passive and active microwave components -...
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MicrowaveComponents
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Passive and Active Microwave Components
8.4.2007
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Objective
I Basic overview of the microwave hardware that is usedin our group (and in all modern communication andnavigation equipment).
I Practical introduction to fundamental test equipment,with an invitation to a hands on experience for thosewho are interested.
I Discussion of problems and possible error sources inmicrowave remote sensing instruments.
www.iapmw.unibe.chMicrowave Physics
Institute of Applied PhysicsUniversitat Bern
Switzerland = Swiss ConfedrationCH (Confoederatio Helvetic)
Population = 8.5 millionGDP (PPP) = $65,000
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Outline
Passive Microwave ComponentsTerminationAttenuatorFilterCouplerFerrite Devices
Active ComponentsDetectorMultiplierMixerAmplifierOscillator
Microwave InstrumentationSpectrum Analyzer
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Topics already covered in WS2007 Lecture”Introduction to Applied Electromagnetism”
I Electromagnetic waves
I Decibel scale for power ratios: 10 · log 10 (P1/P2) [dB]
I Transmission lines: waveguides, cables, micro-strip lines
I Impedance, matching and standing waves
I Vector Network Analyzer
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Introduction
Components in the 22 GHz receiver of the IAP RadiometerPraktikum:
Target
Black BodyCalibration
Atte
nuat
or
NoiseDiode
Mixer Filter AmplifierAmplifierCouplerAntenna Detector
+/− 0.5 GHzRF = 22.2
H2OAtmosphere
DCIF =0 to 0.5 GHz
Local Oscillator
LO =22 GHz
Frequency
Pow
er
LO
IF = |RF +/− LO|
LSB
US
B
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Passive Microwave Components
Definitions
I Linear transfer characteristic– S-parameters do not depend on the power– A continuous wave signal does not get distorted
I Most passive components are reciprocal |S21|2 = |S12|2Ferrite isolators and circulators are an exception
I For lossless two-port devices:– Reflections at both ports are identical |S11|2 = |S22|2– Energy conservation |S11|2 + |S21|2 = 1
Design depends on the frequency range, the requiredperformance and other aspects (e.g. costs, size, mass, powerhandling).
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Lumped Element Devices
I Discrete network of individual components, e.g. coils,capacitors, resistors.
I Dimensions < λ, phase differences from the assemblycan be neglected.
I Usable up to ∼3GHz (and above), but parasitic effectsand radiation losses increase with frequency.
Example for a lumped element 100MHz bandpass filter of aradio amateur receiver.
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Distributed Devices
I All components are connected by transmission lineswith dimensions in the order of λ.
I The connections are an integral part of the circuit, e.g.for tuning or impedance matching.
I Usable up to ∼100 GHz (and above).
I Dielectric and ohmic losses increase with frequency, andmanufacturing becomes very demanding.
Example of an integrated 24 GHz receiver module.
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Quasi-Optics
I At Millimeter and submillimeter wavelengths free spacepropagation provides lowest losses.
I Quasi-optical components with dimensions > λ are usedto guide, split or combine the beams.
Local Oscillatorto Antenna
Image BBHto Cold Sky
Signal BBH
300
mm
FSP Sideband Filterto Cryostat
Quasi-optical module characterized at IAP for the 660 GHzreceiver SMILES, a Japanese remote sensing instrument forthe International Space Station.
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Termination
I Terminates a transmission line (ideally S11= −∞ dB ).
I Tapered absorbing dielectrics in waveguides (a),resistive films in planar or coaxial devices (b).
I Standard coaxial 0-18 GHz terminations specified withreturn loss < -26dB (VSWR<1.1), expensive matchedtermination for VNA calibration have ≥ -36 dB.
I Free space terminations for anechoic chambers orradiometric calibration targets. Often made of lossyfoams with a pyramidal surface to improve thematching.
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Attenuator
I Lossy two-port device to reduce the signal level by -xxdB
I Ideally well matched and frequency independent.
I Resistive networks in coaxial (a) and planar devices,absorbing vane in waveguides.
I Often used to reduce standing waves caused bycomponents with a bad matching.
