLOYEES SHALL BE LIABLE FOR
ANY DIRECT, INDIRECT, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT
OF THE USE OF ITS INSTRUMENTS AND SOFTWARE EVEN IF KEITHLEY INSTRUMENTS, INC., HAS
BEEN ADVISED IN ADVANCE OF THE POSSIBILITY OF SUCH DAMAGES. SUCH EXCLUDED DAM­AGES SHALL INCLUDE, BUT ARE NOT LIMITED TO: COSTS OF REMOVAL AND INSTALLATION,
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2/02

AC Signal Analysis

Using Models 2015, 2015-P, and 2016 THD Multimeters

Quick Reference Guide

©2001, Keithley Instruments, Inc.

All rights reserved.

Cleveland, Ohio, U.S.A.

First Printing, December 2001

Document Number: 2015-903-01 Rev. A

Manual Print History

The print history shown below lists the printing dates of all Revisions and Addenda created
for this manual. The Revision Level letter increases alphabetically as the manual undergoes sub­sequent updates. Addenda, which are released between Revisions, contain important change in­formation that the user should incorporate immediately into the manual. Addenda are numbered
sequentially. When a new Revision is created, all Addenda associated with the previous Revision
of the manual are incorporated into the new Revision of the manual. Each new Revision includes
a revised copy of this print history page.

Revision A (Document Number 2015-903-01)....................................................... December 2001

All Keithley product names are trademarks or registered trademarks of Keithley Instruments, Inc.
Other brand names are trademarks or registered trademarks of their respective holders.

S

afety Precautions

The following safety precautions should be observed before using this product and any associated instrumentation. Although
some instruments and accessories would normally be used with non-hazardous voltages, there are situations where hazardous
conditions may be present.

This product is intended for use by qualified personnel who recognize shock hazards and are familiar with the safety precautions
required to avoid possible injury. Read and follow all installation, operation, and maintenance information carefully before us­ing the product. Refer to the manual for complete product specifications.

If the product is used in a manner not specified, the protection provided by the product may be impaired.

The types of product users are:

Responsible body

ment is operated within its specifications and operating limits, and for ensuring that operators are adequately trained.

Operators

instrument. They must be protected from electric shock and contact with hazardous live circuits.

Maintenance personnel

voltage or replacing consumable materials. Maintenance procedures are described in the manual. The procedures explicitly state
if the operator may perform them. Otherwise, they should be performed only by service personnel.

Service personnel

trained service personnel may perform installation and service procedures.

Keithley products are designed for use with electrical signals that are rated Installation Category I and Installation Category II,
as described in the International Electrotechnical Commission (IEC) Standard IEC 60664. Most measurement, control, and data
I/O signals are Installation Category I and must not be directly connected to mains voltage or to voltage sources with high tran­sient over-voltages. Installation Category II connections require protection for high transient over-voltages often associated with
local AC mains connections. Assume all measurement, control, and data I/O connections are for connection to Category I sourc­es unless otherwise marked or described in the Manual.

Exercise extreme caution when a shock hazard is present. Lethal voltage may be present on cable connector jacks or test fixtures.
The American National Standards Institute (ANSI) states that a shock hazard exists when voltage levels greater than 30V RMS,

42.4V peak, or 60VDC are present.

circuit before measuring.

Operators of this product must be protected from electric shock at all times. The responsible body must ensure that operators
are prevented access and/or insulated from every connection point. In some cases, connections must be exposed to potential
human contact. Product operators in these circumstances must be trained to protect themselves from the risk of electric shock.
If the circuit is capable of operating at or above 1000 volts,

Do not connect switching cards directly to unlimited power circuits. They are intended to be used with impedance limited sourc­es. NEVER connect switching cards directly to AC mains. When connecting sources to switching cards, install protective de­vices to limit fault current and voltage to the card.

Before operating an instrument, make sure the line cord is connected to a properly grounded power receptacle. Inspect the con­necting cables, test leads, and jumpers for possible wear, cracks, or breaks before each use.

