The principle of Pulse Compressor - PowerPoint Presentation

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The principle of Pulse Compressor

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A technical studentship project by Sebastian

Göbel

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Some RF basicsContinuous wave (CW) versus pulsed RFAverage, peak power and duty cycle.

Solid state versus tube based amplifiersHigh demand in LINACS

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IQ and narrowband theory

A narrowband signal can be described as two sine waves in quadrature with separate amplitudes Similar to complex impedance

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Which signals are applicable?

Applicable

Not applicable

Signals that can be described as a slowly changing signal

For example a telecommunications carrier

Narrowband signals

For example a humans voice

Cannot be described a slowly changing signal

Very wideband signals

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Complex description of a signal

The signal can be described as a signal with a slowly changing amplitude and phase

 

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IQ description of signal

Possible to describe phase, frequency and amplitude modulationRepresents the instantaneous amplitude and phase of the signal

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Modulation and demodulationModulation

DemodulationAdding information to a carrierComplex modulation enables two separate information streams I,Q.

Changing I,Q is equivalent to changing amplitude and phase.

Recovering information from a modulated carrier.

Enables separately receiving I,Q channels

Possible to measure amplitude, phase over time simultaneously.

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Basics about resonatorsOne mode of a cavity can be described as an RLC circuit

Q is bandwidth of resonance, high Q means narrowbandFill time, decay time depends on Q, beta

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What is a pulse compressor?

RF equivalent of a capacitor bankStores RF energy to temporarily amplify an RF pulseEnergy stored in cavities or delay lines

Is in theory passive, input is the control

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Basic input for compression

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How does it work?Power leaking from cavity destructively adds with inputQuick input phase doesn’t change cavity phase

Causes cavity power and input to constructive add instead

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Usage of pulse compressors

High power pulsed RF with low duty cycleCould allow for use of solid state over tube amplifiers

Reduces

, increases duty cycle

Faster pulses using Klystron

Klystrons are based around high Q cavities, causing slow reactions

 

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Model of Pulse Compressor

Described as coupled RLC circuit

 

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Simulation of model

Simplify by

Describe input current as sum of R,L,C currents and solve for second order derivative of cavity voltage

 

Solved numerically using

fourth order

Runge-Kutta

method

Stepping through time and approximating cavity voltage and derivative

 

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Simulation results

Simulation using measured values from cavity

Focus on 1us pulse

Program capable of any values

 

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Basic input

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More advanced input is possible

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Analysis of simulated dataAmplitude is easily readable from signal envelope, easily trackedInstantaneous phase is impossible to see from graph.

I,Q Demodulation enables easily extracting instantaneous amplitude and phase over time.20

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IQ Demodulation in softwareUsed resonant frequency of cavity for LOGenerate two arrays with quadrature LO signals, emulates LO and quadrature hybrid.

Mix the two LOs with the input and receive two signals, filter with rolling average of 5 RF cycles.21

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Test of a real Pulse CompressorInteresting to test a real Pulse Compressor and seeing if simulation is accurate.

Single cavity Pulse Compressor chosenMatching of cavities is critical and very temperature dependent.Dual cavity would require active controls to match cavities.

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Equipment required

RF synthesiserFast amplitude and phase modulator(s)

High Q cavity

Q in the many thousands at least for shorter pulses

Circulator

Measuring device(s)

Preferably not just amplitude.

 

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Cavity

Based around copper cavity from amplifier, low loss.Designed for 200 MHzCustom made by Cristiano GagliardiSimulated by Rolf WegnerFeatures a pickup

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Cavity results

Parameter

Measured

Note

F0

197 MHz

Drifts

with temperature and not critical

Q0

7.8E3

Good

Beta

3.6

Good

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β

is measured from the reflection Coefficient at resonance together with loaded Q,

.Q is 3dB-bandwidth of T, is quality factor of circuit connected to a 50 ohm system.Then Undampened Q, is extracted from ,  

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Phase modulatorConsisted of 3 CERN made phase shifters

Not quick enoughPhase rise, fall time around 600nsNot capable of 180deg phase inversionCaps at around 160degRF synth not quick enough eitherMeasured using oscilloscope and IQ demodulation

New solution was found

using

IQ

modulation

Speed less than 100ns

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Test equipment

Type

of equipment

Equipment used

Note

RF synthesizer

Rohde & Schwarz

SMB100A

Good

Cavity

Custom made cavity

Good

Amplitude

modulator

IQ modulator

Controlled by arb. Signal generator.

