Jeff Thomas Tom Holmes Terri Hightower. Learn RF Spectrum Analysis Basics

Transcription

1 Jeff Thomas Tom Holmes Terri Hightower Learn RF Spectrum Analysis Basics

2 Learning Objectives Name the major measurement strengths of a swept-tuned spectrum analyzer Explain the importance of frequency resolution, sensitivity, and dynamic range in making analyzer measurements Outline the procedure making accurate distortion measurements Page 3

3 Spectrum Analyzer Analyzer Definitions A spectrum analyzer measures the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals. Vector Signal Analyzer A vector signal analyzer measures the magnitude and phase of an input signal at a single frequency within the IF bandwidth of the instrument. The primary use is to make in-channel measurements, such as error vector magnitude, code domain power, and spectral flatness, on known signals. Signal Analyzer A signal analyzer provides the functions of a spectrum analyzer and a vector signal analyzer. 3 Back to Basics Training

4 SPECTRUM ANALYZER 9 khz GHz Overview: What is Spectrum Analysis? 8563A Page 4

5 Overview Types of Measurements Available Frequency, power, modulation, distortion & noise Spectrum monitoring Spurious emissions Scalar network analysis Noise figure & phase noise Harmonic & intermodulation distortion Analog, digital, burst & pulsed RF Modulation Wide bandwidth vector analysis Electromagnetic interference Measurement range (-172 dbm to +30 dbm) Frequency range (3 Hz to >>325 GHz) Modulation Noise Distortion Spur Search 7

6 Frequency Versus Time Domain Amplitude (power) frequency time Time domain Measurements Frequency Domain Measurements Page 6

7 Fourier Spectrum Analyzer Fourier analyzer transforms a signal over time into a frequency spectrum Display Amplitude f 1 f 2 Frequency Page 7

8 Swept-Tuned Spectrum Analyzer Filter sweeps over a frequency range Display Amplitude f 1 f 2 Frequency f Page 8

9 Theory of Operation Display terminology Amplitude Reference Level Start Freq. Stop Freq. Freq. Span Center Freq. Back to Basics Training 12

10 Theory of Operation Swept Spectrum Analyzer Block Diagram Input signal RF input attenuator Pre-Selector Or Low Pass Input Filter local oscillator mixer IF gain IF filter (RBW) sweep generator Log Amp envelope detector video filter Crystal Reference Oscillator ADC, Display & Video Processing Back to Basics Training 11

11 Theory of Operation Mixer MIXER f sig 1.5 GHz RF IF LO f sig f LO - f sig f + LO f LO f sig 3.6 GHz f 6.5 GHz LO Back to Basics Training 13

12 Intermediate Frequency (IF) Filter Input IF FILTER IF Bandwidth: also known as resolution bandwidth and RBW Provides shape of frequency domain signal Page 12 Actual f Displayed f

13 Theory of Operation IF Filter (Resolution Bandwidth RBW) IF Filter Input Spectrum IF Bandwidth (RBW) Display A B C Back to Basics Training 14

14 Video Filter Input VIDEO FILTER Without video filtering With video filtering Page 14

15 Specifications? A Definition Specifications describe the performance of parameters covered by the product warranty (temperature = 0 to 55 C, unless otherwise noted). Typical values describe additional product performance information that is not covered by the product warranty. It is performance beyond specification that 80 % of the units exhibit with a 95 % confidence level over the temperature range 20 to 30 C. Typical performance does not include measurement uncertainty. Nominal values indicate expected performance, or describe product performance that is useful in the application of the product, but is not covered by the product warranty. 24 Back to Basics Training

16 Getting the Frequency Range You Need 1 f if 1 f in Page 20 LO IF signal, f if Mixer 2 The input signal is displayed when f LO -f in = f IF f in Range LO Range f LO -f in f LO 0 frequency f LO +f in

17 Getting the Frequency Range You Need LO Feedthrough 3 f LO -f in f LO f LO +f in f if 1 0 f in Range f in IF filter 2 0 LO LO Range Sweep generator 4 0 Display Page 21

18 Getting the Frequency Range You Need Input signal displayed f LO -f in = f IF 3 f LO -f in f LO f LO +f in 1 f in f if 0 f in Range 2 LO f in 0 LO Range 4 0 Page 22

19 Getting the Frequency Range You Need Lower frequency limited by LO feedthrough Upper frequency limited by LO range and IF frequency Microwave frequency measurement uses harmonic mixing Page 23