When installing equipment where access to the main power cord is restricted, such as rack mounting, a separate main input pow­er disconnect device must be provided, in close proximity to the equipment and within easy reach of the operator.

For maximum safety, do not touch the product, test cables, or any other instruments while power is applied to the circuit under
test. ALWAYS remove power from the entire test system and discharge any capacitors before: connecting or disconnecting ca­bles or jumpers, installing or removing switching cards, or making internal changes, such as installing or removing jumpers.

is the individual or group responsible for the use and maintenance of equipment, for ensuring that the equip-

use the product for its intended function. They must be trained in electrical safety procedures and proper use of the

perform routine procedures on the product to keep it operating properly, for example, setting the line

are trained to work on live circuits, and perform safe installations and repairs of products. Only properly

A good safety practice is to expect that hazardous voltage is present in any unknown

no conductive part of the circuit may be exposed.

Do not touch any object that could provide a current path to the common side of the circuit under test or power line (earth) ground. Al­ways make measurements with dry hands while standing on a dry, insulated surface capable of withstanding the voltage being measured.

The instrument and accessories must be used in accordance with its specifications and operating instructions or the safety of the
equipment may be impaired.

Do not exceed the maximum signal levels of the instruments and accessories, as defined in the specifications and operating in­formation, and as shown on the instrument or test fixture panels, or switching card.

When fuses are used in a product, replace with same type and rating for continued protection against fire hazard.

Chassis connections must only be used as shield connections for measuring circuits, NOT as safety earth ground connections.

If you are using a test fixture, keep the lid closed while power is applied to the device under test. Safe operation requires the use
of a lid interlock.

If a screw is present, connect it to safety earth ground using the wire recommended in the user documentation.

!

The symbol on an instrument indicates that the user should refer to the operating instructions located in the manual.

The symbol on an instrument shows that it can source or measure 1000 volts or more, including the combined effect of
normal and common mode voltages. Use standard safety precautions to avoid personal contact with these voltages.

The

WARNING

information very carefully before performing the indicated procedure.

The

CAUTION

ranty.

Instrumentation and accessories shall not be connected to humans.

Before performing any maintenance, disconnect the line cord and all test cables.

To maintain protection from electric shock and fire, replacement components in mains circuits, including the power transformer,
test leads, and input jacks, must be purchased from Keithley Instruments. Standard fuses, with applicable national safety ap­provals, may be used if the rating and type are the same. Other components that are not safety related may be purchased from
other suppliers as long as they are equivalent to the original component. (Note that selected parts should be purchased only
through Keithley Instruments to maintain accuracy and functionality of the product.) If you are unsure about the applicability
of a replacement component, call a Keithley Instruments office for information.

To clean an instrument, use a damp cloth or mild, water based cleaner. Clean the exterior of the instrument only. Do not apply
cleaner directly to the instrument or allow liquids to enter or spill on the instrument. Products that consist of a circuit board with
no case or chassis (e.g., data acquisition board for installation into a computer) should never require cleaning if handled accord­ing to instructions. If the board becomes contaminated and operation is affected, the board should be returned to the factory for
proper cleaning/servicing.

heading in a manual explains dangers that might result in personal injury or death. Always read the associated

heading in a manual explains hazards that could damage the instrument. Such damage may invalidate the war-

11/01

Table of Contents

About this guide ............................................................................ 1

Measurements overview ................................................................ 2

Harmonic distortion measurement ......................................... 2

AC volts and frequency measurements .................................. 2

Frequency spectrum analysis (Model 2015-P only) .............. 2

Measuring harmonic distortion ..................................................... 3

Harmonic distortion calculations: .......................................... 3

Distortion measurement operations ....................................... 4

Distortion measurement examples ....................................... 11

Measuring AC volts and frequency ............................................. 14

Analyzing frequency spectrum (Model 2015-P only) ................. 15

Frequency spectrum ............................................................. 16

Basic peak analysis .............................................................. 16

