Thomas Hasenpusch
Federal Network Agency
Germany www.bundesnetzagentur.de

Types of available Spectrum Analyzers

Sweeping Analyzer

Scans the desired frequency range with a narrow filter

FFT Analyzer
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Captures the time signal and calculates spectrum mathematically
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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Sweeping Analyzer: Principle in Theory

Filter is swept through the desired frequency range
(Span)
A
Thomas Hasenpusch, Bundesnetzagentur f
11.04.2020
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Sweeping Analyzer: Realization of Principle

IF signal is swept through fixed frequency filter (RBW)
A
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IF f
11.04.2020
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Sweeping Analyzer: Simplified Block
Diagram
Input
Mixer
Reference level
IF Amp.
IF Filter Log. Amp.
Detector Video Bandwidth
Envelope
Detector Video Filter
(VBW)
Centre frequency
Resolution
Bandwidth (RBW)
Local
Oscillator
Detector
Detector
Display
Trace
Mode
Span,
Sweep time
Sawtooth
Generator
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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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 11.04.2020
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Sweeping Analyzer: Envelope Detector

„Filters“ out the RF, leaves only modulation component
Video signal
A
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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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 s1 s2 s3 s4 s5 s6 s1 s2 s3 s4 s5 s6
A
Average level
RMS level t
A
Pixel 1 peak
AV
RMS sample
Pixel 2 t
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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FFT: Theory

Fourier says: Each random time signal is the sum of discrete, unmodulated sinewaves
2,0
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
-2,0
0 90 180 270 360 450 540
Tim e sum
2,00
1,50
1,00
0,50
0,00
-0,50
-1,00
-1,50
-2,00
0 90 180 270 360 450 540
Tim e
FFT
1,0
0,8
0,6
0,4
0,2
0,0
0
2,0
1,8
1,6
1,4
1,2
2 4 6
Frequency
8 10
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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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)
RF in
X A/D
F
IF
0 f
RF
Low Pass
Thomas Hasenpusch, Bundesnetzagentur
Memory
Display
DSP
FFT
11.04.2020
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FFT: Problems and issues

Usually no seamless acquisition
(blind times during calculation)
Spectrum 1:
Display
Spectrum 2:
Display
Block 1 acquisition
Block 1 processing
Block 2 acquisition
Block 2 processing time blind time blind time

Solution: Deploying two separate processing lanes with alternate timing (one lane acquires while the other one processes previous block)
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Thomas Hasenpusch, Bundesnetzagentur 11.04.2020

FFT with pulsed signals

FFT analyzers are usually fast enough to show the spectrum of even very short pulses (e.g. Radar)
A
0
A
FFT window
A
FFT window
A
FFT window t f f
Thomas Hasenpusch, Bundesnetzagentur f
11.04.2020
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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
A
AV burst level t
Thomas Hasenpusch, Bundesnetzagentur burst duration
11.04.2020
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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
A
99%

100%
0.5% 0.5% f
OBW
Span (100%)
With analyzer: narrow RBW, MaxHold, OBW function
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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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 11.04.2020
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
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
Ref -20 dBm
-20
-30
1 RM *
CLRWR
-40
-50
-60
TRG -70 dBm -70
Att 10 dB
1
Average level:




Span = zero span
RBW ≥ signal bandwidth
Detector = Average or sample
Trace = linear average
-80
-90
-100
-110
T1
-120
Center 410.5 MHz 2 ms/
RBW 30 kHz
VBW 300 kHz
SWT 20 ms
Marker 1 [T1 ]
-37.47 dBm
8.714500 ms
P OWER [T1]
R MS -39.27 dBm
A
SGL
TRG
T2
3DB
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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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 11.04.2020
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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 analysis time acquisition time
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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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:
0 dB
-1 dB
-2 dB
-3 dB
-4 dB
-5 dB
-6 dB
-7 dB
0 dB 5 dB 10 dB 15 dB measured S+N / N
20 dB
Thomas Hasenpusch, Bundesnetzagentur
RMS
AV
Peak
25 dB 30 dB
11.04.2020
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Literature

ITU Spectrum Monitoring Handbook (2011):
Chapter 4.3: RF level measurements
Thomas Hasenpusch, Bundesnetzagentur 11.04.2020
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