➢
Operating Manual
Spectrum Analyzer
R&S®FSU3
1166.1660.03
R&S®FSU8
1166.1660.08
R&S®FSU26
1166.1660.26
Printed in Germany
R&S®FSU31
1166.1660.31
R&S®FSU32
1166.1660.32
R&S®FSU43
1166.1660.43
R&S®FSU46
1166.1660.46
R&S®FSU50
1166.1660.50
R&S®FSU67
1166.1660.67
1166.1725.12-06-
Test and Measurement Division
®
R&S
is a registered trademark of Rohde & Schwarz GmbH & Co. KG
Trade names are trademarks of the owners
1166.1725.12 0.2 E-3
EC Certificate of Conformity
Certificate No.: 2003-35, Page 1
This is to certify that:
complies with the provisions of the Directive of the Council of the European Union on the
approximation of the laws of the Member States
- relating to electrical equipment for use within defined voltage limits
- relating to electromagnetic compatibility
Conformity is proven by compliance with the following standards:
EN61010-1 : 2001-12
EN55011 : 1998 + A1 : 1999 + A2 : 2002, Class B
EN61326 : 1997 + A1 : 1998 + A2 : 2001 + A3 : 2003
For the assessment of electromagnetic compatibility, the limits of radio interference for Class B
equipment as well as the immunity to interference for operation in industry have been used as a basis.
Affixing the EC conformity mark as from 2003
Equipment type
Stock No. Designation
FSU3 1166.1660.03 Spectrum Analyzer
FSU8 1166.1660.08
FSU26 1166.1660.26
FSU31 1166.1660.31
FSU32 1166.1660.32
FSU43 1166.1660.43
FSU46 1166.1660.46
FSU50 1166.1660.50
FSU67 1166.1660.67
(73/23/EEC revised by 93/68/EEC)
(89/336/EEC revised by 91/263/EEC, 92/31/EEC, 93/68/EEC)
ROHDE & SCHWARZ GmbH & Co. KG
Mühldorfstr. 15, D-81671 München
Munich, 2007-06-25 Central Quality Management FS-QZ / Radde
1166.1660.01-s1- CE E-7
EC Certificate of Conformity
Certificate No.: 2003-35, Page 2
This is to certify that:
complies with the provisions of the Directive of the Council of the European Union on the
approximation of the laws of the Member States
- relating to electrical equipment for use within defined voltage limits
- relating to electromagnetic compatibility
Conformity is proven by compliance with the following standards:
EN61010-1 : 2001-12
EN55011 : 1998 + A1 : 1999 + A2 : 2002, Class B
EN61326 : 1997 + A1 : 1998 + A2 : 2001 + A3 : 2003
For the assessment of electromagnetic compatibility, the limits of radio interference for Class B
equipment as well as the immunity to interference for operation in industry have been used as a basis.
Affixing the EC conformity mark as from 2003
Equipment type
Stock No. Designation
FSU-B4 1144.9000.02 OCXO 10 MHz
FSU-B9 1142.8994.02 Tracking Generator
FSU-B12 1142.9349.02 Output Attenuator
FSU-B18 1145.0242.02/.04 Removable Harddisc
FSU-B19 1145.0394.02 Second Harddisc
FSU-B20 1155.1606.08 Extended Environmetal Spec
FSU-B21 1157.1090.02 LO/IF Connections
FSU-B23 1157.0907.02 Preamplifier 20 dB
FSU-B25 1144.9298.02 Electronic Attenuator
FSU-B27 1157.2000.02 FM Output
FSU-B46 1163.0434.02 46 GHz Frequency Extension
FSU-B50 1163.0470.02 50 GHz Frequency Extension
FSU-B73 1169.5696.03 Vector Signal Analysis
FSU-B88 1157.1432.08/.26 RF Hardware
FSU-U73 1169.5696.04 Vector Signal Analysis
FSP-B10 1129.7246.02 External Generator Control
FSP-B28 1162.9915.02 Trigger Port
(73/23/EEC revised by 93/68/EEC)
(89/336/EEC revised by 91/263/EEC, 92/31/EEC, 93/68/EEC)
ROHDE & SCHWARZ GmbH & Co. KG
Mühldorfstr. 15, D-81671 München
Munich, 2007-06-25 Central Quality Management FS-QZ / Radde
1166.1660.01-s2- CE E-7
R&S FSU Tabbed Divider Overview
Tabbed Divider Overview
Safety Instructions are provided on the CD-ROM
Tabbed Divider
Documentation Overview
Chapter 1: Putting into Operation
Chapter 2: Getting Started
Chapter 3: Manual Control
Chapter 4: Instrument Functions
Chapter 5: Remote Control – Basics
Chapter 6: Remote Control – Description of Commands
Chapter 7: Remote Control – Programming Examples
Chapter 8: Maintenance and Instrument Interfaces
Chapter 9: Error Messages
Index
1166.1725.12 0.3 E-2
Tabbed Divider Overview R&S FSU
Documentation Overview
The documentation of the R&S FSU consists of base unit manuals and option manuals. All manuals are
provided in PDF format on the CD-ROM delivered with the instrument. Each software option available for
the instrument is described in a separate software manual.