Delta function ....................................................................... 22

Frequency list analysis ......................................................... 24

Programming examples ............................................................... 26

Programming sine wave source ........................................... 26

Measuring THD, THD+noise, or SINAD. ........................... 27

RMS voltage measurement .................................................. 28

Bandwidth-limited RMS voltage measurement ................... 29

Background noise measurement .......................................... 30

Fast frequency sweep RMS voltage measurements ............. 31

Frequency domain components ........................................... 32

Measurement of individual harmonics ................................. 33

Peak measurements .............................................................. 34

Measuring differences between two peaks .......................... 35

THD — Total harmonic distortion .................................. 3

THD+N — Total harmonic distortion plus noise ........... 3

SINAD ............................................................................ 4

Distortion measurement notes ...................................... 10

THD measurement example ......................................... 11

Distortion and RMS volts sweep example .................... 12

Frequency bins .............................................................. 16

Frequency boundaries ................................................... 16

MAX? and NEXT? commands ..................................... 16

SFR <freq> and LOC? commands ............................... 18

LEFT? and RIGHT? commands ................................... 19

Peak analysis example .................................................. 20

Acquiring reference frequency and amplitude .............. 22

Move frequency pointer and calculate delta ................. 22

Delta function example ................................................. 23

Creating frequency list .................................................. 24

Returning amplitudes for listed frequencies ................. 25

Frequency list example ................................................. 25

List of Illustrations

Figure 1 Source connections to input ...................................................... 11

Figure 2 Frequency spectrum example ................................................... 16

Figure 3 Peak analysis example using MAX? and NEXT? commands .. 17

Figure 4 Peak analysis example using SFR and LOC? commands ........ 18

Figure 5 Peak analysis example using RIGHT? command ..................... 19

Figure 6 DELTA? peak analysis example ............................................... 23

Figure 7 Connections to double generator amplitude ............................. 26

AC Signal Analysis

Using Models 2015, 2015-P, and

2016 THD Multimeters

About this guide

This guide is designed to familiarize users with basic Model 2015/2015-P/2016 operations
to analyze an AC input signal. Operations include harmonic distortion measurements, standard
AC volts and frequency measurements, and frequency spectrum analysis (Model 2015-P only).

This guide covers front panel operation and remote operation over the GPIB or RS-232. The
SCPI commands provided are generic; actual syntax depends on the test programming lan­guage being used.

Information in this guide is organized as follows:

•

Measurements overview

signal.

Measuring harmonic distortion:

•

Harmonic distortion calculations

three measurement types; THD, THD + noise, and SINAD.

Distortion measurement operations

remote commands) used to configure and measure distortion.

Distortion measurement examples

distortion (THD), and an example that uses a source sweep to measure distortion and
RMS volts.

•

Measuring AC volts and frequency

volts or frequency.

Analyzing frequency spectrum (Model 2015-P only):

•

Basic peak analysis

frequencies in the spectrum. Includes a remote command sequence example to find and
measure the frequencies that have the three highest amplitudes.

Delta function

amplitude between two frequency pointer positions. Includes a command sequence
example.

Frequency list analysis

amplitudes of the listed frequencies. Includes a command sequence example.

– Covers the commands to measure the difference in frequency and

– Summarizes the measurements used to analyze an AC input

– Shows how harmonic distortion is calculated for the

– Covers the various operations (front panel and

– Includes an example to measure total harmonic

– Provides the basic procedure to measure AC

– Covers the commands used to measure the peak amplitudes of the

– Covers the commands to create a frequency list and return the

2 Using Models 2015, 2015-P, and 2016 THD Multimeters

Measurements overview

Harmonic distortion measurement

The Model 2015/2015-P/2016 uses a DSP (Digital Signal Processor) to perform a Fast Fou­rier Transform (FFT) on a voltage signal applied to the front or rear voltage inputs. It then ana­lyzes the levels of the harmonics present in the signal to calculate THD, THD+noise, SINAD,
or RMS volts. The instrument can measure harmonic distortion from 20Hz to 50kHz.

