EMG3056 Microwave Measurement Techniques
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EXPERIMENT MM-1: SPECTRUM ANALYSIS
OBJECTIVE
 To demonstrate Spectrum Analysis measurement techniques of a signal source.
 To illustrate frequency and amplitude measurement using spectrum analyzer.
 To determine distortion and harmonic frequencies of a signal source.
 To analyse changes due to the effect of resolution and video bandwidths.
 To examine phase noise of a signal source.
 To show amplitude modulation sidebands using spectrum analyzer.
APPARATUS
1. Spectrum Analyser (MS2665C)
2. RF Signal Generator (SG-5150)
3. Audio Generator (GAG-809)
4. Coaxial cable, 1-m, BNC m-m
5. Coaxial cable, 1-m, BNC(m) to clip
6. Adapter N(m)-to-BNC(f)
INTRODUCTION
Spectrum Analyser is perhaps the most widely used test equipment for microwave
measurements. It can be used for measuring:
 RF Power (relative and absolute)
 Frequency
 Noise
 Spectral purity (phase noise)
 Modulation
 Distortion
A Spectrum Analyser makes measurement in the frequency domain rather than the
time domain. On an oscilloscope, a sine waveform is traced with respect to time. One may not
be sure whether the waveform is a pure sinewave with no distortion. If the signal is connected
to a Spectrum Analyser, the harmonic distortion will appear on the display as spectral lines
with integral multiple of the fundamental frequency. Spurious noise that is not harmonically
related to the fundamental frequency may also be detected if its level is higher than the
Spectrum Analyser's noise floor.
A block diagram of a typical Spectrum Analyser is shown in Figure 1. The Local
Oscillator (LO) is tuned to sweep across the desired frequency range. The LO signal mixes
with the RF input and hence down-converts the RF signal to the intermediate frequency (IF).
The IF Filter (which sets the Resolution Bandwidth) selects the sideband to be analysed by the
detector. The output passes through a Video Filter (which sets the Video Bandwidth) before
being applied to the vertical deflection system of the CRT display. The result is a display of
signal amplitude as a function of frequency. As the electron beam scans from left to right of
the screen, the LO frequency is swept-tuned to detect spectrum of different frequencies.
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Video
Filter
RF
input
Detector
IF Filter
LO
Sawtooth
Generator
Y
X
Display
Figure 1: Block diagram of a Spectrum Analyser.
Ideally the spectral line for a stable pure sinewave shall be an infinitesimally narrow
line on the power-frequency plot. On a practical Spectrum Analyser, a bell-shape response
with finite spreading on both sides of the sinewave frequency is displayed. This may be due to
the frequency instability of the sinewave source which causes the energy of the carrier to
distribute over a finite bandwidth centred around the carrier. However, in most cases it is due
to the Spectrum Analyser's IF filter frequency response. The bandwidth of this IF filter, which
determines the analyser's Resolution Bandwidth (RBW), is usually user selectable. The
display is actually a tracing of the IF filter shape as the LO sweeps across the desired
frequency range. Decreasing the RBW improves the ability of the Spectrum Analyser to
resolve 2 frequency components that are closely spaced. Since a narrower band-pass filter
limits the measurement to a narrower bandwidth, the displayed measurement noise will be
lowered.
The Video Filter is a low-pass filter which is used to smooth the trace seen on the
screen. By reducing the Video Bandwidth (VBW), the peak-to-peak variation of the noise is
decreased. When signal level is close to noise level, display smoothing can be used to help
find the signal that otherwise might be obscured in the noise floor.
A modulated waveform will contain sidebands in addition to the carrier signals. In
radio broadcasting and wireless telecommunication systems, Frequency Division Multiple
Access (FDMA) is commonly used for multiple radio stations or users to operate at the same
time without mutual interference. A Spectrum Analyser must be used to measure the
bandwidth occupied by a particular programme or modulated signal to ensure that the
sidebands do not extend to the neighbouring channel.
Measuring the spectral content of a communication signal and study the spectral
profile (the shape of the spectral distribution in frequency domain) is important to ensure that
the information is transmitted correctly. For example the percentage modulation of AM signal
and modulation index of FM signal can be determined from the spectral distribution. A
Spectrum Analyser usually has a built-in frequency counter which can be activated to make
accurate frequency measurement for a selected spectral line on the display. The stability of the
carrier signal can be measured in term of phase noise (for example, in dBc/1Hz RBW at
10kHz offset). This is a measurement of noise sideband level at a frequency of 10kHz above /
below the carrier frequency (unmodulated carrier). Hence the measurement result is in dBc,
i.e. dB relative to carrier level. The noise sideband specification is typically normalised to a 1
Hz Resolution Bandwidth (RBW). Built-in firmware is usually available in the Spectrum
Analyser to convert the reading measured using other RBW to the required value with 1 Hz
RBW.
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LAB PROCEDURE
A) Illustrations of Amplitude and Frequency Measurement
1. Turn on the RF Signal Generator. Set the carrier frequency to 100 MHz, with modulation
OFF, and output level –40 dBm. Connect the signal to RF Input of the Spectrum Analyser.
2. Set the Spectrum Analyser to a known state by pressing [Preset][Preset All].
3. Select the frequency range: press [Frequency][Start Freq][98][MHz] [Stop
Freq][102][MHz].
4. Press [Peak Search]. Record the signal level and frequency reading from the Spectrum
Analyser.
Signal Level: _________________ dBm
Frequency: ___________________ MHz
Why the signal level shown on the spectrum analyzer differs from the output level of the
signal generator (–40 dBm)? Explain briefly.
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5. For the greatest frequency accuracy, use the Spectrum Analyser's built-in frequency
counter to read the frequency and amplitude of the signal under test: [Measure][Freq
Count][Count On]. Discuss briefly the reasons of using built-in frequency counter.