The base unit documentation comprises the following manuals:
• R&S FSU Quick Start Guide
• R&S FSU Operating Manual
• R&S FSU Service Manual
Apart from the base unit, these manuals describe the following models and options of the R&S FSU
Spectrum Analyzer. Options that are not listed are described in separate manuals. These manuals are
provided on an extra CD-ROM. For an overview of all options available for the R&S FSU visit the
R&S FSU Spectrum Analyzer Internet site.
Base unit models:
• R&S FSU3 (20 Hz to 3.6 GHz)
• R&S FSU8 (20 Hz to 8 GHz)
• R&S FSU26 (20 Hz to 26.5 GHz)
• R&S FSU31 (20 Hz to 31 GHz)
• R&S FSU32 (20 Hz to 32 GHz)
• R&S FSU43 (20 Hz to 43 GHz)
• R&S FSU46 (20 Hz to 40 GHz)
• R&S FSU50 (20 Hz to 50 GHz)
• R&S FSU67 (20 Hz to 67 GHz)
Options described in the base unit manuals:
• R&S FSU-B4 (OCXO - reference oscillator)
• R&S FSU-B9 (tracking generator)
• R&S FSP-B10 (external generator control)
• R&S FSU-B12 (attenuator for tracking generator)
• R&S FSU-B18 (removable hard disk)
• R&S FSU-B19 (second hard disk for R&S FSU-B18 option)
• R&S FSU-B20 (extended environmental spec)
• R&S FSU-B21 (external mixer)
• R&S FSU-B23 (RF preamplifier)
• R&S FSU-B25 (electronic attenuator)
• R&S FSU-B27 (broadband FM demodulator)
• R&S FSP-B28 (trigger port)
1166.1725.12 0.4 E-3
R&S FSU Tabbed Divider Overview
The operating manual is subdivided into the following chapters:
Chapter 1 Putting into Operation
see Quick Start Guide chapters 1 and 2
Chapter 2 Getting Started
gives an introduction to advanced measurement tasks of the R&S FSU which are
explained step by step.
Chapter 3 Manual Control
see Quick Start Guide chapter 4
Chapter 4 Instrument Functions
forms a reference for manual control of the R&S FSU and contains a detailed
description of all instrument functions and their application.
Chapter 5 Remote Control - Basics
describes the basics for programming the R&S FSU, command processing and the
status reporting system.
Chapter 6 Remote Control - Description of Commands
lists all the remote-control commands defined for the instrument.
Chapter 7 Remote Control - Programming Examples
contains program examples for a number of typical applications of the R&S FSU.
Chapter 8 Maintenance and Instrument Interfaces
describes preventive maintenance and the characteristics of the instrument’s
interfaces.
Chapter 9 Error Messages
gives a list of error messages that the R&S FSU may generate.
Index contains an index for the chapters 1 to 9 of the operating manual.
Service Manual - Instrument
This manual is available in PDF format on the CD delivered with the instrument. It informs on how to check
compliance with rated specifications, on instrument function, repair, troubleshooting and fault elimination.
It contains all information required for repairing the R&S FSU by the replacement of modules. The manual
includes the following chapters:
Chapter 1 Performance Test
Chapter 2 Adjustment
Chapter 3 Repair
Chapter 4 Software Update / Installing Options
Chapter 5 Documents
1166.1725.12 0.5 E-2
Tabbed Divider Overview R&S FSU
1166.1725.12 0.6 E-2
R&S FSU Putting into Operation
1 Putting into Operation
For details refer to the Quick Start Guide chapters 1, "Front and Rear Panel", and 2, "Preparing for Use".
1166.1725.12 1.1 E-2
Putting into Operation R&S FSU
1166.1725.12 1.2 E-2
R&S FSU Getting Started
2 Getting Started
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2
Measuring the Spectra of Complex Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3
Intermodulation Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3
Measurement Example – Measuring the R&S FSU’s intrinsic intermodulation
distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5
Measuring Signals in the Vicinity of Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10
Measurement example – Measuring the level of the internal reference generator
at low S/N ratios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13
Noise Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.16
Measuring noise power density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.17
Measurement example – Measuring the intrinsic noise power density of the
R&S FSU at 1 GHz and calculating the R&S FSU’s noise figure . . . . . . . . . . . . . . . 2.17
Measurement of Noise Power within a Transmission Channel . . . . . . . . . . . . . . . . . . . . . 2.20
Measurement Example – Measuring the intrinsic noise of the R&S FSU at
1 GHz in a 1.23 MHz channel bandwidth with the channel power function . . . . . . . 2.20
Measuring Phase Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.24
Measurement Example – Measuring the phase noise of a signal generator at a
carrier offset of 10 kHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.24
Measurements on Modulated Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.27
Measurements on AM signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.28
Measurement Example 1 – Displaying the AF of an AM signal in the time
domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.28
Measurement Example 2 – Measuring the modulation depth of an AM carrier in
the frequency domain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.29
Measurements on FM Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.31
Measurement Example – Displaying the AF of an FM carrier . . . . . . . . . . . . . . . . . 2.31
Measuring Channel Power and Adjacent Channel Power . . . . . . . . . . . . . . . . . . . . . . . . . 2.33
Measurement Example 1 – ACPR measurement on an IS95 CDMA Signal . . . . . . 2.34
Measurement Example 2 – Measuring the adjacent channel power of an IS136
TDMA signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.38
Measurement Example 3 – Measuring the Modulation Spectrum in Burst Mode
with the Gated Sweep Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.41
Measurement Example 4 – Measuring the Transient Spectrum in Burst Mode
with the Fast ACP function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.43
Measurement Example 5 – Measuring adjacent channel power of a W-CDMA
uplink signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.45
Amplitude distribution measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.49
Measurement Example – Measuring the APD and CCDF of white noise
generated by the R&S FSU . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.49
1166.1725.12 2.1 E-2
Introduction R&S FSU
Introduction
This chapter explains how to operate the R&S FSU using typical measurements as examples.