A DDS (Direct Digital Synthesis) module is included in the THD circuitry to provide a pro­grammable sine source. The source has a second output which can be set to provide the inverse
of the sine output (shifted 180 degrees), or to put out a 0-5V logic level pulse in phase with the
main output.

A sweep feature enables the user to program a list of up to 200 sweep frequencies and
amplitudes, then have the unit return distortion and/or RMS volts for each frequency.

AC volts and frequency measurements

The Model 2015/2015-P/2016 can measure true RMS AC voltage from 0.1µV to 750V
(1000V peak). It can also measure frequency from 3Hz to 500kHz.

Frequency spectrum analysis (Model 2015-P only)

Using remote command programming, the Model 2015-P can measure the amplitudes of
frequencies in a spectrum. The lower bound for the frequency spectrum can be set as low as
20Hz, while the upper bound can be set as high as 20480Hz. The 20Hz and 20480Hz bound­aries are the default settings.

The Model 2015-P can perform the following operations:

• Determine the amplitude for any individual frequency in the spectrum. Values are
returned for the frequency (in Hz) and the amplitude (in dBV).

• Calculate the difference in frequency and amplitude between two different frequencies.
The results of the calculation are returned in Hz and dBV.

• Determine the amplitudes of a specified list of frequencies. Frequency/amplitude pairs
are returned for every frequency in the list.

Using Models 2015, 2015-P, and 2016 THD Multimeters 3

Measuring harmonic distortion

Harmonic distortion calculations:

THD — Total harmonic distortion

THD is the default type for the THD measurement and is defined as follows:

2

V2

V32... Vi

THD% 100*

Where V2 through Vi represent the RMS levels of the harmonics of the signal, and V1 is the

RMS level of the fundamental frequency.

THD can also be expressed in dB, as follows:

THD dB()20*log

=

+++

---------------------------------------------------------=
V1

2



---------------------------------------------------------




V2

V32... Vi

V1

2

2

+++





THD+N — Total harmonic distortion plus noise

THD+N is what conventional (analog) THD meters display. A conventional THD meter has
a notch filter that removes the fundamental frequency from the signal, and measures THD
based on what is left. This includes all of the harmonics, but also includes any random noise in
the signal. Since the instrument uses a digital signal processor to perform a Fourier transform
on the signal, noise can be eliminated from the THD measurement, thus providing a true read­ing. The THD+noise feature was included to duplicate what a conventional THD meter would
read.

Mathematically, THD+noise is expressed as follows:

THD Noise+dB()20*log

=

2



----------------------------------------------------------------------




V32... Vi2N

V2

++ ++

V1

2





Where N is the RMS level of the noise in the signal. THD+noise can also be expressed in
percent.

4 Using Models 2015, 2015-P, and 2016 THD Multimeters

SINAD

This is another way of expressing THD+noise. It is expressed as the ratio of the total signal
(fundamental, noise, and harmonics) to the total signal minus the fundamental. This measure­ment is only expressed in dB.



2

V1

V2

V22V32... Vn2N2+++++

2

V32... Vn2N2++++

SINAD dB()20*log

=



----------------------------------------------------------------------------------------




Distortion measurement operations

1 Selecting distortion mode

Front panel:

Press SHIFT and then ACV

Remote command:

:SENS:FUNC "DIST" ' Selects distortion mode.

2 Selecting THD measurement type (THD, THD+Noise or SINAD)

Front panel:

a. Press THD-MEAS to display “TYPE:”.

b. Use the right arrow key to highlight the present type selection, then use the up or down

range keys to select THD, THD+N or SINAD.

c. Press ENTER, then press EXIT.

Remote command:






:SENS:DIST:TYPE THD|THDN|SINAD ' Selects THD measurement type.

