ilsf - 3-llrf - francis...

RF systems

Francis Perez

Part III:

Low

Level

RF

RF signal generatorRF signal generator

LowLow --level RF controllevel RF control

RF power distributionRF power distribution

Accelerating structureAccelerating structure

RF power AmplifierRF power Amplifier

Main Functionalities

� Control of amplitude and phase of the RF Cavity Voltage to be

synchronized with the bunch beams.

� Control of resonance frequency of the cavity (Tuning)

� RF Diagnostics

� RF interlocks

Why do we need LLRF?

� Ripples of amplifiers.

� Gain and Phase of amplifiers not linear.

� Beam loading (current of beam changes voltage of the cavity).

� Cavity drifts

LLRF

Three magnitudes have to be regulated in the RF system

Three main control loops:

• Amplitude

Precision 0.05 - 1 %

• Phase

Precision 0.01 - 0.5 °

• FrequencyPrecision 1 - 100 Hz

LLRF main loops

RF Drive

Plunger

Cav

Volt

IOT

Pre

Amp

Analog Front End

SR LLRF

Analog Front End

RF ( 500 MHz )

IF ( 20 . MHz )

IF ( 20 . MHz )

RF Control

Beam

General concept:

- Measure the relevant

parameter

- Compare with reference

- Use the difference and PI

control to define the action.

- Act in the amplifier chain

LLRF control loops

LLRF amplitude/phase loop

- Pick up a sample of the voltage inside the

cavity, get amplitude and phase.

- Compute the error (difference measured

respect the setting)

- Use the error signal to drive the amplifier

chainPickup loop

LLRF tuning loop

- A way to measure the mismatch of cavity with amplifier

- Compute the error (difference measured respect the setting)

- Use the error signal to drive the tuner motor

RF Drive

Beam

Plunger Plunger

Cav

Volt

Cell 2

Volt

Cell 4

Volt

Booster LLRF

IOT

Cav Fw

Pre

Amp

Other loops

Field flatness loop in a multicell cavity.Maintain the voltage along the cells constant

Act on plunger in cells 2 and 4.

ALBA

The idea is to pickup a sample of the voltage in the cavity and, after

proper amplification, combine it, 180° dephased, with the driving

signal of the amplitude loop.

CavityPickup

PowerCouplerAmplitude

Loop

RF Drive Combiner VariableAttenuator

VariableAttenuator

180Phase shifter

o

Amplifier

Amplifier Gain K

Directive Coupler

Other loops

Fast RF feedback for beam loading compensation

ANKA

Analogue LLRF

vs

Digital LLRF

Nowadays, digital is the chosen solution, analogue

is becoming obsolete.

Only, in some specific cases, where high speed is

required – low loop delay (fast RF loops), analogue

is still competitive

» Higher precision, lower noise

amplitude down to 0.01% and phase 0.01 degrees

» Flexible, modifications in “software”

by reprogramming the FPGA

» Allows the parameterization of the loopsfor example, change the loop gains on the flight

» Allows better diagnostics

one can check how the loop is working with inside FPGA diagnostics

» Nowadays, can even be cheaper than analogue

Advantages of the digital system

As an example

the

ALBA Digital LLRF system

by Angela Salom

Analog Front End

Down Conversion

FPGA

Digital IQ Demodulation

8DACs80MHz

Analog Front End

UpConversion

IOT2

RF Cavity Voltage (500MHz)

IF

RF Forward Cavity Voltage(500MHz)

Analog Timing System520MHz 500MHz

80MHz 80MHz

CAVITY

Tuning Control Loop

Digital Timing SystemPCI Bus

ICEPAPMotor Controller

13 RF Diagnostic Signals (500MHz)

IQ Ctrl IOT1& IOT2

IOT1CACO

DC

500MHz

8 ADCs80MHz

8 ADCs80MHz

FPGA

Digital IQ Demodulation and Control Loops

Digital Timing System

Analog Front End

Down Conversion

FPGA


8DACs80MHz

Analog Front End

UpConversion

IOT2


IF



80MHz 80MHz

CAVITY

Tuning Control Loop

cPCI Bus



IQ Ctrl IOT1& IOT2

IOT1CACO

DC

500MHz

8 ADCs80MHz

8 ADCs80MHz

FPGA


Commercial Digital Board: cPCI

Analog Front End

Down Conversion

FPGA


8DACs80MHz

Analog Front End

UpConversion

IOT2


IF



80MHz 80MHz

CAVITY

Tuning Control Loop




IQ Ctrl IOT1& IOT2

IOT1CACO

DC

500MHz

8 ADCs80MHz

8 ADCs80MHz

FPGA


Analog Front End

Down Conversion

FPGA


8DACs80MHz

Analog Front End

UpConversion

IOT2


IF



80MHz 80MHz

CAVITY

Tuning Control Loop




IQ Ctrl IOT1& IOT2

IOT1CACO

DC

500MHz

8 ADCs80MHz

8 ADCs80MHz

FPGA


Control Inputs

Diagnostics Inputs

Digital LLRF Conceptual Design

• Digital Commercial Board: cPCI with 16 ADCs, 8 DACs and Virtex-4 FPGA

• Analog Front Ends for Downconversion (RF to IF) and Upconversion (DC to RF)

