Rohde & Schwarz ZNL Series User manual

R&S®ZNL
I/Q Analyzer
User Manual
User Manual
Version 06
1178598902
(;ÜÉç2)

This manual applies to the following R&S ZNL models with firmware version 1.30 and higher:
●R&S®ZNL3 (2 ports, 9 kHz to 3 GHz, N connectors), order no. 1311.6004K12
●R&S®ZNL6 (2 ports, 5 kHz to 6 GHz, female N connectors), order no. 1323.0012.06
The following firmware options are described:
●R&S®ZNL3-B1 Spectrum Analysis (1323.1802.02)
●R&S®ZNL6-B1 Spectrum Analysis (1323.2067.02)
In addition to the I/Q Analyzer application, the following options are described:
●R&S®FPL1-B5, Additional Interfaces (1323.1883.02)
●R&S®FPL1-K9 Power Sensor Support (1323.1754.02)
●R&S®FPL1-B40, Bandwidth extension 40 MHz (1323.1931.02)
© 2019 Rohde & Schwarz GmbH & Co. KG
Mühldorfstr. 15, 81671 München, Germany
Phone: [phone] - 0
Fax: [phone]
Email: [email protected]
Internet: www.rohde-schwarz.com
Subject to change – Data without tolerance limits is not binding.
R&S® is a registered trademark of Rohde & Schwarz GmbH & Co. KG.
Trade names are trademarks of the owners.
1178.5989.02 | Version 06 | R&S®ZNL
Throughout this manual, products from Rohde & Schwarz are indicated without the ® symbol, e.g. R&S®ZNL is indicated as R&S
ZNL.

Contents
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3User Manual 1178.5989.02 ─ 06
Contents
1 Preface.................................................................................................... 5
1.1 For Your Safety..............................................................................................................5
1.2 Documentation Overview............................................................................................. 5
1.3 Conventions Used in the Documentation...................................................................7
2 Welcome to the I/Q Analyzer Application............................................ 9
2.1 Starting the I/Q Analyzer Application..........................................................................9
2.2 Understanding the Display Information....................................................................10
3 Measurement and Result Displays.....................................................13
4 Basics on I/Q Data Acquisition and Processing............................... 17
4.1 Processing Analog I/Q Data from RF Input.............................................................. 17
4.2 Basics on FFT..............................................................................................................20
4.3 Basics on Input from I/Q Data Files...........................................................................25
4.4 I/Q Data Import and Export.........................................................................................26
5 Configuration........................................................................................27
5.1 Configuration Overview..............................................................................................27
5.2 Import/Export Functions............................................................................................ 29
5.3 Receiving Data Input and Providing Data Output.................................................... 32
5.4 Amplitude.....................................................................................................................41
5.5 Frequency Settings.....................................................................................................46
5.6 Trigger Settings...........................................................................................................47
5.7 Data Acquisition and Bandwidth Settings................................................................51
5.8 Display Configuration.................................................................................................57
5.9 Adjusting Settings Automatically..............................................................................57
6 Analysis................................................................................................ 60
6.1 Trace Settings..............................................................................................................60
6.2 Spectrogram Settings.................................................................................................64
6.3 Trace / Data Export Configuration............................................................................. 69
6.4 Marker Usage...............................................................................................................72
7 How to Perform Measurements in the I/Q Analyzer Application..... 95

Contents
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7.1 How to Capture Baseband (I/Q) Data as RF Input....................................................95
7.2 How to Analyze Data in the I/Q Analyzer.................................................................. 96
8 How to Export and Import I/Q Data.....................................................97
9 Remote Commands to Perform Measurements with I/Q Data......... 99
9.1 Introduction................................................................................................................. 99
9.2 Common Suffixes......................................................................................................104
9.3 Activating I/Q Analyzer Measurements...................................................................104
9.4 Configuring I/Q Analyzer Measurements................................................................109
9.5 Configuring the Result Display................................................................................143
9.6 Capturing Data and Performing Sweeps................................................................ 150
9.7 I/Q Analysis................................................................................................................156
9.8 Retrieving Results.....................................................................................................204
9.9 Importing and Exporting I/Q Data and Results...................................................... 214
9.10 Programming Examples........................................................................................... 216
Annex.................................................................................................. 219
A Formats for Returned Values: ASCII Format and Binary Format.. 219
B Reference: Format Description for I/Q Data Files........................... 220
C I/Q Data File Format (iq-tar)...............................................................222
C.1 I/Q Parameter XML File Specification......................................................................223
C.2 I/Q Data Binary File................................................................................................... 226
List of Commands..............................................................................228
Index....................................................................................................234

