Super-Heterodyne
Signal Analyzers
Description and Applications
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Contents
1. Signal Analysis Background
................................................................................................................. 6
Introduction
.............................................................................................................................................. 6
Time and Frequency Domain Representations of Signals
.............................................................. 6
Why View Signals in the Frequency Domain?
................................................................................... 9
Using the Oscilloscope for Frequency Domain Analysis
............................................................... 11
Spectrum Analysis versus Vector Signal Analysis
.......................................................................... 12
Super-Heterodyne versus Direct Conversion Architectures
.......................................................... 13
2. Super-Heterodyne Principle
............................................................................................................... 14
Brief History of the Super-Heterodyne Receiver
............................................................................. 14
Frequency Shift Property
.................................................................................................................... 14
Frequency Shift Property Applied to the Super-heterodyne Receiver
......................................... 15
Moving to a Non-ideal Receiver
..................................................................................................... 17
Mixing Process
..................................................................................................................................... 17
Image Responses
................................................................................................................................ 19
IF Subharmonics
.................................................................................................................................. 20
General M,N Spurs
.............................................................................................................................. 21
LO Feedthrough
................................................................................................................................... 23
LO Emissions
........................................................................................................................................ 24
Residual Responses
............................................................................................................................ 24
Cautionary Note on Nomenclature
.................................................................................................... 25
3. Super-Heterodyne Signal Analyzer Structures
............................................................................... 26
Single Conversion Stage Structure
................................................................................................... 26
Multiple Conversion Lowband Structure
........................................................................................... 27
Multiple Conversion Highband Structure
.......................................................................................... 28
Multiple Conversion Block Converter Structure
............................................................................... 30
Final IF Frequency Selection
.............................................................................................................. 31
Variable Bandwidth Final IF Filters
.................................................................................................... 32
4. RF Chain Signal Processing
.............................................................................................................. 34
Amplitude Representation in Signal Analyzers
................................................................................ 34
RF/IF Path Amplitude Control Elements
........................................................................................... 35
2
Reference Level and Gain Setting Equations
.................................................................................. 36
Mixer Level Effect on Frontend Noise
............................................................................................... 37
ADC Dynamic Range
........................................................................................................................... 38
Preamplifier
........................................................................................................................................... 39
Phase Noise
.......................................................................................................................................... 41
5. IF Chain Signal Processing
................................................................................................................ 44
Analog IF Signal Processing
.............................................................................................................. 44
Resolution Bandwidth Filter
............................................................................................................ 44
Logarithmic Amplifier
....................................................................................................................... 46
Envelope Detector
............................................................................................................................ 47
Video Bandwidth Filter
..................................................................................................................... 47
Sweep Speed Considerations
............................................................................................................ 48
Viewing Modulation
.............................................................................................................................. 48
Detector Modes
.................................................................................................................................... 50
Challenges with the All Analog IF
...................................................................................................... 50
IF Signal Processing with the Digital IF
............................................................................................ 51
Signal Processing Chain: Digital Hardware
..................................................................................... 51
Signal Processing Chain: Software
................................................................................................... 52
Spectral Leakage in the FFT Process
............................................................................................... 53
Resolution Bandwidth Using Windowing Functions
........................................................................ 56
Windowing Function Figures of Merit
................................................................................................ 57
Bin Width
........................................................................................................................................... 57
Equivalent Noise Bandwidth
........................................................................................................... 59
Scalloping Loss
................................................................................................................................. 60
Sidelobe Attenuation
........................................................................................................................ 61
Windowing Function Comparison
...................................................................................................... 61
Trace Averaging
................................................................................................................................... 62
Vector Averaging
.............................................................................................................................. 62
RMS Averaging
................................................................................................................................. 62
Peak-Hold Averaging
....................................................................................................................... 63
Averaging Mode Comparisons
....................................................................................................... 63
Video Bandwidth Filter Emulation
...................................................................................................... 63
3
Unit Conversion
.................................................................................................................................... 65
Log Mode Trace Averaging
................................................................................................................ 65
6. Dynamic Range
.................................................................................................................................... 68
Dynamic Range Definitions
................................................................................................................ 68
Gain Compression
............................................................................................................................... 69
Harmonic Distortion
............................................................................................................................. 72
Optimizing the Signal Analyzer’s Dynamic Range Performance
.................................................. 77
Dynamic Range versus Mixer Level Chart
....................................................................................... 77
Noise Floor Curve on the Dynamic Range Chart
........................................................................ 79
Distortion Curves on the Dynamic Range Chart
............................................................................. 80
Phase Noise Curve on the Dynamic Range Chart
...................................................................... 82
Complete Dynamic Range Chart
....................................................................................................... 83
A Few Observations Regarding the Dynamic Range Chart
.......................................................... 85
Preamplifier Dynamic Range
.............................................................................................................. 86
Near Noise Distortion Measurements
............................................................................................... 88
RF Input Attenuator Effect on Dynamic Range
............................................................................... 89
ADC Contribution to Dynamic Range
............................................................................................ 91
ADC Distortion Associated with Vector Signal Analysis.................................................................
92
ADC Distortion Associated with Spectrum Analysis
....................................................................... 94
Effective TOI Improvement for Analog Devices...........................................................................
95
Effective TOI Improvement for ADCs
............................................................................................ 97
Reference Level Re-Ranging
......................................................................................................... 98
Dynamic Range Considerations for Digitally Modulated Signals.
................................................. 99
SNR for Digitally Modulated Signal
............................................................................................. 102
IMD for Digitally Modulated Signal
.............................................................................................. 103
Phase Noise for Digitally Modulated Signal
............................................................................... 105
ACLR using Vector Signal Analysis Mode
..................................................................................... 106
ACLR using Spectrum Analysis Mode
............................................................................................ 106
Summarizing the Dynamic Range Information
.............................................................................. 107
Attenuator Test
............................................................................................................................... 107
7. Amplitude Accuracy
........................................................................................................................... 108
Absolute Amplitude Accuracy at the Calibration Reference Frequency
.................................... 108
4
Frequency Response
......................................................................................................................... 108
IF Amplitude Response
..................................................................................................................... 108
RF Input Attenuator Switching Uncertainty
.................................................................................... 110
YIG Tuned Filter Amplitude Response
........................................................................................... 110
Near Noise Amplitude Error
.............................................................................................................. 111
Coherent Signal Addition
.................................................................................................................. 112
Mismatch Uncertainty
........................................................................................................................ 114
Worst Case and Root Sum Squared Uncertainty
.......................................................................... 117
Power Meter Leveling
........................................................................................................................ 118
8. Spectrum Monitoring Applications
................................................................................................... 124
The Spectrum Monitoring Dynamic Range Problem
.................................................................... 124
Spectrum Monitoring Receiver
......................................................................................................... 125
9. Summary
............................................................................................................................................ 127
References
.............................................................................................................................................. 128
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