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6. Record the signal frequency (to a resolution of 1 Hz): [Setup][1 Hz]
Frequency: ___________________ MHz
Outline TWO advantages of using low resolution such as 1 Hz for measurement.
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7. To reduce the sweep time of the Spectrum Analyser, turn off the frequency count
function: [Measure][Freq Count][Count Off]. Examine the advantages (if any) of reducing
the sweep time of the analyzer.
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B) Determining the Distortion/Harmonic Frequencies of a signal source
8. Press [Frequency][Stop Freq][500][MHz]. Set [RBW] to [Manual][3][kHz].
9. Set the Signal Generator output level to –20 dBm.
10. Sketch the displayed spectrum, including any harmonics / spurious signals. You can use
[Marker][Normal] and [Peak Search][Next Peak] to step through the spectral lines and
record the frequencies (in MHz) and amplitudes (in dBm). Record also the noise floor
level.
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Analyser Setting
AT
ST
RLV
RB
VB
dB/Div
CF
SP
(AT: Attenuation, ST: Sweep Time, RLV: Reference Level, RB: Resolution Bandwidth, VB:
Video Bandwidth, CF: Centre Frequency, SP: Frequency Span.)
Attenuation
f1 =…………. MHz
f2 =…………. MHz
f3 =…………. MHz
f4 =…………. MHz
Noise floor level
AT=10 dB
dBm
dBm
dBm
dBm
dBm
AT=20 dB
dBm
dBm
dBm
dBm
dBm
11. Increase the RF Attenuation setting of the Spectrum Analyser to 20 dB:
[Atten][Manual][20][dB].
12. Use [Marker][Normal] and [Peak Search][Next Peak] to step through the spectral lines
and record the frequencies (in MHz) and amplitudes (in dBm). Record also the noise floor
level.
Explain the impacts of using higher attenuation (such as AT = 20 dB) on the measured
fundamental frequency, harmonic frequencies and noise floor level. Justify your answers.
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C) Showing the changes due to the effect of RBW setting
13. Press [Frequency][Centre Freq][100][MHz] and [Span][200][kHz].
14. Set the video bandwidth: [VBW][Manual][3][kHz].
15. Set [Marker][Zone Width] to [Spot]. Press [Peak Search] and [Marker][Delta Marker].
16. Vary the RBW and notice the changes in the displayed trace: [RBW][Manual] [100][kHz]
followed by [30][kHz], [10][kHz], [3][kHz], and [1][kHz]. Use the marker function to
measure the 3-dB bandwidth of the displayed result. Record the 3-dB bandwidth and
sweep time for each setting of RBW.
Lower 3-dB
Upper 3-dB
3-dB BW =
Sweep Time
frequency, fL
frequency, fH
fH - fL
(ST)
RBW=100kHz
RBW= 30kHz
RBW= 10kHz
RBW= 3kHz
RBW= 1kHz
Explain the impacts of using lower RBW (such as 1kHz) on the measured fundamental
frequency, noise floor level and sweep time (ST). Justify your answers.
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D) Examining the Phase Noise of a signal source
17. Set the [RBW] at [1][kHz]. Change [Span] to [50][kHz].
18. Use the noise measurement capability of the Spectrum Analyser to measure the carrier
signal phase noise at a 10-kHz offset.
19. Press [Measure][C/N Ratio Measure][Meas On]. Press [Marker] and then rotate the entry
knob to move the delta marker to a frequency which is 10 kHz higher than the carrier
frequency. Record the phase noise.
Phase Noise: ___________________ dBc/Hz @ 10 kHz offset
Explain the significant of phase noise of a signal source.
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E) Showing the changes due to effect of VBW setting
20. Press [Measure][Off] and [Marker][Marker Off]. Set [Span] to [100][kHz].
21. Set VBW at 10 Hz and RBW at 1 kHz.
22. Reduce the RF Signal Generator output level to about 3 dB above the noise floor level of
the Spectrum Analyser.
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23. Vary the VBW and notice the changes in the sweep time: [VBW][Manual] [10][Hz]
followed by [100][Hz], [1][kHz], [10][kHz]. Observe the changes in the displayed trace.
Record the sweep time for each VBW setting.
Sweep Time (ST)
VBW= 10Hz
VBW= 100Hz
VBW= 1 kHz
VBW=10 kHz
Interpret the observation of the displayed trace:
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F) Showing the Amplitude Modulation Sidebands
24. Turn on AM modulation of the RF Signal Generator, with External modulating source, at
30% modulation index. Set the output level to -30 dBm.
25. Connect the Audio Generator output to the “External Modulation” input of the RF Signal
Generator.
26. Set the Audio Generator frequency to 10 kHz and the Attenuator level to –20 dB. Increase
the Amplitude of the Audio Generator output to a suitable level (in which both the HIGH
and LOW lights next to the Modulation Index reading of the RF Signal Generator are off).
27. Sketch the displayed trace of the Spectrum Analyser (RBW = 1 kHz, Span = 100 kHz).
Analyser Setting
AT
ST
RLV
RB
VB
dB/Div
CF
SP
28. Change the RBW to 10 kHz. Sketch the displayed trace on the same graph of step 27.
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Explain the impacts of using RBW = 10 kHz on the measured modulated signal (on
carrier and sidebands). Justify your answers.
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Overall Discussion and Conclusion
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NOTE
No report is needed for this experiment. You would be evaluated on-the-spot during
the experiment. Your are required to submit all the observations, results and
discussions in this sheet at the end of the experiment.
Evaluation Schemes (20%)
Understanding of fundamental concepts – 5%
Ability to explain the procedures involved – 5%
Ability to identify or determine expected results – 5%
Ability to explain or elaborate the results – 5%
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