Preface
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1 Preface
This chapter provides safety-related information, an overview of the user documenta-
tion and the conventions used in the documentation.
1.1 For Your Safety
The R&S ZNL is designated for use in industrial, administrative, and laboratory envi-
ronments. Use the R&S ZNL only for its designated purpose. Observe the safety and
usage instructions documented in the user manual, as well as operating conditions and
performance limits stated in the data sheet.
The product documentation helps you use the R&S ZNL safely and efficiently. Keep the
product documentation in a safe place and pass it on to subsequent users.
Safety information is part of the product documentation. It warns you about potential
dangers and gives instructions how to prevent personal injury or damage caused by
dangerous situations. Safety information is provided as follows:
●In the "Basic Safety Instructions", safety issues are grouped according to subjects.
For example, one subject is electrical safety. The "Basic Safety Instructions" are
delivered with the R&S ZNL in different languages in print.
●Throughout the documentation, safety instructions are provided when you need to
take care during setup or operation. Always read the safety instructions carefully.
Make sure to comply fully with them. Do not take risks and do not underestimate
the potential danger of small details such as a damaged power cable.
1.2 Documentation Overview
This section provides an overview of the R&S ZNL user documentation. Unless speci-
fied otherwise, you find the documents on the R&S ZNL product page at:
www.rohde-schwarz.com/manual/ZNL or www.rohde-schwarz.com/manual/ZNLE.
1.2.1 Getting Started Manual
Introduces the R&S ZNL and describes how to set up and start working with the prod-
uct. Includes basic operations, typical measurement examples, and general informa-
tion, e.g. safety instructions, etc.
A printed version is delivered with the instrument. A PDF version is available for down-
load on the Internet.
Documentation Overview

Preface
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6User Manual 1178.5989.02 ─ 06
1.2.2 User Manuals and Help
Separate user manuals are provided for the base unit and the firmware applications:
●Base unit manual
Contains the description of all instrument modes and functions. It also provides an
introduction to remote control, a complete description of the remote control com-
mands with programming examples, and information on maintenance, instrument
interfaces and error messages. Includes the contents of the getting started manual.
●Firmware application manual
Contains the description of the specific functions of a firmware application, includ-
ing remote control commands. Basic information on operating the R&S ZNL is not
included.
The contents of the user manuals are available as help in the R&S ZNL. The help
offers quick, context-sensitive access to the complete information for the base unit and
the firmware applications.
All user manuals are also available for download or for immediate display on the Inter-
net.
1.2.3 Service Manual
Describes the performance test for checking the rated specifications, module replace-
ment and repair, firmware update, troubleshooting and fault elimination, and contains
mechanical drawings and spare part lists.
The service manual is available for registered users on the global Rohde & Schwarz
information system (GLORIS):
https://gloris.rohde-schwarz.com
1.2.4 Instrument Security Procedures
Deals with security issues when working with the R&S ZNL in secure areas. It is availa-
ble for download on the Internet.
1.2.5 Basic Safety Instructions
Contains safety instructions, operating conditions and further important information.
The printed document is delivered with the instrument.
1.2.6 Data Sheets and Brochures
The data sheet contains the technical specifications of the R&S ZNL. It also lists the
firmware applications and their order numbers, and optional accessories.
The brochure provides an overview of the instrument and deals with the specific char-
acteristics.
Documentation Overview