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Filter
I Used to reject certain frequency bands
I Realized as low-, high or bandpass filter (and alsoband-reject)
1.32 1.34 1.36 1.38 1.4 1.42 1.44 1.46 1.48 1.5−80
−70
−60
−50
−40
−30
−20
−10
0
10
FWHM
Frequency GHz
Am
plitu
de [d
B]
Bandpass Filter for a L−Band Radiometer
S11S12S21S22
Insertion Loss −0.39 dB
Out−of−bandRejection
CenterFrequency
Measurement example of a cavity filter with four sections.FWHM (full width at half maximum)
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Filter Types and Specifications
Selection depends on frequency and relative bandwidth.
Online tool of the manufacturer K&Lhttp://www.klfilterwizard.com
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Cavity Filter Example
7.8 GHz high pass filter made out a series of iris coupledwaveguide resonators. Mesh of the finite element model andsimulation results.
Simulated E-fields in the rejection and transmission band.
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Software for Cavity Filter Design at IAP
I Finite Elements: COMSOL Multphysics, Agilent EMDS
I Mode Matching: S&P (written by P. Fuholz)MICIAN ”Microwave Wizard”
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Planar Filter
Steps to get from a lumped element lowpass filter (a) to anequivalent microstrip design (d).
Inductors and capacitors are replaced by microstrip ”stubs”.Easy to integrate in a circuit, but degraded out of bandperformance.
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Planar Filter Design using ADS
Agilent Advanced Design System (ADS) is a powerfulelectronic design automation software, which includeslibraries and optimizers for planar filters.
Design flow for a bandpass filterfrom the schematic to the layoutand simulation result.
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Power Splitter
Used to distribute an input signal at port 1 equally and inphase between the two output ports 2 and 3. An example isa simple waveguide or microstrip T-junction.
It can be shown, however, that it is not possible to match allports of a symmetric, reciprocal and lossless device, i.e. theSii parameters cannot be zero.
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Resistive Power Splitter
I A simple resistive power splitter is matched at all portsand has a wide bandwidth, but it has additional -3dBloss and ports 2 and 3 are not isolated.
I The Wilkinson power divider has a limited bandwidth,but it is lossless for S21 and S31, and ports 2 and 3 are
isolated. For an ideal device [S ] = −j√2
0 1 11 0 01 0 0
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Directional Coupler
I 4-port device, input port 1 is isolated from port 4.
I Splits the power coming from port 1 equally or with adifferent coupling ratio between ports 2 and 3.
I Most important characteristics:Directivity, bandwidth, phase and amplitude balance
I Very usefull to measure the return loss of a device.
Reflectometer setup with a directional coupler to measurethe return loss ρL of a device. which corresponds to thepower ration P4/P3.
Directional Coupler
10-dB 1.7-2.2 dB directional coupler. From left to right: input, coupled, isoated, and transmitted port
Coupling = - 10 log(P3/P1)
Insertion loss = - 10 log (P2/P1)
Coupling loss = -10 log (1-P3/P1)
Isolation = - 10 log (P4/P1)
Directivity = - 10 log (P4/P3)
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Hybrid Coupler
I Input power is split equally between port 2 and 3.
I For a matched and lossless device the phase differencehas to be either 90 or 180 degrees.
180 degree ”rat-race” coupler 90 degree ”quadrature” coupler
[S ] = 1√2
0 1 1 01 0 0 −11 0 0 10 −1 1 0
[S ] = 1√2
0 1 j 01 0 0 jj 0 0 10 j 1 0
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Multihole Waveguide Coupler
I Coupling holes connect two parallel waveguides.
I Bandwidth increases with number of holes.
3 4
1 2
Submm devices tested at IAP:micromachined 350 GHz hybridand etched 600 GHz hybrid
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Examples of Microstrip Couplers
return loss
-30
-20
-10
0
10
20
30
40
0 20 40 60 80 100 120
frequency [GHz]
[dB
]
directivity
insertion loss
coupling
Simple proximity coupler with wide bandwidth
Optimized step-design with λ/4 matching sections.
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Ferrites
I Ferromagnetic ceramic (Fe2O3+impurities) withhigh resistivity, µr > 1000, εr < 10.
I Can be magnetized permanently by an externalmagnetic field.
I Electromagnetic waves interact with the magneticdipoles.
I Propagation parallel to−→H results
in different effective permeabilityµ+
r and µ−r for left- and right-handed circular polarization, andthus in different propagation con-stants (Faraday rotation):
µ± = µ0
(1 + γµ0MS
ω0±ω
)Larmor frequency ω0 = γB0
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Faraday Isolator
I Non-reciprocal two-port device to reduce standingwaves (ideally S21 = 1 and S12 = 0)
I Resistive vanes at both ports of a circular waveguide areoriented at an angle of 45 to each other and absorbenergy when they are parallel to the E field.
