Super-Heterodyne Signal Analyzers Description and Applications ©National Instruments. All rights reserved. LabVIEW, National Instruments, NI, ni.com, the National Instruments corporate logo, and the Eagle logo are trademarks of National Instruments. See ni.com/trademarks for other NI trademarks. Other product and company names are trademarks or trade names of their respective companies. For patents covering National Instruments products, refer to the appropriate location: Help>>patents in your software, the patents.txt file on your CD, or ni.com/patents. 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 ...................................................................................................................................