Preface
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7User Manual 1178.5989.02 ─ 06
See www.rohde-schwarz.com/brochure-datasheet/ZNL or www.rohde-schwarz.com/
brochure-datasheet/ZNLE.
1.2.7 Release Notes and Open Source Acknowledgment (OSA)
The release notes list new features, improvements and known issues of the current
firmware version, and describe the firmware installation.
The open source acknowledgment document provides verbatim license texts of the
used open source software.
See www.rohde-schwarz.com/firmware/ZNL or www.rohde-schwarz.com/firmware/
ZNLE.
1.2.8 Application Notes, Application Cards, White Papers, etc.
These documents deal with special applications or background information on particu-
lar topics.
See www.rohde-schwarz.com/application/ZNL or www.rohde-schwarz.com/application/
ZNLE.
1.2.9 Calibration Certificate
The document is available on https://gloris.rohde-schwarz.com/calcert. You need the
device ID of your instrument, which you can find on a label on the rear panel.
1.3 Conventions Used in the Documentation
1.3.1 Typographical Conventions
The following text markers are used throughout this documentation:
Convention Description
"Graphical user interface ele-
ments"
All names of graphical user interface elements on the screen, such as
dialog boxes, menus, options, buttons, and softkeys are enclosed by
quotation marks.
[Keys] Key and knob names are enclosed by square brackets.
Filenames, commands,
program code
Filenames, commands, coding samples and screen output are distin-
guished by their font.
Input Input to be entered by the user is displayed in italics.
Conventions Used in the Documentation

Preface
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8User Manual 1178.5989.02 ─ 06
Convention Description
Links Links that you can click are displayed in blue font.
"References" References to other parts of the documentation are enclosed by quota-
tion marks.
1.3.2 Conventions for Procedure Descriptions
When operating the instrument, several alternative methods may be available to per-
form the same task. In this case, the procedure using the touchscreen is described.
Any elements that can be activated by touching can also be clicked using an addition-
ally connected mouse. The alternative procedure using the keys on the instrument or
the on-screen keyboard is only described if it deviates from the standard operating pro-
cedures.
The term "select" may refer to any of the described methods, i.e. using a finger on the
touchscreen, a mouse pointer in the display, or a key on the instrument or on a key-
board.
1.3.3 Notes on Screenshots
When describing the functions of the product, we use sample screenshots. These
screenshots are meant to illustrate as many as possible of the provided functions and
possible interdependencies between parameters. The shown values may not represent
realistic usage scenarios.
The screenshots usually show a fully equipped product, that is: with all options instal-
led. Thus, some functions shown in the screenshots may not be available in your par-
ticular product configuration.
Conventions Used in the Documentation

Welcome to the I/Q Analyzer Application
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2 Welcome to the I/Q Analyzer Application
The R&S FPL1 I/Q Analyzer is a firmware application that adds functionality to perform
I/Q data acquisition and analysis to the R&S ZNL.
The R&S FPL1 I/Q Analyzer features:
●Acquisition of analog I/Q data
●Import of stored I/Q data from other applications
●Spectrum, magnitude, I/Q vector and separate I and Q component analysis of any
I/Q data on the instrument
●Export of I/Q data to other applications
Availability of the I/Q Analyzer
The I/Q Analyzer becomes available when you equip the R&S ZNL with the optional
Spectrum Analyzer hardware (R&S ZNL-B1).
This user manual contains a description of the functionality that the application pro-
vides, including remote control operation.
All functions not discussed in this manual are the same as in the base unit and are
described in the R&S ZNL User Manual. The latest version is available for download at
the product homepage http://www.rohde-schwarz.com/product/ZNL.
Additional information
Several application notes discussing I/Q analysis are available from the Rohde &
Schwarz website:
1EF85: Converting R&S I/Q data files
1EF92: Wideband Signal Analysis
1MA257: Wideband mm-Wave Signal Generation and Analysis
1EF84: Differential measurements with Spectrum Analyzers and Probes
Installation
The R&S FPL1 I/Q Analyzer application is part of the optional Spectrum application
and requires no further installation.
2.1 Starting the I/Q Analyzer Application
The I/Q Analyzer is an application on the R&S ZNL.
To activate the I/Q Analyzer application
1. Select the [MODE] key.
Starting the I/Q Analyzer Application