I Ferrite rod in the center rotates the polarization by±45, depending on the propagation direction.
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Circulator
I Non-reciprocal three-port device with a ferrite post atthe junction.
I Allows to use the same antenna for transmission andreception (radar, communications).
I Absorbers for low frequencies.
Circulator example simulated with COMSOL Multiphysics
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Isolator Example
I Measured performance of a high quality 1.4 GHzisolator, which will be used in an L-band radiometer forSMOS validation
I Good performance only over a very narrow bandwidth
I Isolation, loss and matching degrade outside of thespecified frequency band
1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3−60
−50
−40
−30
−20
−10
0 −0.07 dB
Frequency GHz
Am
plitu
de [d
B]
Measurement of a 1.4 GHz narrow−band isolator
S11S12S21S22
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Other Ferrite Devices
I Waveguide switch by reversing the magnetic fieldof a circulator.
I Variable phase shifters for electronic beam steering
I Attenuator for low frequencies
I Tunable filters and oscillators
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Common Symbols for Passive Devices
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Active Components
I Nonlinear transfer characteristic leads to signaldistortions and frequency conversion (b), which is notthe case on a linear curve (a).
I Nonlinear devices can still have an almost linearbehavior for small scale signals (c)
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Power Measurements
I Different ways to measure electric power,depending on the frequency range:
DC −→ voltmeter + amperemeterAC to ∼ GHz −→ oscilloscopeAC to ∼ 0.1 THz −→ diode detectorAC to > THz −→ bolometer
I Other selection criteria:I Power range (nW or kW?)I Accuracy (absolute or relative?)I Linearity (required dynamic range?)I Time constant (continuous wave or modulated?)
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Bolometric Detection
I Microwave energy is absorbed and heats the device, thetemperature change ∆T = R · P is measured with athermometer.
Thermometer
Thermal conductance RRadiation
PAbsorber T(P)
Heat sinkT = const
0
I Advantages: good power handling, no fundamentalfrequency limit, possibility for absolute calibration.
I Disadvantages (which can be overcome):relative slow, not very sensitive, thermal drift.
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Cryogenic BolometersMost sensitive detectors used in radio astronomy:
I Cooled below 0.5 K
I ”Spiderweb” geometry to minimize mass, heat capacityand thermal conductivity
I Used in many cosmic background experiments
Complete bolometer array and close-up
views of the spiderweb bolometers.
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Diode Detector
I Junction betwee semiconductors with different doping(p-n diode) or metal-semiconductor (Schottky diode).
I Non-linear I/V curve rectifies the RF signal.For small signals it can be approximated by a quadraticcurve, and the DC output signal is linear with the inputpower.
n
p_ _ _+ + +
For
war
d di
rect
ion
brea
kdow
n vo
ltage reverse current I
0
V
I reverse bias forward bias
I = I0 [exp(V/V
0)−1]
⟨ I(t)⟩ > 0
V(t)
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Characteristics of Diode Detectors
I Advantages:
I Very fast (rise times < ns), relative sensitive
I Disadvantages:
I Easily destroyed by ESD (electrostatic discharge)I Moderate linearity and temperature stabilityI Upper frequency cut-off given by the parasitic capacity
of the junction
Response of a typical diode de-tector. Only in the square-lawregion the output signal is pro-portional to the input power.
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Diode Layout
To use diodes at THz frequen-cies the junction area needsto be as small as possible,which is achieved by point-likewhisker contacts or very smallplanar devices.
F i g . 1 . S c a n n i n g e l e c t r o n m i c r o g r a p h o f a p l a n a r S c h o t t k yb a r r i e r d i o d e . C h i p d i m e n s i o n s a p p r o x i m a t e l y 1 8 0 x 8 0 x 4 0 m .
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Frequency Multiplication
A nonlinear device generates harmonics of an input signalwith the fundamental frequency f0.