The basic operating steps such as selecting the menus and setting parameters are described in the Quick
Start Guide, chapter 4, "Basic Operation". Furthermore, the screen structure and displayed function
indicators are explained in this chapter.
Chapter “Instrument Functions” describes all the menus and R&S FSU functions.
All of the following examples are based on the standard settings of the Spectrum Analyzer. These are set
with the PRESET key. A complete listing of the standard settings can be found in chapter “Instrument
Functions”, section “R&S FSU Initial Configuration – PRESET Key” on page 4.6. Examples of more basic
character are provided in the Quick Start Guide, chapter 5, as an introduction.
1166.1725.12 2.2 E-2
R&S FSU Measuring the Spectra of Complex Signals
Measuring the Spectra of Complex Signals
Intermodulation Measurements
If several signals are applied to a DUT with non-linear characteristics, unwanted mixing products are
generated – mostly by active components such as amplifiers or mixers. The products created by 3
intermodulation are particularly troublesome as they have frequencies close to the useful signals and,
compared with other products, are closest in level to the useful signals. The fundamental wave of one
signal is mixed with the 2
f
= 2 × fu1 – fu2 (1)
s1
= 2 × fu2 – fu1 (2)
f
s2
nd
harmonic of the other signal.
rd
order
where f
and fs2 are the frequencies of the intermodulation products and fu1 and fu2 the frequencies of
s1
the useful signals.
The following diagram shows the position of the intermodulation products in the frequency domain.
Level
u2
P
a
D3
P
s2
∆
f
∆
f
f
u2
f
s2
Frequency
P
s1
∆
f
s1
Fig. 2-1 3
P
u1
f
f
u1
rd
order intermodulation products
Example:
fu1 = 100 MHz, fu2 = 100.03 MHz
f
= 2 × fu1 – fu2 = 2 × 100 MHz – 100.03 MHz = 99.97 MHz
s1
f
= 2 × fu2 – fu1 = 2 × 100.03 MHz – 100 MHz = 100.06 MHz
s2
The level of the intermodulation products depends on the level of the useful signals. If the level of the two
useful signals is increased by 1 dB, the level of the intermodulation products is increased by 3 dB. The
intermodulation distance d
signals and the 3
rd
order intermodulation products are related.
is, therefore, reduced by 2 dB. Fig. 2-2 shows how the levels of the useful
3
1166.1725.12 2.3 E-2
Measuring the Spectra of Complex Signals R&S FSU
Output
level
Intercept
point
Compression
Carrier
level
1
1
Fig. 2-2 Level of the 3
D3
a
1
rd
order intermodulation products as a function of the level of the useful signals
Intermodulation
products
3
Input level
The behavior of the signals can explained using an amplifier as an example. The change in the level of
the useful signals at the output of the amplifier is proportional to the level change at the input of the
amplifier as long as the amplifier is operating in linear range. If the level at the amplifier input is changed
by 1 dB, there is a 1 dB level change at the amplifier output. At a certain input level, the amplifier enters
saturation. The level at the amplifier output does not increase with increasing input level.
The level of the 3
signals. The 3
rd
order intermodulation products increases 3 times faster than the level of the useful
rd
order intercept is the virtual level at which the level of the useful signals and the level of
the spurious products are identical, i.e. the intersection of the two straight lines. This level cannot be
measured directly as the amplifier goes into saturation or is damaged before this level is reached.
rd
The 3
d
TOI = a
with TOI (Third Order Intercept) being the 3
order intercept can be calculated from the known slopes of the lines, the intermodulation distance
and the level of the useful signals.
2
/ 2 + Pn (3)
D3
rd
order intercept in dBm and Pn the level of a carrier in dBm.
With an intermodulation distance of 60 dB and an input level, P
, of –20 dBm, the following 3rd order
w
intercept is obtained:
TOI = 60 dBm / 2 + (-20 dBm) = 10 dBm.
