1GA43_0E
Products: Audio Analyzer R&S UPL
Signal Generator R&S SML
Measurements on RF radio tuners
with Audio Analyzer R&S UPL and
Signal Generator R&S SML with option -B5
1GA43_0E
A great number of measurements have to be performed to determine the quality of FM tuners. This
Application Note presents a program that permits these measurements to be carried out in line with
DIN EN60315-4.
Subject to change – 1GPP Betz 01.2003 – 1GA43_0E
Measurements on FM Tuners
Contents
1 Overview..................................................................................................2
2 Operating Principle ..................................................................................2
3 Hardware and Software Requirements ...................................................3
Required Measuring Instruments and Accessories.......................3
4 Test Assembly .........................................................................................3
5 Installing the Software .............................................................................5
Starting the Application Software ..................................................5
Setup Conversion for Firmware Updates ......................................6
6 Measurements to Standard .....................................................................7
Standard Test Conditions ...................................................................7
Standard Test Frequency..............................................................7
Standard Frequency Deviation ......................................................7
Preemphasis .................................................................................7
Standard Modulation Frequency ...................................................8
Standard Input Level .....................................................................8
Filters.............................................................................................8
Configuration (SETUP menu).............................................................9
Operation..........................................................................................10
7 Measurement Functions ........................................................................13
Audio Frequency Response ........................................................13
THD and Noise as a Function of Modulation Frequency.............14
THD and Noise as a Function of Modulation Deviation...............15
Crosstalk as a Function of Modulation Frequency ......................16
Crosstalk Attenuation as a Function of Modulation Deviation .....17
Crosstalk as a Function of RF Level / Stereo Switching
Threshold ....................................................................................17
S/N Ratio as a Function of Input Level........................................19
Input Signal / Output Signal Characteristic..................................20
Characteristic of audio output signal.................................21
Noise signal.......................................................................21
Max. S/N ratio ...................................................................22
Noise-limited sensitivity.....................................................22
Suppression of Pilot Tone and Subcarrier ..................................22
8 Demo Programs for Production Tests ...................................................23
9 References ............................................................................................24
10 Ordering Information..............................................................................24
1 Overview
A great number of measurements have to be performed to determine the
audio quality of FM tuners. With the aid of the program described in this
Application Note, these measurements can be performed in line with
DIN EN60315-4.
2 Operating Principle
The Audio Analyzer R&S UPL generates all required test signals. The
signals are applied to the stereo coder (option R&S SML-B5) in the signal
generator and modulated onto the transmitter signal. The modulated signal
is forwarded to the antenna input of the DUT. The demodulated audio
signals at the DUT output are transferred to the audio analyzer for
measurements.
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Measurements on FM Tuners
When used together with the optional Universal Sequence Controller R&S
UPL-B10, the Audio Analyzer R&S UPL automatically executes complete
measurement sequences and controls the signal generator via the
IEC/IEEE-bus or RS-232-C interface. Thus, even measurements versus the
RF signal level, for instance, can be automatically performed .
3 Hardware and Software Requirements
Required Measuring Instruments and Accessories
An Audio Analyzer R&S UPL with option R&S UPL-B10 is required for
generating and measuring audio signals. RF signals are generated by the
Signal Generator R&S SML (alternatively R&S SMV) which must be
equipped with the Stereo/RDS Coder Option R&S SML-B5.
An external keyboard and, if necessary, a printer are needed in addition.
The BASIC programs required for the automatic measurements are stored
on a diskette available from your local Rohde & Schwarz representative or
can be downloaded in the form of a file from the Rohde & Schwarz website,
unpacked and then stored on a diskette. The R&S UPL should meet the
following software requirements:
• UPL firmware version 3.01 or higher
• Universal Sequence Controller Option UPL-B10 installed
• R&S UPL configured with 64 Kbyte program memory and 32 Kbyte data
memory (using configuration tool UPLSET setting 3)
4 Test Assembly
Required are the Audio Analyzer R&S UPL, Signal Generator R&S SML
and probably an HP-deskjet-compatible printer for result documentation.
Link the audio analyzer and the signal generator via the IEC/IEEE bus.