Time
V(t
)
Time
|V(t
)|
−40
−30
−20
−10
0
Frequency
Am
plitu
de [d
B] f
0
−40
−30
−20
−10
0
Frequency
Am
plitu
de [d
B] 0
2f0
4f0
6f0 8f
0
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Examples of Frequency Multipliers
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Heterodyne Principle
I Superposition of a strong local oscillator (LO) signalwith a weaker radio frequency (RF ) signal on nonlineardevice generates an intermediate frequencyIF = |LO ± RF |
I Normal double sideband mixers (DSB) convert bothsidebands, single sideband conversion (SSB) requires aRF filter or a special mixer.
LO
MixerRF IF
US
B
IF
LSB
2 LO
RF + LOLO
up−conversiondown−conversion
Pow
er
Frequency
LO−RF
RF−LO
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Mixers Designs
I Single ended mixer (a): Common for mm wavelengths.No isolation between RF and LO.
I Balanced mixer (b): Two mixing elements, 3dB hybridcombines LO and RF. Good LO to RF isolation, LOnoise and spurious harmonics are rejected.
I Double balanced mixer (c): Also IF port is isolated,dynamic range is improved.
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Subharmonic Mixer
I Antiparallel diode pair down-converts with the secondLO harmonic IF = |RF − 2LO|
I Advantages:Lower LO frequency and good LO/RF isolation.
I Disadvantages:Higher conversion loss and LO power requirement.
RF
LO
IF
RF filter
diode pair
IF Filter
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SIS Mixer
Superconductor-Isolator-Superconductor tunnel junction:
I Two Niobium layers at 4K (ρΩ = 0),separated a 2 nm thick Al2O3 barrier
I Cooper-pairs (2e−) tunnel through the barrier,resulting in a sharp bend in the I/V curve
0 2 4 60
100
200
300
400
bia
s c
urr
ent I 0
[µA
]
bias voltage U0 [mV]
V0
superconductor
insulator
superconductor
I0
a) b)
V0
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SIS Mixer
Band structure and photoassisted tunneling in a SIS junction.
superconductor"bandgap"
SS
I
energy
2ephoton
unfilled energy states
filled energy states
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SIS Mixer Characteristics
I Very low noise, close to the quantum limit hν/kB
I Upper frequency limit from the bandgap voltageNiobium: 1.4 THz
65 µm
NbTiN
ground
plane
SiO2
dielectric
Nb top-wiring
(1)
(2)
(3)
48 µm
junction
1µm2
feed point
Example of an SIS mixer forthe HIFI instrument. Thejunction has an area of only1µm2, most parts in the im-age are tuning elements forthe impedance matching.
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HEB MixerI Hot Electron Bolometers (HEB) are extremely fast
bolometers, which can be used as mixing element.I Superconducting microbridge (d <10 nm) close to the
transition temperature.I No fundamental RF frequency limit (>2THz)I Limited IF bandwidth (∼ 5 GHz) given by cooling rate
of the electrons.
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Amplifier
I Increase signal amplitude
I Made with bipolar or FETtransistors
I Tradeoff between low noiseand high power
bias out
in
CB
E
n AlGaAs
undoped GaAs
n+
HEMT−FET transistorbipolar transistor
source gate drainbaseemitter
collectorGaAs
n p
quantum−well with 2DEG
Schematic of a bipolar npn transistor and a High ElectronMobility (HEMT) field effect transistor, which works with a2D electron gas in a quantum well.
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Amplifier Specifications
I Gain (amplification in dB)
I Frequency range and gain flatness
I Noise figure (how much noise is added)
I Maximum output power and 1dB compression point
Examples:
Power amplifierG=45dB (±2dB), NF=8dBf=0.8-2 GHz, 1dB Gc = +36dBmVSWR = 1.7dB Bias supply 24V, 2A
Lownoise amplifierG=15dB (±1dB), NF=0.4dBf=1-1.4 GHz, 1dB Gc = +12.5dBmVSWR = 1.7dBBias supply 12V, 40mA
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Oscillator
Active element (1) with a resonant feedback (2)
(1) Transistor, electrons in a vacuum tube (for high power),Gunn diode (semiconductor with negative resistance), ...
(2) LC-circuit, microstrip and dielectric resonator,waveguide cavity, quartz crystal, ...
L C
Schematic of a LC oscillator with ω0 = 1√LC
and
example of a dielectric oscillator in stripline technology.
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49
Magnetron
I Microwave generator for high output power (>1MW)with good efficiency (>80%)
I A high electric field accelerates electrons in a circularcavity, a magnetic field forces them on a spiral pathwhich excites microwave resonances.