1166.1725.12 2.4 E-2
R&S FSU Measuring the Spectra of Complex Signals
Measurement Example – Measuring the R&S FSU’s intrinsic
intermodulation distance
To measure the intrinsic intermodulation distance, use the following test setup.
Test setup:
Signal
Generator 1
Coupler R&S FSP
Signal
Generator 2
FSU
Signal generator settings (e.g. R&S SMIQ):
Level Frequency
Signal generator 1 -10 dBm 999.9 MHz
Signal generator 2 -10 dBm 1000.1 MHz
Measurement using the R&S FSU:
1. Set the Spectrum Analyzer to its default settings.
➢ Press the PRESET key.
The R&S FSU is in its default state.
2. Set center frequency to 1 GHz and the frequency span to 1 MHz.
➢ Press the FREQ key and enter 1 GHz.
➢ Press the SPAN key and enter 1 MHz.
3. Set the reference level to –10 dBm and RF attenuation to 0 dB.
➢ Press the AMPT key and enter -10 dBm.
➢ Press the RF ATTEN MANUAL softkey and enter 0 dB.
By reducing the RF attenuation to 0 dB, the level to the R&S FSU input mixer is increased.
Therefore, 3
rd
order intermodulation products are displayed.
4. Set the resolution bandwidth to 5 kHz.
➢ Press the BW key.
➢ Press the RES BW MANUAL softkey and enter 5 kHz.
By reducing the bandwidth, the noise is further reduced and the intermodulation products can be
clearly seen.
1166.1725.12 2.5 E-2
Measuring the Spectra of Complex Signals R&S FSU
5. Measuring intermodulation by means of the 3
rd
order intercept measurement function
➢ Press the MEAS key.
➢ Press the TOI softkey.
The R&S FSU activates four markers for measuring the intermodulation distance. Two markers are
positioned on the useful signals and two on the intermodulation products. The 3
rd
order intercept is
calculated from the level difference between the useful signals and the intermodulation products. It
is then displayed on the screen:
Fig. 2-3 Result of intrinsic intermodulation measurement on the R&S FSU. The 3
intercept (TOI) is displayed at the top right corner of the grid
The level of a Spectrum Analyzer’s intrinsic intermodulation products depends on the RF level of
the useful signals at the input mixer. When the RF attenuation is added, the mixer level is reduced
and the intermodulation distance is increased. With an additional RF attenuation of 10 dB, the
levels of the intermodulation products are reduced by 20 dB. The noise level is, however, increased
by 10 dB.
6. Increasing RF attenuation to 10 dB to reduce intermodulation products.
➢ Press the AMPT key.
➢ Press the RF ATTEN MANUAL softkey and enter 10 dB.
The R&S FSU’s intrinsic intermodulation products disappear below the noise floor.
rd
order
1166.1725.12 2.6 E-2
R&S FSU Measuring the Spectra of Complex Signals
Fig. 2-4 If the RF attenuation is increased, the R&S FSU’s intrinsic intermodulation products
disappear below the noise floor.
Calculation method:
The method used by the R&S FSU to calculate the intercept point takes the average useful signal level
P
in dBm and calculates the intermodulation d3 in dB as a function of the average value of the levels of
u
the two intermodulation products. The third order intercept (TOI) is then calculated as follows:
TOI/dBm = ½ d
+ Pu
3
Intermodulation- free dynamic range
The Intermodulation – free dynamic range, i.e. the level range in which no internal intermodulation
products are generated if two-tone signals are measured, is determined by the 3
rd
order intercept point,
the phase noise and the thermal noise of the Spectrum Analyzer. At high signal levels, the range is
determined by intermodulation products. At low signal levels, intermodulation products disappear below
the noise floor, i.e. the noise floor and the phase noise of the Spectrum Analyzer determine the range. The
noise floor and the phase noise depend on the resolution bandwidth that has been selected. At the
smallest resolution bandwidth, the noise floor and phase noise are at a minimum and so the maximum
range is obtained. However, a large increase in sweep time is required for small resolution bandwidths. It
is, therefore, best to select the largest resolution bandwidth possible to obtain the range that is required.
Since phase noise decreases as the carrier-offset increases, its influence decreases with increasing
frequency offset from the useful signals.
The following diagrams illustrate the intermodulation-free dynamic range as a function of the selected
bandwidth and of the level at the input mixer (= signal level – set RF attenuation) at different useful signal
offsets.
1166.1725.12 2.7 E-2
Measuring the Spectra of Complex Signals R&S FSU
Distortion free dynamic range
1MHz carrier offset
-60
-70
-80
-90
-100
Dynamic range dB
-110
-120
-60 -50 -40 -30 -20 -10
Fig. 2-5 Intermodulation-free range of the FSU3 as a function of level at the input mixer and the set
resolution bandwidth (useful signal offset = 1 MHz, DANL = -157 dBm /Hz, TOI = 25 dBm;
typ. values at 2 GHz)
RBW=10 kHz
RBW=1
kHz
RBW=100
Hz
RBW=10
Hz
Mixer level
T.O.I
Thermal noise
The optimum mixer level, i.e. the level at which the intermodulation distance is at its maximum, depends
on the bandwidth. At a resolution bandwidth of 10 Hz, it is approx. –42 dBm and at 10 kHz increases to
approx. -32 dBm.