Routines are available for specific measurement modules, which control the
signal generator via the RS-232-C interface. Measurements can thus be
started in the R&S UPL by means of a macro via the IEC/IEEE bus, and
necessary settings in the R&S SML can be made from the R&S UPL via the
RS-232-C interface. Since control via IEC/IEEE-bus and RS-232-C
interface can be used alternately in the R&S SML, the R&S SML and the
R&S UPL can also be used for other tasks in the system after a
measurement macro has been processed via the IEC/IEEE bus. For details
refer to Appendix A.
The printer is connected to the Centronics interface of the R&S UPL.
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Measurements on FM Tuners
Connect the RF output of the R&S SML to the unbalanced antenna input of
the tuner. Since the R&S SML has an output impedance of 50 Ω, the
generator has to be matched to the tuner. Usually, the coaxial 75 Ω antenna
input is used. We recommend to use the Matching Pad R&S RAM which
can be ordered under 358.5414.02.
If the receiver to be measured is only equipped with a balanced input, an
adequate balun with an impedance of 240 Ω or 300 Ω must be connected in
between. Since the measurements are referenced to the RF level at the
antenna input of the tuner and not to the generator output level, the
insertion loss of these matching elements must always be taken into
account. The actual loss must be entered under SETUP and will then be
considered by the program. For loss values, refer to the matching pad
documentation or to the label on the pads.
Connect generator output 1 of the Audio Analyzer R&S UPL to the external
modulation input (left) and generator output 2 to the modulation input (right)
at the rear of Signal Generator R&S SML. Connect the tuner output for the
left channel to analyzer input 1 and the right channel to input 2 of the R&S
UPL.
Make sure that measurements are performed with adequate grounding, e.g.
to eliminate hum pick-up. Since tuners are normally not grounded and their
outputs are floating, the inputs of the R&S UPL should be grounded
provided a ground connection is not established via the antenna connected
to the R&S SML. A selection can be made under SETUP / R&S UPL Input
Selection.
Signal Generator SML/SMV with SML-B5
SIGNA L GEN ER ATO R . 9 kHz . . . 3.3 GHz . SML
R
1090.3000.13
DATA INPUT
POW ER
L
ME NU / V ARIATION
FREQ
7
8
9
LEVEL
4
5
6
SAVE
1
2
3
RCL
0
.
FUNCTION
MOD
G
n
dBµV
SELECT
HELP
µ
µV
BACK
STATUS
m
mV
MOD
ON/OFF
dB(m)
RF
ON/OFF
M
k
LF
RF 50
QUICK SELECT
ASSIGN
MENU 1
MENU 2
PRESET
ERROR
LOCAL
-
x1
.
ENT ER
RF modulated test signal
including ARI and RDS
MA DE IN GERMAN Y
Antenna in
GPIB
FM Stereo Tuner
Audio Analyzer UPL
AUDIO ANALYZER . DC ... 110 kHz . UPL
CONTROL
DIGITAL AUDIO
UNBAL
OUTPUT
OPTICAL
BAL
UNBAL
INPUT
OPTICAL
BAL
START SINGLE
Signal
Generation
and
Analysis
STOP
CONT
DATA / PANEL
ANLR
FILTER
7
8
9
STATUS
FILE
4
5
2
Right
CURSOR / VARIATION
Left
SELECT
HELP
BACKSP
6
DISPLAY GRAPH
1
OFF
REM
OUTPUT LOCAL
H COPY SYSTEM
EDIT
GEN
CANCEL
3
PgUp
OPTIONSSHOW I/O
0
.
ENTER
+/-
PgDn
ANALOG
GENERATOR
1
ANALYZER
2
2
1
POWER
GEN
OVLD
110 V RMS / 160 V PK CAT I
analog audio signals
Fig. 1: Test assembly for tuner measurement
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Measurements on FM Tuners
5 Installing the Software
The application software is installed with the aid of the INSTALL.BAT
installation program which is also stored on the supplied floppy disk.
• Quit the R&S UPL software by pressing the SYSTEM key on the
instrument or Ctrl F9 on the keyboard.
• Insert floppy disk.
• Select floppy disk drive (enter A:).