I Standard for microwave ovens and radar systems
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Oscillator Specifications
I Frequency accuracy and stability
I Phase noise (specified in dB below carrier [dBc])
I Harmonic and spurious signals
I Phase noise and short term accuracy depends on thequality of the resonator (Q-factor).
Phase Noise
Residual FM
Spurious
non-harmonic spur
~65dBc
harmonic spur
~30dBc
CW output
Residual FM is the integrated
phase noise over 300 Hz - 3
kHz BW phase
noise
0.5 f0 f0 2f0
sub-harmonics
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Oscillator Types
I Atomic clocks (Cs or Rb) for absolute time standardswith ∆f /f < 10−15 (e.g. at METAS, UniNE, NIST)
I Quartz oscillators as reference signals up to 100 MHzreach ∆f /f = 10−6 to 10−9, depending on temperaturecompensation or temperature stabilization.
I All higher frequencies are usually synchronized to aquartz crystal with a phase-locked loop (PLL).
free running phase lockedExample of a 6 GHz cavity oscillator
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Modulation Analog
I Amplitude modulation (AM): VAM = A(t) sin(f0 · t)
Volta
ge
Time
Carrier
Modulation
I Frequency modulation (FM): VFM = A0 sin(f (t) · t)Phase Modulation (PM): VPM = A0 sin(f0 · t + φ(t))
Volta
ge
Time
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53
Modulation Digital
Amplitude
Frequency
Phase
Quadrature phase-shift keying (QPSK):Digital ModulationPolar Display: Magnitude & Phase Represented Together
Magnitude is an absolute value
Phase is relative to a reference signal
Phase
Mag
0 deg
QPSK IQ Diagram
I
Q
0001
1011
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54
Spectrum Analysis:From Time- to Frequency Domain
: ; 4
8 &4
8
To increase the frequency resolution a longer time series hasto be analyzed.
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55
Spectrum Analyzer
I Realtime analyzer measures all channels simultaneously⇒ best signal-to-noise ratio for a given integration time
I Swept spectrum analyzer moves a filter over thespectrum ⇒ flexible standard instrument for mostmeasurement tasks
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Realtime Spectrum Analyzer
I Filterbank spectrometerSize, cost and power consumption increase linear withnumber of channels.
I Acousto Optical Spectrometer (AOS)Bandwidth up to 1 GHz, typically 1-2k channels.
I Digital autocorrelation and FFT spectrometer1 GHz bandwidth with 16k channels in a small unit.Cost effective and rapidly evolving because of the hugemarked for digital technology.
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Acousto Optical Spectrometer (AOS)
I IF signal is converted to an acoustic wave in a crystal(Bragg-cell), which modulates its refractive index.
I A collimated laser beam is diffracted by the resultingphase grating.
I A linear CCD array detects the image, which representsthe IF spectrum.
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Digital FFT Spectrometer
I IF signal is sampled with a fast Analog/DigitalConverter (ADC) with sampling rate fs .
I Time record of N samples is Fourier transformed inrealtime in a fast FPGA processor.
I The averaged power spectra cover a bandwidth of fs/2with a frequency resolution of fs/N.
....
.. .
..
....
.....
.. .ADC FFT
N = Number of sample points (*powers of 2)
Sampling
fs = Sampling frequency (sampling rate) = (N/2) + 1
Spectrum display
n = Number of lines (or bins)
∆ t
T
= N x t = 1/ f
T = Time record length f = Frequency step
= 1/T = fs/Nt = 1/fs = Sample time
∆ f
00 N
Window
Timerecord
Time records1 2 ...n
Time record
Frequency range
Lines (N/2)
0 (fs/2)
Samplesfs
N/fs0
.
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Swept Spectrum Analyzer
I LO of a heterodyne receiver is swept over the frequencyrange of interest
I Resolution bandwidth (RBW) can be adjusted bychanging the IF filter
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60
Effect of RBW and VBW
RBW determines the frequency resolution. Smaller RBWreduces the noise floor, but increases sweep time:
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Video Bandwidth (VBW) determines smoothing of thespectrum:
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Spectrum Analyzer: Filters and Detectors
Digital filters and FFT processing improve speed andchannel selectivity at narrow bandwidths.
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Different ways to analyze the detector output, depending onthe measurement task (e.g. RMS for noise measurements):
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