• Timing systems: 520MHz (500 + 20 MHz) for downconversion synchronized with digital 80MHz clock for digital acquisition

FPGA Board

� Lyrtech: VHS ADAC-4 cPCI format

2 x 8 ADCs 125 MHz 14 bits

8 DACs 125MHz 14 bits

2 x Virtex 4

2 x 128 Mbytes RAM

IQ Demodulation� Demodulation

Removal of the periodic variation of a signal keeping its information, i.e. transform a waveform signal into baseband or DC.

�IQ Demodulation

Comparison between two signals to obtain the quadrature and in phase components of the signal demodulated in comparison to the reference signal

Reference: Master Oscillator Clock (500MHz)

Cavity Voltage: 500MHz

I: In Phase Component (0°)

Q: In Quadrature component (90°)

Amp:

Φ:

22 QI +

)tan(IQ

a

R F C avity Vo ltage

M aster O sc illator

I Q

IQ

I Q

IQ

φ

Amp

I

Q

Master Oscillator

φ

�Phasor Diagram: I&Q

RF Systems of ALBA, March 13th, 2013 15/67

Freq ( MHz )20 1020

cos ( ß ) · cos (α ) = ½ · ( cos (α - ß ) + cos ( α + ß ))

+

Freq ( MHz )500 520

cos ( ß ) · cos (α )

Downconversion: From RF to IF

Ref Signal (520 MHz) = MO + IF

ADCs Clock = 4*IF

Phase shifter to adjust phase

delay lines between RF plants

Upconversion: From DC to RF

IQ Modulator

DC I Control

DC Q Control

MO (500MHz)

RF Drive

The MO signal is modulated, by I & Q,

in amplitude and phase

Timing System

VCXO (160MHz)

1/2

1/8

0/90º

IQ Mod

520MHz for Downconversion

Bandpass filter

Digital Timing System Analog Timing System

20MHz

80MHzCLK

10MHz

MO (500MHz)

Needed for proper digitalization and synchronization

Control Loops

Amplitude and Phase Control Loops� Digital IQ Demodulation

� PI Control Loop for IQ

Accumulator

+

-

+

+

+

+

P action

Integral

action

Set Point Kp

Input

(I,Q)Control

Signal

Ki

error

Settings

Diagnostics

IQ

-I

-Q

IQ

20 MHz

I

Q

I

QIF 20 MHz

Dem

uxADC80MHz

80MHz Clk

80MHz Clk

1 2 3 4 1 2 3 ... SW Reference Counter

ActionIntegral ActionalProportion

k Error

k Error

kError

InputSetPoint

operator) theby set be (to

achieve to VoltageCav

voltageCavity of Components IQ

i

ti

P

+⋅

⋅⋅

−

∑

Inputs:

Set Point:

Error:

P Action:

I Action:

Accum Error:

Control Signal:

Amplitude and Phase Control LoopsDifferent PI Loop responses: Over-damp

Cavity Gain: 0.25; Kp gain = 1; Ki gain = 1

tInput Ref

error

Error accum e * kp

e accum * ki

Control action

0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

1 0.0 100 100 100 100 100 200

2 50 100 50 150 50 150 200

3 50 100 50 200 50 200 250

4 62.5 100 37.5 237.5 37.5 237.5 275

… … … … … … … …

30 99.8 100.0 0.2 399.3 0.2 399.3 399.5

Accumulator

+

-

+

+

+

+

P action

Integral action

Set Point Kp

Input

Control Signal

Ki

error

Cavity

Input = Control Signal * Cavity Transfer Function

Cavity Gain 0.25; Ki 1; Kp 1

0.0

50.0

100.0

150.0

200.0

250.0

300.0

350.0

400.0

450.0

0 5 10 15 20 25 30 35

Input Reference control

Amplitude and Phase Control LoopsDifferent PI Loop responses: Under-damp

Cavity Gain: 0.25; Kp gain = 2; Ki gain = 3

t Input Ref errorError accum e * kp

e accum * ki

Control action

0 0.0 0 0 0 0 0 0

1 0.0 100 100 100 200 300 500

2 125 100 -25 75 -50 225 175

3 43.8 100 56.3 131.3 112.5 393.8 506.3

4 126.6 100 -26.6 104.7 -53.1 314.1 260.9

… … … … … … … …

30 100.4 100 -0.4 133.1 -0.8 399.4 398.6

Accumulator

+

-

+

+

+

+

P action

Integral action

Set Point Kp

Input

Control Signal

Ki

error

Cavity

Input = Control Signal * Cavity Transfer Function

Cavity Gain 0.25; kp 2; ki 3

0.0

100.0

200.0

300.0

400.0

500.0

600.0

0 5 10 15 20 25 30 35

input reference Control action

Characterization of Cavity Transfer Function

Transfer Function: Mathematical representation of a system (model)� to be calculated analytically

