ROSE HULMAN INSTITUTE OF TECHNOLOGY Department of Electrical and Computer Engineering The Spectrum Analyzer Operating Principles and Instructions by Bruce A Black with changes by Mark A Yoder 1 Introduction These instructions apply specifically to the Tektronix 7L5 Spectrum Analyzer plug in but the general operating principles are valid for many other commercially available instruments as well The instructions are divided into three sections Section 2 is a step by step cookbook for turning on the instrument and observing the spectrum of a signal Section 3 is a very brief description of the way spectrum analysis is done Section 4 discusses the structure of the Tektronix 7L5 and describes the operation of the main panel controls 1 1CAUTIONS 1 The spectrum analyzer is a sensitive instrument designed for measuring voltages in the millivolt range It can be burned out by signals that are not large by normal laboratory standards As a rule of thumb input signals should be no larger than one volt peak to peak with a zero DC level Input signals should always be monitored on an oscilloscope before being applied to the spectrum analyzer input 2 Unlike an oscilloscope the spectrum analyzer cannot be used by twiddling the knobs until a stable display is obtained It is possible to obtain false and uncalibrated but stable displays Before using the instrument read the instructions 2 Operating Instructions This section contains a detailed step by step procedure for initializing the spectrum analyzer and displaying the spectrum of a waveform Do not connect an input signal to the spectrum analyzer unless Caution 1 has been complied with 2 1 Initialization To render the spectrum analyzer operational the following procedure should be followed First be sure that the analyzer is plugged into the right hand compartment of a Tektronix 7000 series oscilloscope as Figure 1 The Spectrum Analyzer BAF 12 4 01 11 11 AM SpectrumAnalyzer doc Page 1 of 9 The Spectrum Analyzer Operating Principles and Instructions shown in Fig 1 Set the oscilloscope s VERTICAL MODE to right and the TRIGGER SOURCE to right Turn on the power If necessary adjust the READOUT and INTENSITY controls so that the readout of the control settings and the sweep appear on the CRT When the spectrum analyzer is turned on the dot frequency will be automatically set to zero and the reference level will be automatically set to its maximum value This will be the case even if these controls had been set to other values when the power was turned off Observe that the DOT FREQUENCY and REFERENCE LEVEL controls have no scales Readouts on the top of the CRT indicate the settings of these controls dot frequency is on the right and reference level is in the center Unlike an oscilloscope a spectrum analyzer cannot be operated by twiddling the knobs until a stable trace is obtained As it is possible to obtain false traces and to misinterpret the display the following initial control settings should be used Fig 2 shows the location of the first three settings 1 Dot Frequency Set the FINE TUNING off 2 Reference Level Set the INPUT BUFFER off make sure the VAR control is set for calibrated operation The LOG buttons can be set for 10 dB div 2 dB div or lin as desired but 10 dB div is the scale most frequently used 3 Triggering Set the SOURCE to free run and the MODE to norm Set the LEVEL SLOPE to 0 and adjust until a sweep is obtained Fig 3 shows the location of the rest of the controls 4 Digital Storage Set the DISPLAY for A and B on Set SAVE A off and MAX HOLD off Set the BASELINE CLIPPER full counterclockwise 5 Frequency Span Div Set to max 6 Resolution Set to coupled 7 Time Div Set to auto 8 Input The L3 input module has two BAF 12 4 01 11 11 AM SpectrumAnalyzer doc Figure 2 Location of Left Controls Page 2 of 9 The Spectrum Analyzer Operating Principles and Instructions switches that are used to set the input impedance and reference level The TERMINZ switch should normally be set to 1 M unless a 50 or 600 impedance is specifically required Be careful since 50 can appear as a short circuit to some signal sources The REF switch is normally set to dBV when the TERMINZ switch is set to 1 M 2 2 Calibrating the Analyzer With the analyzer set up as described above do the following to calibrate the instrument 1 Connect the CALIBRATOR signal to the INPUT BNC connector on the plug in module with a short length of coaxial cable The calibrator produces a 500 kHz square Figure 3 Location of Right Controls wave at a level of 40 dBV Set the DOT FREQUENCY to 500 kHz The value of the dot frequency is displayed in the upper right corner of the CRT Set the REFERENCE LEVEL to 0 dBV The value of the reference level is displayed at the top center of the CRT Set the FREQUENCY SPAN DIV to 5 kHz 2 Adjust the HORIZ POSITION and VERTICAL POSITION controls little black knobs at the lower right of the front panel to place the peak of the displayed calibration signal exactly at the center of the CRT screen You are now ready to use the spectrum analyzer You should calibrate the analyzer every time you turn it on Do not change the settings of the position controls while you are using the analyzer or you will have to repeat the calibration procedure 2 3 Display of a Waveform With the spectrum analyzer initialized as described in Section 2 1 with the FREQUENCY SPAN DIV set to max the CRT will display the entire frequency range from DC to 5 MHz There will be a spike called the DC Marker at the left edge of the screen This DC Marker is always present at 0 Hz and must not be confused with the display of an input signal When in doubt disconnect the input the DC Marker is present even when no input signal is applied to the analyzer BAF 12 4 01 11 11 AM SpectrumAnalyzer doc Page 3 of 9 The Spectrum Analyzer Operating Principles and Instructions Above the DC Marker there will be a bright dot This dot is located at the frequency determined by the DOT FREQUENCY control Increasing the dot frequency moves the entire display including the dot to the right After a few seconds the display will reset itself to its original position leaving the dot offset to the right The dot together with the dot frequency readout in the upper right corner of the CRT can be used to make frequency measurements When the FREQUENCY SPAN DIV control is set to any position other than max the dot will appear at the top center of the CRT and the dot frequency will be the frequency at the center of the CRT Normal operation of the
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