Welcome to the I/Q Analyzer Application
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A dialog box opens that contains all applications currently available on your
R&S ZNL.
2. Select the "I/Q Analyzer" item.
The R&S ZNL opens a new channel setup for the I/Q Analyzer application.
The measurement is started immediately with the default settings.
It can be configured in the I/Q Analyzer "Overview" dialog box, which is displayed
when you select the "Overview" softkey from any menu (see Chapter 5.1, "Configura-
tion Overview", on page 27).
Multiple Channel Setups and Sequencer Function
When you activate an application, a new channel setup is created which determines
the measurement settings for that application (channel setup). The same application
can be activated with different measurement settings by creating several channel set-
ups for the same application.
The number of channel setups that can be configured at the same time depends on the
available memory on the instrument.
Only one measurement can be performed at any time, namely the one in the currently
active channel setup. However, in order to perform the configured measurements con-
secutively, a Sequencer function is provided.
If activated, the measurements configured in the currently defined channel setups are
performed one after the other in the order of the tabs. The currently active measure-
ment is indicated by a symbol in the tab label.
The result displays of the individual channel setups are updated in the tabs (as well as
the "MultiView" ) as the measurements are performed. Sequential operation itself is
independent of the currently displayed tab.
For details on the Sequencer function see the R&S ZNL User Manual.
2.2 Understanding the Display Information
The following figure shows a measurement diagram during I/Q Analyzer operation. All
different information areas are labeled. They are explained in more detail in the follow-
ing sections.
Understanding the Display Information

Welcome to the I/Q Analyzer Application
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1
2 3
4
56
Figure 2-1: Screen elements in the I/Q Analyzer application
1 = Channel Setup bar for firmware and measurement settings
2+3 = Window title bar with diagram-specific (trace) information
4 = Diagram area with marker information
5 = Diagram footer with diagram-specific information, depending on result display
6 = Instrument status bar with error messages and date/time display
Channel Setup bar information
In the I/Q Analyzer application, the R&S ZNL shows the following settings:
Table 2-1: Information displayed in the channel setup bar for the I/Q Analyzer application
Ref Level Reference level
(m.+el.)Att (Mechanical and electronic) RF attenuation
Ref Offset Reference level offset
Freq Center frequency
Meas Time Measurement time
Rec Length Defined record length (number of samples to capture)
SRate Defined sample rate for data acquisition
RBW (Spectrum evaluation only) Resolution bandwidth calculated from the
sample rate and record length
In addition, the channel setup bar also displays information on instrument settings that
affect the measurement results even though this is not immediately apparent from the
display of the measured values (e.g. transducer or trigger settings). This information is
displayed only when applicable for the current measurement. For details see the
R&S ZNL Getting Started manual.
Understanding the Display Information

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12User Manual 1178.5989.02 ─ 06
Window title bar information
For each diagram, the header provides the following information:
Figure 2-2: Window title bar information in the I/Q Analyzer application
1 = Window number
2 = Window type
3 = Trace color
4 = Trace number
5 = Detector
6 = Trace mode
Diagram footer information
The information in the diagram footer (beneath the diagram) depends on the evalua-
tion:
●Center frequency
●Number of sweep points
●Range per division (x-axis)
●Span (Spectrum)
Status bar information
Global instrument settings, the instrument status and any irregularities are indicated in
the status bar beneath the diagram.
Understanding the Display Information

Measurement and Result Displays
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3 Measurement and Result Displays
Access: "Overview" > "Display Config"
Or: [MEAS] > "Display Config"
The I/Q Analyzer can capture I/Q data. The I/Q data that was captured by or imported
to the R&S ZNL can then be evaluated in various different result displays. Select the
result displays using the SmartGrid functions.
For details on working with the SmartGrid see the R&S ZNL Getting Started manual.
Result displays for I/Q data:
Magnitude .................................................................................................................... 13
Spectrum ......................................................................................................................13
I/Q-Vector ..................................................................................................................... 14
Real/Imag (I/Q) .............................................................................................................15
Marker Table ................................................................................................................ 15
Marker Peak List .......................................................................................................... 15
Magnitude
Shows the level values in time domain.
Remote command:
LAY:ADD:WIND? '1',RIGH,MAGN, see LAYout:ADD[:WINDow]? on page 144
Results:
TRACe<n>[:DATA]? on page 208
Spectrum
Displays the frequency spectrum of the captured I/Q samples.

Measurement and Result Displays
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The specified Analysis Bandwidth is indicated by vertical blue lines.
Note that a peak search is performed only within the indicated Analysis Bandwidth ,
unless you specify Search Limits ( Left / Right ) in the marker settings.
Remote command:
LAY:ADD:WIND? '1',RIGH,FREQ, see LAYout:ADD[:WINDow]? on page 144
Results:
TRACe<n>[:DATA]? on page 208
I/Q-Vector
Displays the captured samples in an I/Q-plot. The samples are connected by a line.
Note: For the I/Q vector result display, the number of I/Q samples to record ( "Record
Length" ) must be identical to the number of trace points to be displayed ("Sweep
Points"; for I/Q Analyzer: 10001). For record lengths outside the valid range of sweep
points the diagram does not show valid results.