20 E X T E N D U N M A T C H E D P E R F O R M A N C E W I T H E X T E R N A L M I X I N G Extend to 325 GHz and beyond Better close-in phase noise performance than internallymixed 67 GHz analyzers! Supported measurements Spectrum analysis PowerSuite one-button power measurements N9068A phase noise measurement application 89600A VSA Supported external mixers M1970V, M1970E and M1970W Series OML Inc. VDI And other third-party external mixers Page 28

21 Frequency and Amplitude Accuracy Absolute Amplitude in dbm Relative Amplitude in db Relative Frequency Frequency Page 24

22 Specifications Accuracy: Frequency & amplitude Components which contribute to uncertainty are: Input mismatch (VSWR) RF Input attenuator (Atten. switching uncertainty) Mixer and input filter (frequency response) IF gain/attenuation (reference level accuracy) RBW filters (RBW switching uncertainty) Log amp (display scale fidelity) Reference oscillator (frequency accuracy) Calibrator (amplitude accuracy) Back to Basics Training 29

23 Signal Resolution What Determines Resolution? Resolution Bandwidth Residual FM RBW Type and Selectivity Noise Sidebands Page 26

24 IF Filter Bandwidth 3 db Input Spectrum LO Mixer 3 db BW IF Filter/ Resolution Bandwidth Filter (RBW) Sweep Detector RBW Display Page 27

25 Resolving Two Equal-level Signals 10 khz RBW 3 db 10 khz Page 28

26 Resolving Two Unequal-level Signals 3 db bandwidth Selectivity (filter shape) 3 db 3 db BW 60 db 60 db BW Selectivity = 3 db BW 60 db BW Page 29

27 Resolving Two Unequal-level Signals For a RBW of 1 khz and a selectivity of 15:1, the 60 db bandwidth is 15 x 1 khz = 15 khz... so the filter skirt is 7.5 khz away from the filter s center frequency 60 db 10 khz Distortion Page khz

28 Residual FM Page 31 Residual FM "Smears" the Signal

29 Noise Sidebands (Phase Noise) Phase Noise Page 32 Noise Sidebands can prevent resolution of unequal signals

30 Sweep Rate Swept too fast Penalty For Sweeping Too Fast Is An Uncalibrated Display Page 33

31 Analog versus Digital Resolution Bandwidths Analog Filter Digital Filter Typical Selectivity Analog 15:1 Digital 5:1 RES BW 100 Hz SPAN 3 khz Page 34

32 Rules to Analyze By: Use the Analyzer s Automatic Settings Whenever Possible When using the analyzer in its preset mode, most measurements will be easy, fast, and accurate Automatic selection of resolution bandwidth, video bandwidth, sweep time and input attenuation When manually changing the analyzer parameters, check for uncal messages Page 35

33 Sensitivity and Displayed Average Noise Level RF Input Mixer RES BW Filter Detector LO Page 38 Sweep A spectrum analyzer generates and amplifies noise just like any active circuit.

34 RF Input Attenuator Effects Displayed noise is a function of RF input attenuation signal level 10 db Attenuation = 10 db Attenuation = 20 db Signal-to-noise ratio decreases as RF input attenuation is increased Page 39

35 IF Filter (Resolution Bandwidth) Effects Displayed noise is a function of IF filter bandwidth Decreased BW = Decreased Noise 10 db 10 db 100 khz RBW 10 khz RBW 1 khz RBW Page 40 Best sensitivity = narrowest RBW

36 Rules to Analyze By: Getting the Best Sensitivity Requires Three Settings Narrowest resolution bandwidth Minimum RF attenuation Sufficient video filter to smooth noise (VBW < 0.01 Resolution BW) Page 43

37 Where is Distortion Generated? Mixers Generate Distortion Signal to be measured Frequency translated signals Resulting signal Page 44 Mixer-generated distortion

38 Most Influential Distortion is the Second and Third Order < -50 dbc < -40 dbc < -50 dbc Two-Toned Intermod Harmonic Distortion Page 45

39 Summary The RF spectrum analyzer is a heterodyne receiver Offers a narrow resolution capability over a wide frequency range Measures small signals in presence of large signals Remember to: Adjust the measurement procedure for specific application Test for internal distortion Take sideband noise into account Page 56

40 Modern Spectrum Analyzer Block Diagram Pre-amp Analog IF Filter FFT Digital IF Filter Digital Detectors Attenuation YIG ADC Swept vs. FFT Digital Log Amp Replaced by Back to Basics Training 69