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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

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−10

0 −0.07 dB

Frequency GHz

Am

plitu

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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

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−10

0

Frequency

Am

plitu

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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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Passive

Termination

Attenuator

Filter

Coupler

Ferrites

Active

Detector

Multiplier

Mixer

Amplifier

Oscillator

MW Instruments

Spectrum Analyzer

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

MicrowaveComponents

Passive

Termination

Attenuator

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Ferrites

Active

Detector

Multiplier

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Oscillator

MW Instruments

Spectrum Analyzer

50

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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MW Instruments

Spectrum Analyzer

51

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

MicrowaveComponents

Passive

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Attenuator

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MW Instruments

Spectrum Analyzer

52

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

MicrowaveComponents

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Attenuator

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MW Instruments

Spectrum Analyzer

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

MicrowaveComponents

Passive

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Attenuator

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Active

Detector

Multiplier

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Oscillator

MW Instruments

Spectrum Analyzer

54

Spectrum Analysis:From Time- to Frequency Domain

: ; 4

8 &4

8

To increase the frequency resolution a longer time series hasto be analyzed.

MicrowaveComponents

Passive

Termination

Attenuator

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Active

Detector

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MW Instruments

Spectrum Analyzer

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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MicrowaveComponents

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Ferrites

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Detector

Multiplier

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Amplifier

Oscillator

MW Instruments

Spectrum Analyzer

56

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.

MicrowaveComponents

Passive

Termination

Attenuator

Filter

Coupler

Ferrites

Active

Detector

Multiplier

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Amplifier

Oscillator

MW Instruments

Spectrum Analyzer

57

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.

MicrowaveComponents

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MW Instruments

Spectrum Analyzer

58

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

.

MicrowaveComponents

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MW Instruments

Spectrum Analyzer

59

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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MicrowaveComponents

Passive

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Attenuator

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Active

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Multiplier

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Oscillator

MW Instruments

Spectrum Analyzer

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:

MicrowaveComponents

Passive

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Attenuator

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MW Instruments

Spectrum Analyzer

61

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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