Phase noise has a considerable influence on the intermodulation-free range at carrier offsets between 10
and 100 kHz (Fig. 2-6). At greater bandwidths, the influence of the phase noise is greater than it would be
with small bandwidths. The optimum mixer level at the bandwidths under consideration becomes almost
independent of bandwidth and is approx. –40 dBm.
Distortion free dynamic range
-60
-70
-80
-90
-100
Dynamic range dB
-110
-120
-60-50-40-30-20-10
10 to 100 kHz offset
RBW=10 kHz
RBW=1
kHz
RBW=100
Hz
RBW=10
Hz
Mixer level
T.O.I
The rm al noise
Fig. 2-6 Intermodulation-free dynamic range of the FSU3 as a function of level at the input mixer and
of the selected resolution bandwidth (useful signal offset = 10 to 100 kHz, DANL = -157 dBm
/Hz, TOI = 25 dBm; typ. values at 2 GHz).
1166.1725.12 2.8 E-2
R&S FSU Measuring the Spectra of Complex Signals
Aa
Hint
If the intermodulation products of a DUT with a very high dynamic range are to be
measured and the resolution bandwidth to be used is therefore very small, it is best
to measure the levels of the useful signals and those of the intermodulation
products separately using a small span. The measurement time will be reduced–
in particular if the offset of the useful signals is large. To find signals reliably when
frequency span is small, it is best to synchronize the signal sources and the
R&S FSU.
1166.1725.12 2.9 E-2
Measuring Signals in the Vicinity of Noise R&S FSU
z
Measuring Signals in the Vicinity of Noise
The minimum signal level a Spectrum Analyzer can measure is limited by its intrinsic noise. Small signals
can be swamped by noise and therefore cannot be measured. For signals that are just above the intrinsic
noise, the accuracy of the level measurement is influenced by the intrinsic noise of the Spectrum Analyzer.
The displayed noise level of a Spectrum Analyzer depends on its noise figure, the selected RF
attenuation, the selected reference level, the selected resolution and video bandwidth and the detector.
The effect of the different parameters is explained in the following.
Impact of the RF attenuation setting
The sensitivity of a Spectrum Analyzer is directly influenced by the selected RF attenuation. The highest
sensitivity is obtained at a RF attenuation of 0 dB. The R&S FSU’s RF attenuation can be set in 5 dB steps
up to 70 dB (5 dB steps up to 75 dB with option Electronic Attenuator R&S FSU-B25). Each additional 5
dB step reduces the R&S FSU’s sensitivity by 10 dB, i.e. the displayed noise is increased by 5 dB.
Impact of the reference level setting
If the reference level is changed, the R&S FSU changes the gain on the last IF so that the voltage at the
logarithmic amplifier and the A/D converter is always the same for signal levels corresponding to the
reference level. This ensures that the dynamic range of the log amp or the A/D converter is fully utilized.
Therefore, the total gain of the signal path is low at high reference levels and the noise figure of the IF
amplifier makes a substantial contribution to the total noise figure of the R&S FSU. The figure below
shows the change in the displayed noise depending on the set reference level at 10 kHz and 300 kHz
resolution bandwidth. With digital bandwidths (≤100 kHz) the noise increases sharply at high reference
levels because of the dynamic range of the A/D converter.
14
12
10
8
6
4
rel. noise level /dB
2
0
-2
-70 -60 -50 -40 -30 -20 -10
RBW = 10 kHz
RBW = 300 kH
Reference level /dBm
Fig. 2-7 Change in displayed noise as a function of the selected reference level at bandwidths of
10 kHz and 300 kHz (-30 dBm reference level)
1166.1725.12 2.10 E-2
R&S FSU Measuring Signals in the Vicinity of Noise
Impact of the resolution bandwidth
The sensitivity of a Spectrum Analyzer also directly depends on the selected bandwidth. The highest
sensitivity is obtained at the smallest bandwidth (for the R&S FSU: 10 Hz, for FFT filtering: 1 Hz). If the
bandwidth is increased, the reduction in sensitivity is proportional to the change in bandwidth. The
R&S FSU has bandwidth settings in 2, 3, 5, 10 sequence. Increasing the bandwidth by a factor of 3
increases the displayed noise by approx. 5 dB (4.77 dB precisely). If the bandwidth is increased by a factor
of 10, the displayed noise increases by a factor of 10, i.e. 10 dB. Because of the way the resolution filters
are designed, the sensitivity of Spectrum Analyzers often depends on the selected resolution bandwidth.
In data sheets, the displayed average noise level is often indicated for the smallest available bandwidth
(for the R&S FSU: 10 Hz). The extra sensitivity obtained if the bandwidth is reduced may therefore deviate
from the values indicated above. The following table illustrates typical deviations from the noise figure for
a resolution bandwidth of 10 kHz which is used as a reference value (= 0 dB).