• Call the installation program (enter INSTALL).
•
Return to UPL program (enter C:\UPL).
The INSTALL program creates the C:\TUNER directory in the R&S UPL (if
it is not already available) and copies the BASIC programs and all setups
required for the application into this directory.
Starting the Application Software
The application program is executed by way of the automatic sequence
control of R&S UPL. After starting the UPL program, press the F3 key (on
the external keyboard) to select automatic sequence control.
The logging function should be switched off; this is the case when "logging
off" is displayed at the bottom right of the screen. With the logging function
on, all commands entered in the manual mode would be appended to the
program and so use up memory. The logging mode can be switched on and
off with the F2 key of the external keyboard.
The application programs have to be called from path C:\TUNER where all
program modules and setups are searched for. The path can be changed in
the following ways:
• In the R&S UPL with the "Working Dir" command in the FILE panel
• By calling one of the setups required for tuner measurements
• In the R&S UPL-B10 with the BASIC command line UPD OUT
"MMEM:CDIR '\TUNER' "
• Via the SHELL of the automatic sequence control by entering CD TUNER
and then EXIT
• At DOS level by entering CD TUNER
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Measurements on FM Tuners
The program floppy contains the BASIC program TUNER.BAS for
automatic sequence control. The program is loaded and started by entering
•
LOAD"TUNER"
•
RUN
The respective softkeys displayed at the bottom of the screen in the
automatic sequence control mode may be used instead.
• Upon delivery, the setups are configured so that measurement results are
output to a "default" printer. This means that the printer settings used
last by R&S UPL will be used again. The screen of the R&S UPL is set
to colour display and an external monitor is driven (may be used).
IMPORTANT: Correct execution of the software cannot be guaranteed if
changes are made in the setups.
Setup Conversion for Firmware Updates
For updating the UPL firmware, the setups may have to be converted. This
is done automatically when the setup is loaded, but the conversion may
delay the loading. To avoid this, the setups can be converted and stored
before the application software is started. This can be done in two ways:
•
At the DOS level by calling the conversion program
DO_CONV \TUNER, which converts all setups in the TUNER directory
•
In the R&S UPL by loading and storing each setup
IMPORTANT: In the case of READ ONLY setups, the "r" attribute has to be
deleted first (at the DOS level with command ATTRIB -r).
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Measurements on FM Tuners
6 Measurements to Standard
Standard Test Conditions
All measurements are to be performed under test conditions specified by
the standard. In addition to correct supply voltage, specified ambient
temperature, etc, this also means that any squelch that may be used has to
be switched off in order not to impair the measurements.
The antenna signals used for the measurements must meet certain
requirements (standard radio frequency input signal). To avoid description
for each single measurement, all the conditions are listed and described
below:
Standard Test Frequency
Frequency range in MHz
Standard frequency in MHz
65.8 to 73.0
69
76.0 to 90.0
83
87.5 to 104.0
94
87.5 to 108.0
98
The standard test frequency is a function of the frequency range of the
receiver. The desired test frequency in the range from 65 MHz to 108 MHz
can be entered under SETUP.
Standard Frequency Deviation
Operating mode/signal
RMSD ± 50 kHz
RMSD ± 75 kHz
Mono
± 50 kHz
± 75 kHz
Stereo
± 45 kHz
± 67.5 kHz
Pilot tone
± 4.5 kHz
± 6.75 kHz
The standard frequency deviation corresponds to the permissible rms
deviation (RMSD) for mono and to 90% of RMSD for stereo. The maximum
system deviation of ±50 kHz or ±75 kHz can be selected under SETUP.
The permissible useful deviation (90% of RMSD) and the pilot tone
deviation (9% of RMSD) are automatically calculated from this value.
Preemphasis
The fact that the amplitude of voice and music signals goes down when the
frequency increases is normally utilized in sound broadcasting to reduce the
noise. Preemphasis can be used in this case to increase the high signal
frequencies during modulation in the transmitter. The inverse frequency
response in the receiver again reduces these signal components so that a
flat overall frequency response is obtained, but the reduction of the high
frequencies in the receiver significantly reduces the noise. When
measurements are performed, care must however be taken that the
maximum deviation of the system is not exceeded even at high frequencies
while preemphasis is on. Therefore, if measurements are performed at the
modulation frequency with preemphasis on, the deviation should be
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Measurements on FM Tuners
adjusted so that the maximum permissible deviation is only attained at a
modulation frequency of 15 kHz.