Measurement and Result Displays
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Remote command:
LAY:ADD:WIND? '1',RIGH,VECT, see LAYout:ADD[:WINDow]? on page 144
Results:
TRACe<n>[:DATA]? on page 208
Real/Imag (I/Q)
Displays the I and Q values in separate diagrams.
Remote command:
LAY:ADD:WIND? '1',RIGH,RIM, see LAYout:ADD[:WINDow]? on page 144
Results:
TRACe<n>[:DATA]? on page 208
Marker Table
Displays a table with the current marker values for the active markers.
This table is displayed automatically if configured accordingly (see " Marker Table Dis-
play " on page 76).
Tip: To navigate within long marker tables, simply scroll through the entries with your
finger on the touchscreen.
Remote command:
LAY:ADD? '1',RIGH, MTAB, see LAYout:ADD[:WINDow]? on page 144
Results:
CALCulate<n>:MARKer<m>:X on page 175
CALCulate<n>:MARKer<m>:Y on page 213
Marker Peak List
The marker peak list determines the frequencies and levels of peaks in the spectrum or
time domain. How many peaks are displayed can be defined, as well as the sort order.
In addition, the detected peaks can be indicated in the diagram. The peak list can also
be exported to a file for analysis in an external application.

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Remote command:
LAY:ADD? '1',RIGH, PEAK, see LAYout:ADD[:WINDow]? on page 144
Results:
CALCulate<n>:MARKer<m>:X on page 175
CALCulate<n>:MARKer<m>:Y on page 213

Basics on I/Q Data Acquisition and Processing
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4 Basics on I/Q Data Acquisition and Pro-
cessing
Some background knowledge on basic terms and principles used when describing I/Q
data acquisition on the R&S ZNL in general, and in the I/Q Analyzer application in par-
ticular, is provided here for a better understanding of the required configuration set-
tings.
The I/Q Analyzer provides various possibilities to acquire the I/Q data to be analyzed:
●Capturing analog I/Q data from the [RF Input] connector
●Importing I/Q data from a file
Background information for all these scenarios and more is provided in the following
sections.
●Processing Analog I/Q Data from RF Input.............................................................17
●Basics on FFT.........................................................................................................20
●Basics on Input from I/Q Data Files........................................................................ 25
●I/Q Data Import and Export..................................................................................... 26
4.1 Processing Analog I/Q Data from RF Input
Complex baseband data
In the telephone systems of the past, baseband data was transmitted unchanged as an
analog signal. In modern phone systems and in radio communication, however, the
baseband data is modulated on a carrier frequency, which is then transmitted. The
receiver must demodulate the data based on the carrier frequency. When using mod-
ern modulation methods (e.g. QPSK, QAM etc.), the baseband signal becomes com-
plex. Complex data (or: I/Q data) consists of an imaginary (I) and a real (Q) compo-
nent.
Sweep vs sampling
The standard Spectrum application on the R&S ZNL performs frequency sweeps on
the input signal and measurements in the frequency and time domain. Other applica-
tions on the R&S ZNL, such as the I/Q Analyzer, sample and process the individual I
and Q components of the complex signal.
I/Q Analyzer - processing complex data from RF input
The I/Q Analyzer is a standard application used to capture and analyze I/Q data on the
R&S ZNL. By default, it assumes the I/Q data is modulated on a carrier frequency and
input via the "RF Input" connector on the R&S ZNL.
The A/D converter samples the IF signal at a rate of 100 MHz. The digital signal is
down-converted to the complex baseband, lowpass-filtered, and the sample rate is
Processing Analog I/Q Data from RF Input