Noise fi gure
offset /dB
3
digital RBW analog RBW
2
1
0
-1
0,01 0,1 1 10 100 1000 10000
RBW /kHz
Fig. 2-8 Change in R&S FSU noise figure at various bandwidths. The reference bandwidth is 10 kHz
Impact of the video bandwidth
The displayed noise of a Spectrum Analyzer is also influenced by the selected video bandwidth. If the
video bandwidth is considerably smaller than the resolution bandwidth, noise spikes are suppressed, i.e.
the trace becomes much smoother. The level of a sinewave signal is not influenced by the video
bandwidth. A sinewave signal can therefore be freed from noise by using a video bandwidth that is small
compared with the resolution bandwidth, and thus be measured more accurately.
Impact of the detector
Noise is evaluated differently by the different detectors. The noise display is therefore influenced by the
choice of detector. Sinewave signals are weighted in the same way by all detectors, i.e. the level display
for a sinewave RF signal does not depend on the selected detector, provided that the signal-to-noise ratio
is high enough. The measurement accuracy for signals in the vicinity of intrinsic Spectrum Analyzer noise
is also influenced by the detector which has been selected. The R&S FSU has the following detectors:
1166.1725.12 2.11 E-2
Measuring Signals in the Vicinity of Noise R&S FSU
Maximum peak detector
If the max. peak detector s selected, the largest noise display is obtained, since the Spectrum Analyzer
displays the highest value of the IF envelope in the frequency range assigned to a pixel at each pixel in
the trace. With longer sweep times, the trace indicates higher noise levels since the probability of
obtaining a high noise amplitude increases with the dwell time on a pixel. For short sweep times, the
display approaches that of the sample detector since the dwell time on a pixel is only sufficient to obtain
an instantaneous value.
Minimum peak detector
The min. peak detector indicates the minimum voltage of the IF envelope in the frequency range assigned
to a pixel at each pixel in the trace. The noise is strongly suppressed by the minimum peak detector since
the lowest noise amplitude that occurs is displayed for each test point. If the signal-to-noise ratio is low,
the minimum of the noise overlaying the signal is displayed too low.
At longer sweep times, the trace shows smaller noise levels since the probability of obtaining a low noise
amplitude increases with the dwell time on a pixel. For short sweep times, the display approaches that of
the sample detector since the dwell time on a pixel is only sufficient to obtain an instantaneous value.
Autopeak detector
The Autopeak detector displays the maximum and minimum peak value at the same time. Both values
are measured and their levels are displayed on the screen joint by a vertical line.
Sample detector
The sample detector samples the logarithm of the IF envelope for each pixel of the trace only once and
displays the resulting value. If the frequency span of the Spectrum Analyzer is considerably higher than
the resolution bandwidth (span/RBW >500), there is no guarantee that useful signals will be detected.
They are lost due to undersampling. This does not happen with noise because in this case it is not the
instantaneous amplitude that is relevant but only the probability distribution.
RMS detector
For each pixel of the trace, the RMS detector outputs the RMS value of the IF envelope for the frequency
range assigned to each test point. It therefore measures noise power. The display for small signals is,
however, the sum of signal power and noise power. For short sweep times, i.e. if only one uncorrelated
sample value contributes to the RMS value measurement, the RMS detector is equivalent to the sample
detector. If the sweep time is longer, more and more uncorrelated RMS values contribute to the RMS
value measurement. The trace is, therefore, smoothed. The level of sinewave signals is only displayed
correctly if the selected resolution bandwidth (RBW) is at least as wide as the frequency range which
corresponds to a pixel in the trace. At a resolution bandwidth of 1 MHz, this means that the maximum
frequency display range is 625 MHz.
Average detector
For each pixel of the trace, the average detector outputs the average value of the linear IF envelope for
the frequency range assigned to each test point. It therefore measures the linear average noise. The level
of sinewave signals is only displayed correctly if the selected resolution bandwidth (RBW) is at least as
wide as the frequency range which corresponds to a pixel in the trace. At a resolution bandwidth of 1 MHz,
this means the maximum frequency display range is 625 MHz.
1166.1725.12 2.12 E-2
R&S FSU Measuring Signals in the Vicinity of Noise
Quasi peak detector
The quasi peak detector is a peak detector for EMI measurements with defined charge and discharge
times. These times are defined in CISPR 16, the standard for equipment used to measure EMI emissions.
Measurement example – Measuring the level of the internal reference
generator at low S/N ratios
The example shows the different factors influencing the S/N ratio.
1. Set the Spectrum Analyzer to its default state.
➢ Press the PRESET key.
The R&S FSU is in its default state.
2. Switch on the internal reference generator
➢ Press the SETUP key.
➢ Press the softkeys SERVICE - INPUT CAL.
The internal 128 MHz reference generator is on.
The R&S FSU’s RF input is off.
3. Set the center frequency to 128 MHz and the frequency span to 100 MHz.
➢ Press the FREQ key and enter 128 MHz.
➢ Press the SPAN key and enter 100 MHz.