The frequency response of preemphasis has the effect of a 1st order
highpass filter with predefined time constant. A country-specific value of
50 µs or 75 µs is used. Measurements without preemphasis can also be
performed.
Preemphasis with a time constant of 50 µs increases the signal by a factor
of about 4.8 at 15 kHz relative to low frequencies. This yields the maximum
deviation that can be adjusted for frequency response measurements
(approx. 20% of the maximum deviation with 50 µs preemphasis and
approx. 14% with 75 µs preemphasis). This influence must also be taken
into account for measurements with the 1 kHz standard test frequency. In
this case the setting is approx. 95% at 50 µs preemphasis and approx. 90%
at 75 µs preemphasis.
A preemphasis of 0 µs (no preemphasis), 50 µs or 75 µs can be selected
under SETUP.
Standard Modulation Frequency
The 1 kHz standard reference frequency should be used in this case.
Standard Input Level
The standard input level determines the antenna signal at the tuner input. It
is specified with 70 dB(fW) which corresponds to 40 dB(pW). In practice,
the antenna voltage is specified in most cases. A value of 70 dB(fW)
corresponds to 866 µV at an input impedance of 75 Ω.
The antenna test voltage to be used can be entered under SETUP in the
range 0.1 mV to 10 mV. This value is used for all measurements that are
performed without regard to the RF signal level.
Filters
A bandpass filter is required for some measurements at the audio
frequency outputs. The filter's passband range is 200 Hz to 15 kHz. To
suppress residual pilot tones, the attenuation at 19 kHz must be higher than
50 dB. This filter can be directly selected in the R&S UPL under IEC
TUNER. It either meets or exceeds the standard.
The A weighting filter to IEC 60651-1 is used for weighting noise signals.
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Measurements on FM Tuners
Configuration (SETUP menu)
F5
END
F6
F7
F8
F9
F10
F11
FRQ_RESP THDN_FRQ THDN_DEV CRSS_FRQ CRSS_LEV CRSS_DEV
F12
→
Press key F12 to display the next level of the softkey labels.
F5
F6
F7
F8
←
S/N_LEV
IN/OUT
PIL_SUPP
F9
F10
F11
F12
ALL
RECALL
SETUP
Press the SETUP key to select the configuration menu. The following is
displayed:
Enter the desired values and confirm with RETURN. After the last value has
been entered, all values are stored in a file and automatically used each
time the program is restarted.
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Measurements on FM Tuners
Operation
F5
END
F6
F7
F8
F9
F10
F11
FRQ_RESP THDN_FRQ THDN_DEV CRSS_FRQ CRSS_LEV CRSS_DEV
F12
→
Clicking the respective key starts the test routine. Since there are more
selection items than softkeys, the next softkey levels are called with F12.
F5
F6
F7
F8
←
S/N_LEV
IN/OUT
PIL_SUPP
F9
F10
F11
F12
ALL
RECALL
SETUP
The next higher level can be selected with the F12 key as long as the →
arrow is displayed below the key. With F5, the user can return to the next
lower level as long as the ← arrow is displayed below the key. At the lowest
level, END is displayed instead of the arrow. After pressing F5, the query
"Do you really want to quit?" is displayed allowing the test program to be
terminated.
A test routine is started by clicking the respective key, but the tuner to be
measured has to be aligned to the test frequency before the routine is
started. When the program is started, the R&S SML is automatically set to
the desired test frequency and the desired antenna level.
After each measurement, the result is displayed and the softkeys are
labelled as follows:
F5
F6
F7
CONT
F8
F9
F10
EXP-FILE TRC-FILE
F11
F12
PCX-FILE
PRINTER
A measured trace can now be stored in the form of an export file, trace file
or PCX picture, or a hardcopy can be printed. The files are stored in the
C:\TUNER\RESULTS directory. After storage, the respective key label is
blanked to prevent the trace being stored twice.
