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Rohde & Schwarz UPD, Примечание к применению

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1GA16_1L.DOC

ROHDE & SCHWARZ

11

4

4.3 

.3 FFT Noise

FFT Noise

4.3.1 Advantages, Limits and Main Fields of Application

• 

The pure measurement time - without signal generation time - is very short (online response).

• 

An increase (doubling) of the frequency resolution increases the measurement time only
insignificantly.

• 

Up to 3712 measurement points  - 7424 in the UPL - are possible (1024 in the sweep mode) so that a
higher frequency resolution is possible over the entire sound frequency range with linear scaling.

In the present application the FFT noise mode can be used for measuring the impedance and sound
pressure characteristic. Main fields of application are:

• 

production testing (go/nogo tests),

• 

adjustments,

• 

sound level measurements in not completely anechoic rooms.

4.3.2 Setup and Program Settings

Selecting the FFT noise mode (see section 4.6, Selection of Measurement Mode) activates the
SPEAKER.SCO setup with the following settings:

FIG 4: Example of SPEAKER.SCO setup for FFT noise mode

The values for Lower Freq, Upper Freq, VOLT RMS and FFT size are generated from user entries or
taken from the configuration file SPEAKER.CFG.

Remarks:

• 

If an entry is made in the GENERATOR panel which requires a regeneration of the noise signal, the
generator starts the very time-consuming noise signal computation (GEN BUSY in the status display).
Noise-signal generation can be aborted by a keystroke in the case of manual control but this is not
possible when the universal sequence controller is used.

• 

"Window RECTANGULAR" switches off the window function.

• 

"Zooming ON" (with zoom factor 2) is only possible in the UPL, as the UPD generator is not able to
set frequency resolutions below 5.8584 Hz.

Содержание UPD

Страница 1: ...Loudspeaker measurements with Audio Analyzers UPD or UPL Application Note 1GA16_1L replaces 1GPAN16L Subject to change M Schlechter 07 97 Products Audio Analyzer UPD Audio Analyzer UPL ...

Страница 2: ...T MODES 9 4 1 MODES OF FREQUENCY RESPONSE ANALYSIS 9 4 2 FREQUENCY SWEEP 10 4 2 1 ADVANTAGES 10 4 2 2 SETUP AND PROGRAM SETTINGS 10 4 3 FFT NOISE 10 4 3 1 ADVANTAGES LIMITS AND MAIN FIELDS OF APPLICATION 11 4 3 2 SETUP AND PROGRAM SETTINGS 11 4 4 SWEPT BURST MEASUREMENT 12 4 4 1 ADVANTAGES AND MAIN FIELDS OF APPLICATION 12 4 4 2 PROGRAM SETTINGS 12 4 5 GO NOGO TESTS 12 4 6 SELECTING THE MEASUREMEN...

Страница 3: ...LARITY FOR ELECTRICAL SYSTEMS 30 5 4 2 POLARITY CHECK BY COMPARISON TO REFERENCE PHASE MEASUREMENT 30 5 4 3 POLARITY CHECK BY COMPARISON TO REFERENCE SOUND PRESSURE CHARACTERISTIC 31 5 5 DISTORTION 32 5 5 1 SPECIAL FEATURES OF THD N MEASUREMENT WITH POST FFT 33 5 5 2 SPECIAL FEATURES OF ZOOM FFT 33 6 COMBINING SEVERAL MEASUREMENTS 33 6 1 MACRO OPERATION 34 6 1 1 RECORDING A MACRO 34 6 1 2 RUNNING ...

Страница 4: ...stortion as a function of frequency using a variety of measurement procedures and displays Measurement of phase linearity and group delay as well as Various methods for checking the polarity The software not only yields highly accurate results for the development of loudspeakers but also supports measurements in production testing Impedance and frequency response curves can be referenced to a gold...

Страница 5: ...OS level and then the universal sequence controller is selected with key F3 external keyboard Note Before selecting the universal sequence controller make sure that the logging mode is switched off logging off displayed at the very right of the command line as otherwise commands entered in the manual mode are added to the application program and this might cause the BASIC program memory to overflo...