3 Selecting THD frequency type (Auto, Set, or Acquire)

The THD measurement technique used by the Model 2015/2015-P/2016 requires that the
unit know the fundamental frequency of the signal exactly. Even a difference of a few hertz will
cause large errors. The AUTO setting will cause the Model 2015/2015-P/2016 to measure the
frequency of a source every time it makes a THD measurement. This will slow down the read­ing rate, but is useful if the source frequency is unknown or unstable. SET will prompt the user
to manually enter the frequency.

ACQUIRE is similar to AUTO, but only measures the frequency once, then uses that fre­quency for all subsequent THD measurements. The signal source must be applied to the input
when ACQUIRE is selected.

Using Models 2015, 2015-P, and 2016 THD Multimeters 5

Front panel:

a. Press THD-MEAS.

b. Press ENTER once. The display will show “FREQ:”.

c. Using the left/right arrow and up/down range keys, select ACQUIRE, AUTO, or SET.

(Make sure the signal source is connected and on when ACQUIRE is selected.)

d. Press ENTER. If SET was selected, the instrument will prompt the user for the fre-

quency to be used. Use the left/right arrow and up/down range keys to set the frequency,
and press ENTER.

e. Press EXIT.

Remote commands:

:SENS:DIST:FREQ:AUTO ON|OFF ' Turns AUTO on or off.
:SENS:DIST:FREQ:ACQ ' Acquires frequency once.
:SENS:DIST:FREQ <NRf> ' Sets frequency in Hz; disables AUTO

' frequency (if enabled).
' <NRf> = 20 to 20000

4 Setting number of harmonics to use in the THD measurement (2 to 64)

The Model 2015/2015-P/2016 can use up to the 64th harmonic of a signal (64 times the fun­damental frequency) in the THD measurement, but only recognizes frequencies below 50kHz.
Harmonics with frequencies above 50kHz are ignored, therefore acting as an adjustable low­pass filter.

Front panel:

a. Press THD-MEAS.

b. Press ENTER until the display shows “HARMONICS:”.

c. Set the number of the highest harmonic to use in the measurement using the left/right

arrow and up/down range keys.

d. Press ENTER, and then press EXIT.

Remote command:

:SENS:DIST:HARM <NRf> ' Sets the number of harmonics to

' include in the THD measurement.
' <NRf> = 2 to 64

6 Using Models 2015, 2015-P, and 2016 THD Multimeters

5 Selecting THD measurement units (percent or dB)

Front panel:

a. Press THD-MEAS.
b. Press ENTER until the display shows “UNITS:”.
c. Select PERCENT or dB using the left/right arrow and up/down range keys.
d. Press ENTER, and then EXIT.

Remote command:

:UNIT:DIST PERC|DB ' Selects percent or dB units for THD.

6 Selecting the THD shaping filter (None, C-weighting, CCITT,

CCIRARM, A-weighting, or CCIR).

The Model 2015/2015-P/2016 includes digital shaping filters to simulate having the sample

signal pass through various types of telephone lines. The default filter setting is NONE.

Front panel:

a. Press THD-MEAS.
b. Press ENTER until the display shows “SFIL:”.
c. Select NONE, C, CCITT, CCIRARM, A, or CCIR using the left/right arrow and up/

down range keys.

d. Press ENTER.

Remote command:

:SENS:DIST:SFIL NONE|C|CCITT|CCIRARM|A|CCIR ' Selects shaping

' filter for THD.

7 Setting the THD voltage input range

The Model 2015/2015-P/2016 defaults to autoranging for the voltage input range. If using
manual ranging, it is important to use the lowest range possible for the signal level. Not using
the appropriate range will cause inaccurate readings, or the display may show “underflow” or
“overflow”.

Front panel:

With the instrument in the THD mode, press the up range or down range key. The display
will show the chosen range. Otherwise, press AUTO for autoranging.

Remote commands:

:SENS:DIST:RANG <n> ' Specify expected voltage to set

' range; turns off autoranging.
' <n> = 0 to 757.5

:SENS:DIST:RANG:AUTO ON|OFF ' Turns autoranging on or off.