Clicking the CONT key restores the selection menu for the various
measurements.
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Measurements on FM Tuners
When the EXP_FILE key is pressed, the displayed traces can be saved in a
file in ASCII export format. This file has the name EXPxx.EXP, xx
representing a consecutive number (of max. 5 digits). This allows direct
import and processing of measurement results by means of other programs
such as Excel.
When the TRC_FILE key is pressed, the displayed trace is saved in a file in
ASCII format. This file has the name TRCxx.TRC, xx representing a
consecutive number (of max. 5 digits). The TRC files can be reloaded in the
R&S UPL and displayed.
The screen content can be copied into a PCX file with the aid of the
PCX_FILE key. This file has the name PICxx.PCX, xx representing a
consecutive number (of max. 5 digits). Thus the measurement results can
also be used in word processing programs, for instance. The entire screen
except for the softkey line is always copied.
Since both the EXP, TRC and PCX files are consecutively numbered, it is
useful to copy the files of a measurement sequence, for instance, and to
save them under a new name. The original files can then be deleted. Thus,
results can be identified more easily and a mixup between them avoided.
To this end, a DOS shell can be called after termination of the test program
(e.g. with key F5) by entering the command SHELL <RETURN>. The files
can then be copied or renamed with the aid of DOS commands (standard
procedure in the C:\TUNER\RESULTS directory). Entering EXIT
<RETURN> restores BASIC without the program being cleared. The
program can be immediately restarted with RUN.
The screen content can be output to a printer by pressing the PRINTER
key. In this case the desired printer settings are not selected by the
program. The printer remains set as selected last in the manual mode of
the R&S UPL. The desired printer, scaling and format should therefore be
manually set once in the OPTION panel of the R&S UPL prior to the
measurement. It is recommended to select LOW or MEDIUM resolution
and as far as possible integer scale factors for the printer output. If
fractional scale factors (especially values <1) are used, the pixel values are
interpolated and the print quality might be reduced.
When RECALL is selected, all saved data records are displayed. After a
data record has been selected, the data is loaded and results are displayed
in numeric form like after a measurement.
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Measurements on FM Tuners
An automatic sequence of all measurement functions is started with ALL.
All measured traces are temporarily stored and evaluated by the program, if
required. After all measurements have been terminated, the results are
displayed numerically and softkeys are labelled as shown below:
F5
F6
F7
BACK
F8
VIEW
F9
F10
F11
SAVE
F12
REPORT
When the SAVE key is pressed, the user is first asked to enter additional
information for the report, and then the results are stored after a file name
has been entered (max. 8 characters). The following files are created in the
C:\TUNER\RESULTS directory.
•
•
•
•
•
•
•
•
•
•
•
Name.REP
Name.RES
Name.FRQ
Name.TNF
Name.TND
Name.CRF
Name.CRL
Name.CRD
Name.SNL
Name.IOL
Name.PIS
Report information
Numeric measurement results
PCX file of frequency response measurement
PCX file for THD+N versus frequency measurement
PCX file for THD+N versus deviation measurement
PCX file for crosstalk versus frequency measurement
PCX file for crosstalk versus RF level measurement
PCX file for crosstalk versus deviation measurement
PCX file for S/N versus RF level measurement
PCX file for IN/OUT versus level measurement
PCX file for pilot tone suppression
The program temporarily selects the printer settings desired for report
printing. It is assumed that the printer is deskjet-compatible. This printer
type is emulated by laserjet and many other printers. After printout, the
original settings are restored.
Using the VIEW key the grahpics can be reviewed in a sequence.
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Measurements on FM Tuners
7 Measurement Functions
Audio Frequency Response
The audio frequency response of a UHF receiver is influenced by the quality
of the IF section, detector, stereo coder and deemphasis circuit.
The measurement is performed under the conditions specified by the
standard but without a bandpass filter.
The emphasis of 50 µs or 75 µs specified by the standard for VHF FM
transmissions is simulated in the Signal Generator R&S SML. This means
that low-frequency audio signals are modulated with a low deviation. The
frequency deviation is then increased by emphasis to the maximum
permissible deviation at the upper frequency limit.
