Spectrum Analyzer

1
Spectrum Analyzer
2
Another Oscilloscope???

• Like an oscilloscope
• Oscilloscope in time domain
• Spectrum analyzer in frequency domain
  (selectable)

3
Is Frequency Domain Better?

• In time domain, all frequency components of
  signals are summed together
• In freq domain, complex signals are separated
  into their frequency components

4
What does SA do?

• Processes the signal
• Displays the spectral content of the input signal
  within a selected range of frequencies
• i.e., signal power(db) versus frequency
• Measures frequency, power, harmonic content,
  modulation, spurs, and noise, etc

5
Types of RF Spectrum Analyzer

• Two common ways to measure RF signals
• Fourier Transform
• Swept-Tuned or Swept-Toned

6
Fourier Analyzer

• Capture time domain signals information
• Digitizes time domain signal (digital sampling)
• Perform Fourier transform
• Fast, but limits in frequency range, sensitivity

7
Swept Analyzer

• Sweep across frequency range
• Display the frequency of interest
• Large frequency range
• Widely used

8
RF Spectrum Analyzer

• ADVANTEST 9 kHz to 3 GHz, 1 kHz resolution

9
Three Primary Function Keys

• Frequency (Hz)
• Amplitude (Abs dBm, Rel dB)
• Span (Hz) -gt (how big the window we want to
  view)

10
Other Important Keys
Auto Tune
Save
Softkeys
Resolution Bandwidth Video Bandwidth
Peak Search Marker
11
Typical Output Displayed
12
The TDS-220 Tektronix Oscilloscope and the FFT
module
13
Functions

• An instrument for observing a waveform both in
  time and frequency domains
• In time domain works as a digital oscilloscope

14
Main Keys
Cursor
Measure
Autoset
Sec/Div
Volts/Div
Math
15
Frequency Domain

• Takes the FFT of a waveform
• The FFT samples relate to the DTFS samples, i.e.,
  
• Takes a portion of a signal and repeats it

16
Taking FFT

• There 4 steps to take the FFT
• Setting up the time-domain waveform
• Use AUTOSET to display the proper YT waveform
• Need to see a few periods of the waveform clearly

Good scale
Bad scale
17
Taking FFT

• Be careful about the DC values
• Press the MATH key to see the FFT menu
• Use the FFT CH1/2 to switch between the channel ½
  FFT and YT waveforms

18
Taking FFT

• Choose the window type
• Choose the HANNING window for measurements

Point by point multiplication

X
19
Taking FFT

• Use FFT Zoom menu to zoom
• The left graticule line is at 0Hz frequency

20
Measurements Scales

• Measurements on FFT waveforms
• Frequency in Hz from 0Hz to half of the sampling
  rate
• Magnitude in dB where 0dB equals 1 V RMS
• dB and dBm
• dB a relative value
• dBm an absolute value when the reference power is
  1mW

21
References

• Basic Spectrum Analyzer (HP)
• Knowing The Signal Classes Greatly Simplifies
  Spectrum Analysis (TestMart)
• Signal Processing http//www.ece.queensu.ca/hpages
  /courses/Elec421/why.html
• TDS200 Series Digital Real-Time Oscilloscope,
  User Manual (Tektronix)
• TDS200 Series Extension Modules, Instruction
  Manual (Tektronix)