Phase

modulatorIQ ModulatorControlled by arb. Signal generator.Arbitrary signal generatorTektronix AFG3022CTo control amp, phase modulators from software.CirculatorTDK CU281A, 200MHzMeasuring deviceTektronix DPO4104BOscilloscope, IQ demodulation used in software.27

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Comparing simulation and lifeComparing simulated output pulse and measured output pulse

Using same computer generated phase functionAllows more complex phase, amplitude functionsGenerated in Matlab, transferred via USB to arbitrary signal generatorRead using oscilloscope and demodulated to display data similarly to simulated data

Frequency measured using curve-fit during input

pulse

Set reference phase with statistics

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180 Degree step zoomed out

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180 Degree step

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Exponentially rising phase31

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Stable amplitude and phase pulse

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New amplitude and phase shifterIQ Modulation basedAllows phase and amplitude

Modern and widely used in high-speed communication systemsDevelopment kits available360 Degree capable

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IQ Modulator

I,Q equivalent to amplitude and phase

Can change amplitude and phase

 

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ADL5390ADL5390 was chosen

Two matched variable amplifiers with a combinerVoltage input 0-1V, controllable by arbitrary signal gen.FeaturesUp to 230MHz modulation BW, limited to 25MHz by signal generatorFull scale change in around 50ns

Amplifier based, increased isolation

Few external components

Quadrature coupler

Biasing

Power supply

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Expected speed of the modulator

230 MHz BW, limited to 25 MHz by the control signal

Quicker at lower gain as full scale is 50ns

To flip 0 to 180, is to switch from full scale twice

Expected time for 180 phase inversion is less than 100ns, even at maximum gain

Speed could further be improved with external post amplifier, at the cost of SNR

 

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Quadrature couplerInitially a splitter and phase shifter was tested

Also called 3dB hybridTransformer based which reduces size compared to transmission lineHand tuned to 197MHzOutput mismatch, <0.2 dB, <5degImproved amplitude stability during phase change, sign of better quadrature input

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BiasingADL5390 uses differential signals biased around 0.5V

Complementary input has trim potentiometer allowing calibrationDC offset amplifier was tested, not satisfactory resultsCaused by big loop radius, custom PCB or alternative chip needed, was not worth effortWould have provided biasing internally, reducing setup complexity and allowed overvoltage protection

Not a must for functional prototype

Solution was to use offset from the arbitrary signal generator

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Power supplyLab power supplies available are switch mode

Noisy and contains high frequency tonesPost regulator using LT3042Low noise, High PSRR and BW.Input low pass filter and ferriteQuick start-up to enable large reference capacitanceNoise floor lower than

oscilloscope

See datasheet for reference schematic

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Simplified diagram of IQ modulator40

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Finished prototype

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Test of IQ modulatorPhase shifting speed

More complex combinations of I,Q data to see amplitude accuracy over complete rangeAmplitude accuracy affected by angle, amplitudeTest with spiral I,Q formPhase rotates 10 turns linearlyWhile amplitude sweeps linearly from 0 to 1160 Degree phase step

To compare against old setup

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Test setup for IQ modulatorMeasures input for reference

Measures output directlyTrigger of I inputControlled by arbitrary signal gen.Signals read from USB stickIQ Demodulation software reused for reading output

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Polar plot [spiral]

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I,Q over time plot [spiral]

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Amplitude, phase over time [spiral]

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Phase step test comparison160 degree pulse tested to be comparable to old setupDone at half gain as max gain

Significantly fasterUsable for test of pulse compressor

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160deg step comparison

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Possible more tests that could be done

Single side band modulationSeeing ratio of wanted, unwanted sidebandIQ constellation diagram, seeing amount of possible positions in I,Q space without interference

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Thoughts for improvement

Higher resolution controlHigher resolution arbitrary signal generator gives less noiseBetter quadrature coupler, done on PCB for good matchingHigher precision, more stable biasing

Post amplifier for use of lower gain and therefore higher speed

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SummarySimulation of cavity is accurateOld phase shifters were too slow and not capable

New IQ modulator a success, significantly faster and 180 degree capable.Could be a solution in the future if tube and klystron based solutions become undesirable

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Reflection

Project goal was to achieve an understanding of resonators and to simulate the behavior using lumped elementsImplement a phase function to achieve a compressed pulseOptionally focus on new hardware better suited for control of a pulse compressor

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Lessons learnedCERN delivery system can be slow

Three weeks of waiting for componentsCrimping properly is importantFast control loop need be to very small, even on PCB level

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ReferencesVector mod/demodulation diagram, slide 9

E.Jensen, CERN, CAS Chavannes 2013Pulse compressor diagram, slide 11X-band SLED type Pulse Compressor, made in CERN, I.

Syratchev

, 2013

ADL5390 block

diagram, slide 34

Analog devices datasheet for ADL5390

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