This effect is compensated for by the deemphasis circuit in the tuner so that
the frequency response of the audio signal becomes as linear as possible.
Modern instruments have a deviation of max. 1 dB at the lower frequency
limit and of max. 3 dB at the upper end of the transmission range
(referenced to 1 kHz).
The level deviation between the two stereo channels is also a quality
criterion because level differences shift the center for stereo sound
impression.
Fig. 1: Audio frequency response of a stereo receiver referenced to 1 kHz in
the left channel
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Measurements on FM Tuners
THD and Noise as a Function of Modulation Frequency
Distortions are caused by RF and IF sections and by the detector in the
receiver but also by AF amplification circuits. IEC 60315 specifies also
measurements that characterize the effects caused by the amplifier section.
However, most of the distortions are normally produced by the tuner.
For THD measurements, the receiver is operated under standard
conditions. The two stereo channels are modulated simultaneously; the
modulation frequency is swept between 20 Hz and 5 kHz. THD and noise
are measured with reference to the total output signal. The result - in % or
dB - is graphically displayed versus the modulation frequency (THD+N
measurement). The bandpass filter specified by the standard cannot be
used here because frequencies below 200 Hz are also to be measured. In
order not to corrupt measurement results by residual pilot tones, the
measurement bandwidth is limited by a 15 kHz lowpass filter.
Fig. 2: THD+N measurement versus modulation frequency
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Measurements on FM Tuners
THD and Noise as a Function of Modulation Deviation
In Fig. 2, THD was measured versus frequency at a deviation reduced by
preemphasis. Particularly in the detector stage of the tuner, the THD may
considerably vary depending on the actual frequency deviation. This effect
can be determined by a THD measurement at 1 kHz versus the signal
deviation.
Fig. 3: THD+N measurement versus modulation deviation
When the deviation is small, the noise component dominates while the THD
increases with increasing deviation. Fig. 3 shows the result of a high-class
tuner where THD increases only slightly at larger deviations.
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Measurements on FM Tuners
Crosstalk as a Function of Modulation Frequency
Crosstalk occurs when signal components of a channel are coupled into
another audio channel. This reduces channel separation and thus impairs
the stereo effect. Crosstalk attenuation is the level ratio of the wanted signal
in a channel to the unwanted signal coupled into the other channel. It is
specified as attenuation in dB. Crosstalk is measured in both directions.
The measurement is performed under the conditions specified by the
standard. As with measurements of the audio frequency response,
emphasis is switched on so that a smaller deviation is set at low
frequencies. Only the left channel is modulated at first with a modulation
frequency that is varied between 200 Hz and 15 kHz. The level is measured
in both channels and the ratio is formed. To suppress the noise
components, a selective measurement is carried out. The measurement is
repeated in the modulated right channel. Results are graphically displayed
as shown in Fig. 4.
Common crosstalk values at 1 kHz are within 30 to 40 dB.
Fig. 4: Crosstalk attenuation as a function of modulation frequency
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Measurements on FM Tuners
Crosstalk Attenuation as a Function of Modulation
Deviation
The crosstalk attenuation may also be a function of modulation deviation.
To investigate this effect, the crosstalk attenuation can be measured as a
function of the deviation. The measurement is performed analogously to
that versus the modulation frequency.
Fig. 5: Crosstalk attenuation as a function of modulation deviation
Crosstalk as a Function of RF Level / Stereo Switching
Threshold
Another test is crosstalk measurement as a function of antenna input level.
The receiver is operated under conditions specified by the standard but is
set to maximum deviation. Starting at 100 nV, the input level is increased to
10 mV. Results are graphically displayed as is shown in Fig. 6.
This measurement shows the behaviour of the tuner when weak stereo
signals are received. In the case of very weak antenna signals, reception is
in the mono mode, i.e. the same signal is transmitted in both channels. In
the diagram, this can be identified by the absence of crosstalk attenuation
(0 dB). When the antenna voltage is increased, the stereo decoder starts
operating at a certain level. This stereo threshold is clearly visible in Fig. 6
because of the sudden increase in crosstalk attenuation. The level of the
stereo threshold is specified in the test report. For this purpose, a point is
evaluated that has a crosstalk attenuation of 10 dB in both directions.
