University of Jordan
Electrical Engineering Department
EE 429
Communications Lab
EXPERIMENT 6
Using the Spectrum Analyzer
Lab Supervisor: Dr. Mohammed Hawa
Dr. Ahmad Mustafa
Lab Engineer: Eng. Reem Al-Debs
Prepared By: Eng. Noor Awad
Revised and updated by: Dr. Mohammed Hawa
EXPERIMENT 5
USING THE SPECTRUM ANALYZER
DISCUSSION
A spectrum analyzer is a measurement device used to examine the spectral composition of an
electrical waveform (i.e., display its power spectral density).
There are analog and digital spectrum analyzers: An analog spectrum analyzer uses a variable
bandpass filter whose mid-frequency is automatically swept through the range of frequencies
of which the spectrum is to be measured. A digital spectrum analyzer computes the Fast
Fourier transform (FFT), a mathematical process that transforms a waveform into the
components of its frequency spectrum.
The figure below shows the HP 8568A spectrum analyzer you will use in this lab.
This spectrum analyzer has the following features:
Frequency range: 100Hz to 1.5 GHz.
Amplitude range: -137dBm (0.02 pW) to +30 dBm (1000 mW).
Coupling: Input 1 (DC coupled), Input 2 (AC coupled).
THE FRONT PANEL
The front panel keys on the spectrum analyzer provide convenient control over what you see
on the spectrum analyzer’s DISPLAY, such as the center frequency, frequency span, reference
level, resolution bandwidth, sweep time, etc. Any of these function can be selected by pressing
its corresponding key and then changed by using the data controls.
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THE FUNCTION KEYS
The available FUNCTION KEYS are shown in the Figure below. They are:
CENTER FREQUENCY: this key changes the center frequency on the
display.
FREQUENCY SPAN: this key changes the total display frequency range
symmetrically about the center frequency. Frequencies within the
frequency span are shown on the display.
START and STOP FREQUENCY: If you do not want to specify a center
frequency and a frequency span, you can still specify the frequency
range on the display using the Start Frequency (left-most frequency on
the display graticule) and Stop Frequency (right-most frequency on the
display graticule) keys. Start and Stop Frequencies are mutually
exclusive with the Center Frequency and Frequency Span functions.
REFERENCE LEVEL: this key changes the absolute amplitude level of the top graticule line.
THE DATA CONTROLS
The DATA controls are located to the right and left of the FUNCTION keys. They are used to
change the values of the center frequency, frequency span, start/stop frequency and reference
level. These controls are:
The Data knob allows the continuous change of center frequency,
frequency span or start/stop frequencies, reference level, and the position of the marker.
Clockwise rotation of the DATA knob will increase the function value.
The DATA STEP Keys allow rapid increase or decrease of
the active function value. The step size is dependent upon the analyzer's measurements range.
Each press results in a single step.
The DATA Number/Units Keyboard allows
exact value entries to center frequency, frequency span, start/stop frequency, reference level,
log scale and the positions of the markers and display line.
THE MARKER
The NORMAL button in the MARKER group (see Figure below)
activates the marker, which is a bright spot that lies directly on the
display trace. The horizontal position of an activated marker is
controlled by the Data controls. It is used for direct readout of
amplitude and frequency of the trace. You can deactivate the marker
by activating another function, such as CENTER FREQUENCY.
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PROCEDURE A – AM/FM Radio Stations
Each AM/FM radio station occupies a predetermined bandwidth around a governmentallocated center frequency. In this procedure, you will pick up a radio station on the spectrum
analyzer and observe its characteristics:
1. Switch the spectrum analyzer ON.
2. Connect one end of a BNC probe to the spectrum analyzer’s input, and hold the other end in
your hand. This probe will act as an antenna to pick up radio signals present on the air. You
should see a group of signals appearing on the display. Each signal represents a radio station.
3. Press the CENTER FREQUENCY button and use the keyboard to set the frequency to
99MHz (“Amman FM” radio station). Set the FREQUENCY SPAN to 2 MHz.
4. You will see a single chunk of frequencies appear on the display.
5. To measure the actual frequency and power of the signal, activate the marker by pressing
the NORMAL button. Using the bright spot (marker) you now see on the display read the
center frequency of the station, its bandwidth and maximum power level. Record your
answers.
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PROCEDURE B – A single sinusoidal Signal
1. Use the VCO-HI block on your ANALOG COMMUNICATIONS kit to generate a pure
sinusoidal signal. What does the power spectral density of a sinusoidal signal look like?
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2. Connect the spectrum probe to the output of the VCO-HI (make sure you also connect the
GND of the probe to a GND terminal on the kit).
3. Turn the POSITIVE SUPPLY knob on the kit fully CW and the VCO-HI potentiometer knob
fully CW.
4. Set the CENTER FREQUENCY of the spectrum analyzer to 1455 kHz, and its FREQUENCY
SPAN to 700 kHz.
5. Activate the marker by pressing the NORMAL button, and move the bright spot to the top
of the pulse. Read the actual frequency of the sinusoidal signal generated by VCO-HI.
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6. Turn the POSITIVE SUPPLY knob on the kit fully CCW, and read the frequency of the signal
one more time.
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7. Turn the potentiometer (amplitude) knob on the VCO-HI block CCW and then CW. How
does this affect the signal you see on the display?
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PROCEDURE C – An FM modulated Signal
1. Use the function generator combined with the VCO-LO block on the ANALOG
COMMUNICATIONS kit to generate an FM-modulated
signal just like you did in Experiment 2 (see Figure).
2. Connect a two-post connector on the 452 kHz terminal.
Turn the potentiometer (amplitude) knob of VCO-LO fully
CW. Turn the NEGATIVE SUPPLY knob fully CW.
3. Set the frequency of the function generator (message signal frequency) to 40 kHz and the
amplitude to maximum.
4. Connect the spectrum analyzer probe to the FM output on VCO-LO.
5. Set the spectrum’s CENTER FREQUENCY to 534.9 kHz, and its FREQUENCY SPAN to
437.1 kHz.
6. Explain what you see on the spectrum display.
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7. How many Significant Side Pairs (SSP) appear on the display?
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8. With the help of the marker, measure the frequency of each one of the SSP’s.
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9. What is the distance between each impulse in the FM spectrum?
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10. What is the bandwidth of this FM spectrum?
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