4. Set the RF attenuation to 60 dB to attenuate the input signal or to increase the intrinsic noise.
➢ Press the AMPT key.
➢ Press the RF ATTEN MANUAL softkey and enter 60 dB.
The RF attenuation indicator is marked with an asterisk (*Att 60 dB) to show that it is no longer
coupled to the reference level. The high input attenuation reduces the reference signal which can
no longer be detected in noise.
Fig. 2-9 Sinewave signal with low S/N ratio. The signal is measured with the autopeak detector
and is completely swamped by the intrinsic noise of the Spectrum Analyzer.
1166.1725.12 2.13 E-2
Measuring Signals in the Vicinity of Noise R&S FSU
5. To suppress noise spikes the trace can be averaged.
➢ Press the TRACE key.
➢ Press the AVERAGE softkey.
The traces of consecutive sweeps are averaged. To perform averaging, the R&S FSU automatically
switches on the sample detector. The RF signal, therefore, can be more clearly distinguished from
noise.
Fig. 2-10 RF sinewave signal with low S/N ratio if the trace is averaged.
6. Instead of trace averaging, a video filter that is narrower than the resolution bandwidth can be
selected.
➢ Press the CLEAR/WRITE softkey in the trace menu.
➢ Press the BW key.
Press the VIDEO BW MANUAL softkey and enter 10 kHz.
The RF signal can be more clearly distinguished from noise.
Fig. 2-11 RF sinewave signal with low S/N ratio if a smaller video bandwidth is selected.
1166.1725.12 2.14 E-2
R&S FSU Measuring Signals in the Vicinity of Noise
7. By reducing the resolution bandwidth by a factor of 10, the noise is reduced by 10 dB.
➢ Press the RES BW MANUAL softkey and enter 300 kHz.
The displayed noise is reduced by approx. 10 dB. The signal, therefore, emerges from noise by
about 10 dB. Compared to the previous setting, the video bandwidth has remained the same, i.e. it
has increased relative to the smaller resolution bandwidth. The averaging effect is, therefore,
reduced by the video bandwidth. The trace will be noisier.
Fig. 2-12 Reference signal at a smaller resolution bandwidth
1166.1725.12 2.15 E-2
Noise Measurements R&S FSU
Noise Measurements
Noise measurements play an important role in spectrum analysis. Noise e.g. affects the sensitivity of radio
communication systems and their components.
Noise power is specified either as the total power in the transmission channel or as the power referred to
a bandwidth of 1 Hz. The sources of noise are, for example, amplifier noise or noise generated by
oscillators used for the frequency conversion of useful signals in receivers or transmitters. The noise at
the output of an amplifier is determined by its noise figure and gain.
The noise of an oscillator is determined by phase noise near the oscillator frequency and by thermal noise
of the active elements far from the oscillator frequency. Phase noise can mask weak signals near the
oscillator frequency and make them impossible to detect.
1166.1725.12 2.16 E-2
R&S FSU Noise Measurements
Measuring noise power density
To measure noise power referred to a bandwidth of 1 Hz at a certain frequency, the R&S FSU has an easyto-use marker function. This marker function calculates the noise power density from the measured
marker level.
Measurement example – Measuring the intrinsic noise power density
of the R&S FSU at 1 GHz and calculating the R&S FSU’s noise figure
1. Set the Spectrum Analyzer to its default state.
➢ Press the PRESET key.
The R&S FSU is in its default state.
2. Set the center frequency to 1 GHz and the span to 1 MHz.
➢ Press the FREQ key and enter 1 GHz.
➢ Press the SPAN key and enter 1 MHz.
3. Switch on the marker and set the marker frequency to 1 GHz.
➢ Press the MKR key and enter 1 GHz.
4. Switch on the noise marker function.
➢ Press the MEAS key.
➢ Press the NOISE MARKER softkey.
The R&S FSU displays the noise power at 1 GHz in dBm (1Hz).
Since noise is random, a sufficiently long measurement time has to be selected to obtain stable
measurement results. This can be achieved by averaging the trace or by selecting a very small
video bandwidth relative to the resolution bandwidth.
5. The measurement result is stabilized by averaging the trace
➢ Press the TRACE key.
➢ Press the AVERAGE softkey.
The R&S FSU performs sliding averaging over 10 traces from consecutive sweeps. The
measurement result becomes more stable.
Conversion to other reference bandwidths
The result of the noise measurement can be referred to other bandwidths by simple conversion. This is
done by adding 10 · log (BW) to the measurement result, BW being the new reference bandwidth.
Example:
A noise power of –150 dBm (1 Hz) is to be referred to a bandwidth of 1 kHz.
P
= -150 + 10 · log (1000) = -150 +30 = -120 dBm(1 kHz)
[1kHz]
1166.1725.12 2.17 E-2
Noise Measurements R&S FSU
Calculation method:
The following method is used to calculate the noise power:
If the noise marker is switched on, the R&S FSU automatically activates the sample detector. The video
bandwidth is set to 1/10 of the selected resolution bandwidth (RBW).