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Measurements on FM Tuners
Fig. 6: Crosstalk attenuation as a function of the antenna level
Since the pilot-tone method is used, the noise increases when the stereo
decoder is switched on. This is shown by the input signal / output signal
characteristic in Fig. 8. The sudden noise increase is often disturbing
particularly if this area is repeatedly traversed as the transmitter input signal
varies. This is often the case in moving vehicles. For this reason smooth
stereo switchover was developed where the crosstalk between the stereo
channels is gradually increased with increasing antenna level. The use of
such a circuit can be seen in the diagram where the crosstalk increases
gradually above the stereo switching threshold.
Stereo operation is signalled on many FM broadcast receivers. However,
the response threshold for this stereo indication need not be identical with
the level at which the stereo coder starts to operate. Particularly when
smooth stereo switchover is used, stereo reception is often signalled only
when the received field strength is sufficiently high for adequate channel
separation.
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Measurements on FM Tuners
S/N Ratio as a Function of Input Level
The S/N ratio is the ratio of the audio frequency voltage of the signal to the
noise voltage. According to IEC 60315-4, different weighting filters may be
used with these measurements, but the test method with A-weighting in the
HiFi sector has been adopted as standard and is the basis for the
measurements described below.
The S/N ratio of receivers can be determined in different ways:
• When the sequential method is used and a modulated signal is received,
the audio output voltage is measured, then modulation is switched off
and the noise is measured. This corresponds to the in/out
measurement shown in Fig. 9.
• When the simultaneous method is used and a modulated signal is
received, the level of the 1 kHz audio signal is measured. The noise
voltage is then determined with the aid of appropriate bandstop filters
while a modulated RF signal is present. Since the noise output voltage
of an FM receiver increases under certain circumstances when a
modulated signal is present, this method more accurately represents
the conditions encountered in practical applications.
In this application, the S/N ratio is measured with the simultaneous method.
The rms value is measured with an A-weighting filter. This corresponds to
the conditions used in the HiFi world.
The receiver is operated under the conditions specified by the standard and
set to maximum deviation. The signal is stereo-modulated with a 1 kHz
signal. The use of the bandpass filter reduces the measurement range to
between 200 Hz and 15 kHz; effects of hum or insufficient pilot tone
suppression are not taken into account. After the audio output voltage has
been determined, the 1 kHz component is separated by a notch filter and
the noise voltage is measured. In order not to influence the noise by the
THD of the 1 kHz signal, a measurement is chosen in the Audio Amplifier
R&S UPL, where any harmonics are also ignored. The S/N ratio is
calculated from the signal voltage and the noise voltage and graphically
displayed as a function of the RF input level (Fig. 7).
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Fig. 7: S/N ratio as a function of the antenna voltage
Input Signal / Output Signal Characteristic
The in/out characteristic illustrates the relation between the antenna input
voltage and the audio signal generated by the tuner. This is one of the most
important measurements because the diagram provides a great deal of
information particularly when the noise output voltage is considered in the
measurement.
The receiver is operated under the conditions specified by the standard. For
measuring the audio output voltage, the signal is modulated with maximum
deviation; for the noise measurement with 0 deviation. The antenna level is
logarithmically swept from 100 nV to 10 mV. The audio output signal is
measured and graphically displayed with the maximum output voltage set to
0 dB. The level sweep is repeated and the noise output voltage is recorded.
A diagram as shown in Fig. 8 is obtained.
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Fig. 8: Output / input signal characteristic of tuner with 50 dB S/N ratio
The following information can be obtained from the diagram:
• Characteristic of audio output signal
Only above a certain antenna input voltage will the tuner be able to detect
an audio signal in the RF signal. This is at the point in the diagram where
the signal characteristic and the noise characteristic separate. The
associated level can be referred to as the absolute sensitivity of the tuner
but plays only a minor role in amplifier characteristics.
The output signal characteristic always shows a steep rise when the
antenna signal increases and then continues at a constant level.
Depending on the receiver, this maximum level (which is also the reference
level for the measurement) is attained with different RF levels.