Страница 6: ...ould not be stored the application should be quit with CTRL BREAK SPEAKER SCO Setup file used as the basis for all commands of the application software Most of the settings in this file have to remain unchanged or are overwritten by the application software A few settings only may be or have to be updated by the user The program reads the setup but does not change it The final version of the setup...

Страница 7: ...equencies may also be selected but in this case the following aspects have to be considered the lower limit frequency of the analyzer is higher the resolution of the 8k FFT is only 41 67 Hz 100 kHz analyzer or 125 Hz 300 kHz analyzer the measurement speed is lower With the preliminary configuration terminated the setup has to be stored as complete setup under the same name and may be assigned the ...

Страница 8: ... four hierarchical softkey levels Level 1 main level selection of measurement implicit loading of a specific measurement module Level 2 selection of measurement mode frequency sweep or FFT noise see section 4 Measurement Modes with implicit re loading of SPEAKER SCO setup Level 3 entry of parameters and start of measurement Level 4 post processing and restart of measurement D RUN EXIT BACK BACK BA...

Страница 9: ...ed in detail in section 4 VOLTAGE F6 level 3 total voltage FFT noise or sinewave voltage frequency sweep at the same time DC voltage for measuring the DC resistance FREQ RNG F7 level 3 Lower Freq in Hz and Upper Freq in Hz frequency range within which sinusoidal lines are generated FFT noise or sweep points set frequency sweep With FFT noise mode reducing the frequency range also reduces the numbe...

Страница 10: ...be continued without any transitions that no window function is required for suppressing the FFT continuation error and an optimum frequency and level accuracy of the FFT is obtained Measurement time and frequency resolution depend on the selected FFT size or on the frequency stepwidth of the noise generator The smallest step of the 25 kHz generator is 5 86 Hz This is therefore the maximum permiss...

Страница 11: ...lution can be improved without any restrictions by reducing the frequency range with FFT the smallest step is fixed to 5 86 Hz UPD or 2 93 Hz UPL The generator setting time for each sweep point is very short while a time consuming noise signal generation is required with FFT noise Frequency sweep is possible with all measurements FFT noise can be used for voltage measurements only 4 2 2 Setup and ...

Страница 12: ...tely anechoic rooms 4 3 2 Setup and Program Settings Selecting the FFT noise mode see section 4 6 Selection of Measurement Mode activates the SPEAKER SCO setup with the following settings FIG 4 Example of SPEAKER SCO setup for FFT noise mode The values for Lower Freq Upper Freq VOLT RMS and FFT size are generated from user entries or taken from the configuration file SPEAKER CFG Remarks If an entr...

Страница 13: ...RMS mode TRIGGERED where the measurement is automatically triggered on detection of the signal level Same as with the sinewave sweep vary the frequency depending on the values set for start stop points and spacing The measurement time is a multiple of one period but at least one period Towards the upper end the number of periods is limited such that the measurement is terminated before the first e...

Страница 14: ...WP the frequency sweep for determining the phase THD N measurement ZOOM FFT Softkey 2 F6 THD TOTL selects the THD measurement forall harmonics d2 to d9 for determining the distortion display as a BARGRAPH for single measurements or sweep trace Softkey 3 F7 THD d2 selects the THD measurement for thefirst harmonic for determining the distortion display as a BARGRAPH for single measurements or sweep ...

Страница 15: ...s Impedance and Associated Parameters The impedance curve of a loudspeaker chassis or system gives information on important parameters which sufficiently characterize the response at low frequencies The parameters include the quality factor Qt resonance frequency fr and equivalent volume Veq Equations used in this Application Note are in line with DIN IEC 268 part 5 Since UPD and UPL comprise a DC...

Страница 16: ...level 3 it can be determined how many outliers runaways are tolerated by the algorithm for extreme value determination resonance and tuning frequency In this case the relative maximum or minimum values have to be determined For instance when searching for the maximum it may happen particularly with very flat impedance curves that a lower value is measured although the curve slightly rises To avoid...

Страница 17: ...According to DIN IEC 268 it is to be determined at least in the range from 20 Hz to 20 kHz Either logarithmic frequency sweep or linear FFT noise may be used for the measurement Results should in any case be displayed logarithmically If there are no special reasons for selecting FFT noise eg online response frequency sweep should be given preference that in this case a smaller number of measuremen...