To calculate the noise, the R&S FSU takes an average over 17 adjacent pixels (the pixel on which the
marker is positioned and 8 pixels to the left, 8 pixels to the right of the marker). The measurement result
is stabilized by video filtering and averaging over 17 pixels.
Since both video filtering and averaging over 17 trace points is performed in the log display mode, the
result would be 2.51 dB too low (difference between logarithmic noise average and noise power). The
R&S FSU, therefore, corrects the noise figure by 2.51 dB.
To standardize the measurement result to a bandwidth of 1 Hz, the result is also corrected by –10 · log
(RBW
Detector selection
The noise power density is measured in the default setting with the sample detector and using averaging.
Other detectors that can be used to perform a measurement giving true results are the average detector
or the RMS detector. If the average detector is used, the linear video voltage is averaged and displayed
as a pixel. If the RMS detector is used, the squared video voltage is averaged and displayed as a pixel.
The averaging time depends on the selected sweep time (=SWT/625). An increase in the sweep time
gives a longer averaging time per pixel and thus stabilizes the measurement result. The R&S FSU
automatically corrects the measurement result of the noise marker display depending on the selected
detector (+1.05 dB for the average detector, 0 dΒ for the RMS detector). It is assumed that the video
bandwidth is set to at least three times the resolution bandwidth. While the average or RMS detector is
being switched on, the R&S FSU sets the video bandwidth to a suitable value.
), with RBW
noise
being the power bandwidth of the selected resolution filter (RBW).
noise
The Pos Peak, Neg Peak, Auto Peak and Quasi Peak detectors are not suitable for measuring noise
power density.
Determining the noise figure:
The noise figure of amplifiers or of the R&S FSU alone can be obtained from the noise power display.
Based on the known thermal noise power of a 50 Ω resistor at room temperature (-174 dBm (1Hz)) and
the measured noise power P
NF = P
+ 174 – g,
noise
the noise figure (NF) is obtained as follows:
noise
where g = gain of DUT in dB
Example:
The measured internal noise power of the R&S FSU at an attenuation of 0 dB is found to be –155 dBm/1
Hz. The noise figure of the R&S FSU is obtained as follows
NF = -155 + 174 = 17 dB
1166.1725.12 2.18 E-2
R&S FSU Noise Measurements
p
Aa
Co rrectio n
factor in dB
Note
If noise power is measured at the output of an amplifier, for example, the sum of
the internal noise power and the noise power at the output of the DUT is measured.
The noise power of the DUT can be obtained by subtracting the internal noise
power from the total power (subtraction of linear noise powers). By means of the
following diagram, the noise level of the DUT can be estimated from the level
difference between the total and the internal noise level.
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Total
ower/intrinsic noise power in dB
Fig. 2-13 Correction factor for measured noise power as a function of the ratio of total power to the
intrinsic noise power of the Spectrum Analyzer.
1166.1725.12 2.19 E-2
Noise Measurements R&S FSU
Measurement of Noise Power within a Transmission Channel
Noise in any bandwidth can be measured with the channel power measurement functions. Thus the noise
power in a communication channel can be determined, for example. If the noise spectrum within the
channel bandwidth is flat, the noise marker from the previous example can be used to determine the noise
power in the channel by considering the channel bandwidth. If, however, phase noise and noise that
normally increases towards the carrier is dominant in the channel to be measured, or if there are discrete
spurious signals in the channel, the channel power measurement method must be used to obtain correct
measurement results.
Measurement Example – Measuring the intrinsic noise of the
R&S FSU at 1 GHz in a 1.23 MHz channel bandwidth with the channel
power function
Test setup:
The RF input of the R&S FSU remains open-circuited or is terminated with 50 Ω.
Measurement with the R&S FSU:
1. Set the Spectrum Analyzer to its default state.
➢ Press the PRESET key.
The R&S FSU is in its default state.
2. Set the center frequency to 1 GHz and the span to 2 MHz.
➢ Press the FREQ key and enter 1 GHz.
➢ Press the SPAN key and enter 2 MHz.
3. To obtain maximum sensitivity, set RF attenuation on the R&S FSU to 0 dB.
➢ Press the AMPT key.
➢ Press the RF ATTEN MANUAL softkey and enter 0 dB.
4. Switch on and configure the channel power measurement.
➢ Press the MEAS key.
➢ Press the CHAN POWER ACP softkey.
The R&S FSU activates the channel or adjacent channel power measurement according to the
currently set configuration.
➢ Press the CP/ACP CONFIG ! softkey.
The R&S FSU enters the submenu for configuring the channel.
➢ Press the CHANNEL BANDWIDTH softkey and enter 1.23 MHz.
The R&S FSU displays the 1.23 MHz channel as two vertical lines which are symmetrical to the
center frequency.
➢ Press the PREV key.
The R&S FSU returns to the main menu for channel and adjacent channel power measurement.
➢ Press the ADJUST SETTINGS softkey.
The settings for the frequency span, the bandwidth (RBW and VBW) and the detector are
automatically set to the optimum values required for the measurement.
1166.1725.12 2.20 E-2
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