• Noise signal
As the RF signal increases, the noise goes down until it attains its
minimum. This minimum value is lower in the mono mode than in the stereo
mode.
In the stereo mode, the level sweep of the input signal first shows the same
characteristic as for a mono signal. When a certain signal level is attained,
the stereo decoder starts operating (stereo threshold). This is first noticed
by a clear increase of noise. As the input signal level rises, the noise is
reduced again but normally does not attain the minimum value attained in
mono reception.
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• Max. S/N ratio
The maximum S/N ratio can be determined from the maximum audio output
signal level and the minimum noise level. This value is also documented in
the test report.
• Noise-limited sensitivity
The noise-limited sensitivity is the antenna level at which an audio signal
with defined S/N ratio is obtained. The sensitivity value is at the same time
a measure of the replay quality of the audio signal.
If HiFi stereo receivers are used, this value should be tested at an S/N ratio
of 50 dB.
For instance, sensitivities of approximately 3 µV for mono reception and of
30 to 40 µV for stereo can be assumed for modern, high-quality tuners. The
sensitivity for an S/N ratio of 40 dB and 30 dB is also determined. These
values are often attained only below the stereo switchover threshold. They
are documented in the report for information only.
Suppression of Pilot Tone and Subcarrier
A pilot tone is transmitted at 19 kHz to identify stereo broadcast
transmissions. In order not to disturb instruments such as amplifiers and
recorders connected to the tuner, the pilot tone and its subcarriers must be
sufficiently suppressed in the tuner. This is done by circuits in the stereo
coder or by means of filters at the tuner output.
Another quality criterion of a tuner is the suppression of pilot tone, auxiliary
carrier or other interfering products. The audio frequency voltage normally
measured at maximum deviation and at conditions specified by the
standard is then used as a 0 dB reference in the display. Subsequently, the
useful signal modulation is set to 0 and the remaining frequency
components are displayed. The pilot signal suppression is measured and
documented in the report. At the same time, the highest signal component
is searched for and also documented in the report as interference
suppression.
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Fig. 9 shows the signal spectrum. The individual frequencies can be clearly
identified.
High-quality tuners should suppress all frequency components above the
transmission range by at least 50 dB.
Fig. 9:
Unmodulated output spectrum of a tuner with residual pilot tone,
subcarrier and interfering components
8 Demo Programs for Production Tests
With the installation of the tuner program automatically the programs
MULTFREQ.BAS and FASTDIST.BAS are generated. Those programs are
intended as examples for fast frequency response and distortion
measurements in production line use.
After loading and starting the program MULTFREQ it will be asked if the
measurement should be done with approximately 20, 50 or 100 frequency
points. Then after each hit of the SPACE key a measurement is done and
displayed and the measured frequency response is PASS/FAIL checked
versus the given frequency tolerance limits. The program can be stopped
using the ESC key.
The program FASTDIST works in a similar way but after start it asks for the
limit value of the measured distortion. The measurement is started with the
SPACE key, the measured distortion value (THD+N) will be displayed and
checked against the verdict value. The program can be stopped using the
ESC key.
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9 References
Methods of measurement on radio receivers for various classes of emission
- Part 4: Receivers for frequency-modulated sound broadcasting emissions.
IEC 60315-4: 1997
10 Ordering Information
Audio Analyzer
Universal Sequence Controller
(option)
R&S UPL
R&S UPL-B10
1078.2008.06
1078.3904.02
Signal Generator
Signal Generator
Signal Generator
Signal Generator
Stereo/RDS Coder (option)
R&S SML01
R&S SML02
R&S SML03
R&S SMV03
R&S SML-B5
1090.3000.11
1090.3000.12
1090.3000.13
1147.7509.13
1147.8805.02
Matching Pad 50/75 Ohm
R&S RAM
0358.5414.02
ROHDE & SCHWARZ GmbH & Co. KG . Mühldorfstraße 15 . D-81671 München . Postfach 80 14 69 . D-81614 München .
Tel (089) 4129 -0 . Fax (089) 4129 - 13777 . Internet: http://www.rohde-schwarz.com
This application note and the supplied programs may only be used subject to observance of the conditions of use set forth
in the download area of the Rohde & Schwarz website.
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