Страница 18: ... are obtained by scrolling up through the obtained impedance value array The maximum is considered found if N RUNAWAYS 1 see section 5 1 2 Special Notes on Entry of Parameters are found in successive values in the descending order 5 1 6 Absolute Minimum and Maximum Impedance The absolute maximum of impedance Z max is normally not identical with the resonance impedance Using softkeys Cursor set to ...

Страница 19: ...sing direction 5 1 8 Q Factor The quality factor Qt is derived from the impedance curve according to the following formula BASIC syntax Qt Fr F2 F1 R0 SQR R0 2 R1 2 SQR R1 2 1 where Zr Fr Resonance impedance and resonance frequency see section 5 1 5 Zt Ft Tuning impedance and tuning frequency see section 5 1 7 R0 Resonance impedance standardized to R DC see section 5 1 3 DC Impedance R0 Zr RDC R1 ...

Страница 20: ...yed TRACE B the relative impedance characteristic is displayed as trace B while the absolute impedance characteristic is displayed as trace A see Fig 7 OFF switches the reference off so that only the absolute impedance characteristic is displayed 5 5 2 2 Sound Pressure Measurements Sound Pressure Measurements Sound pressure variations versus frequency allow conclusions to be drawn on the tone qual...

Страница 21: ... may amplify or attenuate the sound level at certain frequencies The strongest amplification or attenuation occurs when the in phase or quadrature signals are superimposed This effect cannot be completely avoided but it can be reduced by means of the following measures Covering all walls with absorbing material carpets curtains etc reduces the echo level Minimizing the microphone distance This inc...

Страница 22: ...tomatic control and make necessary entries In both cases the nominal loudspeaker impedance has to be entered REF IMPE F6 The selection between the manual and automatic mode is made by pressing softkey MANU F11 or AUTO F12 which starts the measurement or control After the start a new softkey level is displayed which offers only the STOP softkey F10 for terminating the measurement Manual mode After ...

Страница 23: ...n mV Pa SENSITIV F6 gain factor of microphone amplifier in dB GAIN F7 with no microphone amplifier connected a GAIN of 0 dB must be set distance between standard microphone and loudspeaker in m DISTANCE F8 so that the sound travel time can be taken into account For sweep measurements FREQ SWP or BRST SWP the room parameters are entered in addition Shortest echo path in meters ECHOPATH F9 Loudspeak...

Страница 24: ... voltage generated by the microphone should be stored as a 0 dB line Note In the case of sensitivity measurements the units µBar and Pascal cannot be entered If they are already set via a response measurement they are automatically changed to dBspl 5 2 5 Sensitivity Measurement and Radiation Angle The sensitivity measurement is a sound pressure measurement at a fixed frequency measurement mode SNS...

Страница 25: ...h sinewave signals for measurements under open field conditions eg in an anechoic chamber and near field measurements BRST SWP is a frequency sweep using burst signals for far field measurements also in not completely sound absorbing rooms Although the FFT noise measurement ensures a fastdisplay of the transmission range it can only be used with restrictions as far as numericcalculations are conce...

Страница 26: ...periods is measured to obtain a high accuracy measurements are only performed before the first echo arrives Echofree measurements can only be performed in the short time between the echo delay and the signal delay The frequency of exactly one measurement period during this time is the theoretical lower frequency limit f T T m s L L e a e a min 1 330 where La microphone distance Le shortest echo pa...

Страница 27: ...on range is calculated defined by a lower limit of 10 dB When this limit is exceeded for more than 1 3 third 1 9 octave the limit of the transmission range is attained Shorter drops are ignored The transmission range determined by the application program is indicated next to the graphics display Determination of the transmission range is started at the frequency specified under SNS FREQ If the lev...

Страница 28: ... any key the actual transition frequency is confirmed and the two curves are combined the near field measurement is rescaled to the level value of the far field measurement at the transition frequency Note The two curves are still available as separate data records after they have been combined They can be combined again by entering a new TRANSFRQ F11 in level 4 A combined near far field measureme...