Basics on I/Q Data Acquisition and Processing
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18User Manual 1178.5989.02 ─ 06
reduced. The analog filter stages in the analyzer cause a frequency response which
adds to the modulation errors. An equalizer filter before the resampler compensates
for this frequency response. The continuously adjustable sample rates are realized
using an optimal decimation filter and subsequent resampling on the set sample rate.
A dedicated memory (capture buffer) is available in the R&S ZNL for a maximum of
25 Msamples (25*1000*1000) of complex samples (pairs of I and Q data). The number
of complex samples to be captured can be defined (for restrictions refer to Chap-
ter 4.1.1, "Sample Rate and Maximum Usable I/Q Bandwidth for RF Input",
on page 18).
The block diagram in Figure 4-1 shows the analyzer hardware from the IF section to
the processor.
Figure 4-1: Block diagram illustrating the R&S ZNL signal processing for analog I/Q data (without
bandwidth extension options)
4.1.1 Sample Rate and Maximum Usable I/Q Bandwidth for RF Input
Definitions
●Input sample rate (ISR): the sample rate of the useful data provided by the device
connected to the input of the R&S ZNL
●(User, Output) Sample rate (SR): the user-defined sample rate (e.g. in the "Data
Acquisition" dialog box in the "I/Q Analyzer" application) which is used as the basis
for analysis or output
●Usable I/Q (Analysis) bandwidth: the bandwidth range in which the signal
remains undistorted in regard to amplitude characteristic and group delay; this
range can be used for accurate analysis by the R&S ZNL
●Record length: Number of I/Q samples to capture during the specified measure-
ment time; calculated as the measurement time multiplied by the sample rate
For the I/Q data acquisition, digital decimation filters are used internally in the
R&S ZNL. The passband of these digital filters determines the maximum usable I/Q
bandwidth. In consequence, signals within the usable I/Q bandwidth (passband)
Processing Analog I/Q Data from RF Input

Basics on I/Q Data Acquisition and Processing
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19User Manual 1178.5989.02 ─ 06
remain unchanged, while signals outside the usable I/Q bandwidth (passband) are
suppressed. Usually, the suppressed signals are noise, artifacts, and the second IF
side band. If frequencies of interest to you are also suppressed, try to increase the out-
put sample rate, which increases the maximum usable I/Q bandwidth.
As a rule, the usable I/Q bandwidth is proportional to the output sample rate. Yet, when
the I/Q bandwidth reaches the bandwidth of the analog IF filter (at very high output
sample rates), the curve breaks.
●Relationship Between Sample Rate, Record Length and Usable I/Q Bandwidth... 19
4.1.1.1 Relationship Between Sample Rate, Record Length and Usable I/Q Bandwidth
Up to the maximum bandwidth, the following rule applies:
Usable I/Q bandwidth = 0.8 * Output sample rate
Regarding the record length, the following rule applies:
Record length = Measurement time * sample rate
Maximum record length for RF input
The maximum record length is the maximum number of samples that can be captured.
Table 4-1: Maximum record length
Sample rate Maximum record length
100 Hz to 100 MHz 25 Msamples
The Figure 4-2 shows the maximum usable I/Q bandwidths depending on the output
sample rates.
Usable I/Q
bandwidth
[MHz]
10
Output sample
rate fout [MHz]
2010
5
15
20
30 40 50 60 70 80 90 100
25
30
35
40
RF-Input:
BW = 0.80 *
fout
16
12.8
Without
bandwidth
extension
With 40 MHz
bandwidth
ext. option
Figure 4-2: Relationship between maximum usable I/Q bandwidth and output sample rate
Processing Analog I/Q Data from RF Input

Basics on I/Q Data Acquisition and Processing
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4.2 Basics on FFT
The I/Q Analyzer measures the power of the signal input over time. To convert the time
domain signal to a frequency spectrum, an FFT (Fast Fourier Transformation) is per-
formed which converts a vector of input values into a discrete spectrum of frequencies.
t[s]
FFT
f[Hz]
4.2.1 Window Functions
The Fourier transformation is not performed on the entire captured data in one step.
Only a limited number of samples is used to calculate an individual result. This process
is called windowing.
After sampling in the time domain, each window is multiplied with a specific window
function. Windowing helps minimize the discontinuities at the end of the measured sig-
nal interval and thus reduces the effect of spectral leakage, increasing the frequency
resolution.
Various different window functions are provided in the R&S ZNL to suit different input
signals. Each of the window functions has specific characteristics, including some
advantages and some trade-offs. Consider these characteristics to find the optimum
solution for the measurement task.
Ignoring the window function - rectangular window
The rectangular window function is in effect not a function at all, it maintains the origi-
nal sampled data. This may be useful to minimize the required bandwidth. However, be
aware that if the window does not contain exactly one period of your signal, heavy
sidelobes may occur, which do not exist in the original signal.
Basics on FFT