Measurements of digital signals with spectrum analyzers

1. Measurements of digital signals with spectrum analyzers Thomas Hasenpusch Federal Network Agency Germany

2. Types of available Spectrum Analyzers • Sweeping Analyzer • Scans the desired frequency range with a narrow filter • FFT Analyzer • Captures the time signal and calculates spectrum mathematically Thomas Hasenpusch, Bundesnetzagentur

3. A f Sweeping Analyzer: Principle in Theory • Filter is swept through the desired frequency range (Span) Thomas Hasenpusch, Bundesnetzagentur

4. A IF f Sweeping Analyzer: Realization of Principle • IF signal is swept through fixed frequency filter (RBW) Thomas Hasenpusch, Bundesnetzagentur

5. Envelope Detector Mixer IF Amp. IF Filter Log. Amp. Video Filter Input Detector Local Oscillator Display Sawtooth Generator Sweeping Analyzer: Simplified Block Diagram Detector Video Bandwidth (VBW) Reference level Resolution Bandwidth (RBW) Detector Centre frequency Trace Mode Span, Sweep time Thomas Hasenpusch, Bundesnetzagentur

6. Sweeping Analyzer: RBW 3dB „dip“ RBW = frequency spacing is not always sufficient to separate two signals Optimum (best frequency resolution): RBW = span / display pixels Thomas Hasenpusch, Bundesnetzagentur

7. A Sweeping Analyzer: Envelope Detector • „Filters“ out the RF, leaves only modulation component Video signal Thomas Hasenpusch, Bundesnetzagentur

8. s1 s2 s3 s4 s5 s6 s1 s2 s3 s4 s5 s6 A A Average level RMS level t t Pixel 1 Pixel 2 Sweeping Analyzer: Detectors • Analyzer measures much faster than it can display • Multiple measurement results lie behind each display pixel • Detector determines which of the measured values is displayed peak AV RMS sample Thomas Hasenpusch, Bundesnetzagentur

9. sum FFT FFT: Theory • Fourier says: Each random time signal is the sum of discrete, unmodulated sinewaves Thomas Hasenpusch, Bundesnetzagentur

10. RF in DSP A/D X Low Pass FFT Memory fRF 0 FIF Display FFT Analyzer: Principle • Fourier formulas allow calculation of the spectrum of each time signal • Fast Fourier Transform (FFT) greatly reduces calculations, but work only under certain assumptions • Time signal is captured (acquired) for a certain time, digitized and stored in memory • FFT spectrum is then calculated from the stored time samples by a Digital Signal Processor (DSP) Thomas Hasenpusch, Bundesnetzagentur

11. Spectrum 1: Display Spectrum 2: Display Block 1 acquisition Block 2 acquisition Block 1 processing Block 2 processing time blind time blind time FFT: Problems and issues • Usually no seamless acquisition (blind times during calculation) • Solution: Deploying two separate processing lanes with alternate timing (one lane acquires while the other one processes previous block) Thomas Hasenpusch, Bundesnetzagentur

12. A 0 FFT window FFT window FFT window t A A A f f f FFT with pulsed signals • FFT analyzers are usually fast enough to show the spectrum of even very short pulses (e.g. Radar) Thomas Hasenpusch, Bundesnetzagentur

13. A AV burst level burst duration t Important levels of digital signals • Peak: maximum possible level over a long meas. time • Applies when assessing interference potential • RMS (continuous signals): average power a over long meas. time • Applies when checking reception capability, coverage and licence conditions • AV burst (pulsed signals): average power during burst only • Applies as RMS, but in case of bursted signals Thomas Hasenpusch, Bundesnetzagentur

14. A 99% 100% 0.5% 0.5% f OBW Span (100%) Bandwidth Measurement (Direct Method) • Most important for monitoring stations: 99% bandwidth (equal to occupied bandwidth) • Definition: bandwidth in which 99% of all transmitted energy lies • With analyzer: narrow RBW, MaxHold, OBW function Thomas Hasenpusch, Bundesnetzagentur

15. Level Measurement: Procedure With Sweeping Analyzer (1) • Peak level: • Span ≥ signal bandwidth or zero span • RBW ≥ signal bandwidth • Detector: peak • MaxHold • Read highest level with Marker • RMS-level: • Span ≥ signal bandwidth • Narrow RBW (span/display points) • Detector = RMS or sample • ClearWrite • Channel Power measurement function • If reading is instable: increase sweep time (never use MaxHold!) Thomas Hasenpusch, Bundesnetzagentur

16. R B W 3 0 k H z M a r k e r 1 [ T 1 ] V B W 3 0 0 k H z - 3 7 . 4 7 d B m R e f - 2 0 d B m A t t 1 0 d B S W T 2 0 m s 8 . 7 1 4 5 0 0 m s - 2 0 P O W E R [ T 1 ] R M S - 3 9 . 2 7 d B m A - 3 0 S G L 1 1 R M * T R G C L R W R - 4 0 - 5 0 - 6 0 T R G - 7 0 d B m - 7 0 3 D B - 8 0 - 9 0 - 1 0 0 - 1 1 0 T 2 T 1 - 1 2 0 C e n t e r 4 1 0 . 5 M H z 2 m s / Level Measurement: Procedure With Sweeping Analyzer (2) • AV-burst level: • Span = zero span • RBW ≥ signal bandwidth • Detector = RMS or sample • ClearWrite, trigger on burst • Sweep time ≥ burst time • Time domain power measurement • Average level: • Span = zero span • RBW ≥ signal bandwidth • Detector = Average or sample • Trace = linear average Thomas Hasenpusch, Bundesnetzagentur

17. Level Measurement: Procedure with FFT Analyzer (1) • Peak level: • Capture bandwidth = signal bandwidth • Time domain analysis • Select shortest possible acquisition time • MaxHold over multiple acquisitions or amplitude vs. time together with long analysis time • Read highest value • RMS level: • Capture bandwidth ≥ signal bandwidth • Channel power function • Long acquisition time or average over multiple short acquisition times • If reading is instable: increase acquisition time or number of averages Thomas Hasenpusch, Bundesnetzagentur

18. analysis time acquisition time Level Measurement: Procedure with FFT Analyzer (2) • AV-burst level: • Capture bandwidth ≥ signal bandwidth • Trigger analysis on burst start • Channel power function • Acquisition time (or analysis time) = burst time Thomas Hasenpusch, Bundesnetzagentur

19. Level Measurement Under Low S/N Ratios • For accurate Pk measurement, S/N ≥ 20 dB is necessary • For accurate RMS, AV, AV-burst measurement, 10 dB S/N is sufficient • Corrections to indicated level for measurements under lower S/N values: Thomas Hasenpusch, Bundesnetzagentur

20. Literature • ITU Spectrum Monitoring Handbook (2011):Chapter 4.3: RF level measurements Thomas Hasenpusch, Bundesnetzagentur