Страница 29: ...ponse of Phase The phase linearity should be checked in a linear frequency sweep where the phase is measured between the loudspeaker input and the generated sound wave In the case of a linear phase response a line with a negative slope would ideally be obtained In order to obtain a continuous phase response without 360 shifts the sweep points displayed in the range from 0 to 360 have to be further...

Страница 30: ...points 1024 The condition f f Hz d m stop start 1023 150 should be fulfilled or for the microphone distance d m f f Hz stop start 153450 Example A microphone distance of 1 m and a frequency sweep of 20 to 20000 Hz yield a frequency increment of 150 Hz and therefore 135 sweep points As the example shows this restriction has relevance only in theory If the sweep should be carried out with a greater ...

Страница 31: ...cteristic 5 5 4 4 Polarity Polarity Correct polarity of crossover units loudspeaker arrays and systems becomes important if partly or completely identical signals are applied to several loudspeakers While an incorrectly poled mono broadband loudspeaker would hardly be noticed superposition of the 180 phase shifted sound waves of two stereo loudspeakers produces audible cancellations This also appl...

Страница 32: ...UT is then set up at the same distance and the phase is measured at the same frequency If the measured value is outside the selected tolerance the loudspeaker is incorrectly poled The inaccuracy of the distance between loudspeaker and standard microphone must not exceed 1 8 of a wavelength 45 360 For this reason this method cannot be used at high frequencies For the upper limit frequency flim the ...

Страница 33: ...r measuring the total harmonic distortion or any combination of discrete harmonics with or without display of the harmonics line spectrum This measurement function ignores the noise for measuring the 2nd and 3rd order distortion in line with DIN IEC 268 part 5 d 2 and d3 have to be selected THD N with or without post FFT of total signal FFT with or without zoom FFT for increasing the frequency res...

Страница 34: ...asurements to be made in the FAST mode In this mode of the THD N measurement useful signal suppression to obtain precision is left out This halves the measurement time which is particularly important for swept measurements Since the useful signal is displayed in its original size post FFT shows harmonics and useful signal in their correct proportion For measurements without post FFT display the FF...

Страница 35: ...he application software where the common start point is stored in the menu under UTILITY MACRO Any number of macros can be defined 7 sample macros SPEAKER0 to SPEAKER6 have been defined and assigned to the softkeys MACRO 0 to MACRO 5 They can be called up via USER MAC and may be overwritten any time Macros can be prepared and stored under any name with the aid of the USER MAC softkey 6 1 1 Recordi...

Страница 36: ...r compliance with tolerances printout termination of the macro MACRO 1 Frequency response measurement by means of FFT noise with automatic setting of a 1 W output power into 4 Ω and determining the sensitivity in the frequency band from 500 Hz to 1 5 kHz MACRO 2 Frequency response measurement by means of a burst sweep A near field measurement is optional MACRO 3 Sweep of THD measurement d2 to d9 w...

Страница 37: ...function as for storing Note With USER CFG a macro can be used with different configuration files Loading a configuration overwrites the currently used configuration If the currently used configuration should be saved it must first be stored in one of the configuration files and then called up again If a configuration file has been loaded by mistake which should not become the new default configur...

Страница 38: ...1GA16_1L DOC ROHDE SCHWARZ 37 ...

Страница 39: ...inter interface printing out the graphics window together with a special status panel and comments displaying limit violations in tabulated form OPERATION LIM REP Return to the application software is possible with the F3 key After the return all application softkeys are ready for use again 7 7 2 2 Printout Printout To print hardcopies a printer has to be connected to the UPD UPL and configured It...

Страница 40: ...O F6 are offered Thus it is possible to display or printout measured values with graphics window or full screen graphics Comments cannot be entered but will be printed provided the respective settings have been made in the OPTIONS panel 7 7 3 3 Restart Restart 7 3 1 Restarting a Measurement At the end of each measurement a new measurement can be triggered by means of the START softkey F12 Modifica...

Страница 41: ... means of the ESC key To avoid inadvertent termination of the application the query Exit this application y n is displayed before the application is quit and has to be confirmed with Y Any other entry causes the application to be continued The application can be aborted any time by means of the key combination CTRL BREAK Softkeys and variables remain unchanged likewise the configuration file SPEAK...

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