� to be measured experimentally

Experimentally: Step response in Open Loop

Cav Volt

Reference

� Group Delay = 1.93µs� ζ (63%) = 9.7 µs� Gain = 0.8� Filling time = 29.3 µs

Transfer Function

s.

e.s

eK sTs

6

102

10791

80

1

6

−

⋅⋅−−

⋅+⋅=

τ+⋅

−

System Characterization (1st Order)

(DAMPY Cavity)

Amplitude and Phase Control Loop

Step response in Close Loop (Cav&DACs)

0.95 1 1.05 1.1 1.15

x 10-3

30

40

50

60

70

80

90

100

t(s)

Cav

Sig

nals

(m

V)

ICavQCavCavAmpIRefCavRef

0.95 1 1.05 1.1 1.15 1.2

x 10-3

20

40

60

80

100

120

140

160

t(s)

DA

Cs

Out

put

(mV

)

IDACQDAC

Diagnostics signals of Loops

IQ Cav IQ FwCav Amplitude Fw AmplitudeCav Phase Fw Phase

IQ Error Control AmplitudeIQ Integral Action Control PhaseIQ PID Output

fRF

freq

Tuning Loop

� Loop required to adjust resonance frequency of the cavity

� How it works?

� How do we measure detuning of the cavity? (Δf0)

Cavity geometry defines Resonance Frequency (f0)

Cavity should be matched to RF, without beam f0 = fRF

Power dissipation (heat) and beam loading requires that

resonance frequency is adjusted ∆f0 � cavity geometry

change � Action: modification of the geometry by a plunger

in the cavity.

Af0

freqfRF

Detuning effects:

� Voltage of the cavity will decrease

� Phase of the cavity will vary

Tuning Loop Input: Phase difference between Forward

Power and Cavity Voltage

φ

A

Tuning Loop� Cordic Algorithm to calculate Cav – Fw phase difference

Iterative process to calculate phase without employing any multipliersResolution better than 0.001º after 16 iterations (1/80MHz * 16 = 0.2 µs)

� Tuning Loop not always active to avoid plunger oscillations around 0ºphase

t=t0 ∆φ < -3º Tuning Ont=t1 ∆φ = 1º Tuning Offt=t2 ∆φ < -3º Tuning Ont=t3 ∆φ = 1º Tuning Off

� Tuning Outputs: Two LVTTL signals (Pulse and direction)

Motor Driver Control : IcepapFrequency of the pulses adjustable (from 100Hz to 2kHz)

Results on real cavity @ 20kW

t0 t1 t2 t3-3

-101

3

Fw&Cav Dephase

Cavity Reflected Power

t(s)

Pha

se (

º)

Pow

er (

a.u.

)

High Power testUp to 80 kW

y = 0.0004x2 - 0.0007x

R2 = 1

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350 400 450 500

IQ Drive [mV]

Pow

er [k

W]

Power Tests

Power tests at ALBA High Power RF Lab from 80W to 80kW

0 50 100 150 200 250

0

10

20

30

40

50

60

70

80

90

100

t(s)

mV

- D

egre

es

Reflected Cav

Cav Volt

Fw Cav Volt

Amp & Ph loop OFFAmp & Ph loop ON -

Slow PID

Amp & Ph loop ON -Fast PID

Dynamic range: 30 dB

Loops performance:

Amplitude, Phase and Tuning loops simultaneously in

operation

Close Loop

Open Loop

Booster Ramping� Ramping to accelerate from 100MeV to 3GeV

100MeV

3GeV

333ms

BTS extraction

LTB Injection

333ms

3Hz TRG

t1 t2 t3 t4 t1 t2 t3 t4

Cav Volt Init

Cav Volt End

Tuning Pulses

Ramping Timing

� Parameters to be set by LLRFTiming

t1: Time to start ramp up after trigger

t2: Ramp up timet3:Top ramping time

t4: Ramping down time

Cavity Voltage

Cavity voltage at Ramping startCavity Voltage at Top Ramp

� Amplitude and Phase loops always active

�Tuning Loop only active at Top of the ramp

Booster Field Flatness (FF)� Booster Cavity: 5 cells

� Field Flatness loop: to keep the same voltage at 5 cells

� How does it work?

RF Drive

Beam

Plunger Plunger

Cav Volt

Cell2 Volt

Cell4 Volt

Booster LLRF

IOT

Cav Fw

Pre Amp

Cell 3: Input of Amplitude and Phase loop (Cavity Voltage)

Cell 2 and Cell 4: Inputs of Field Flatness loop

Actuator: Two Plungers

φ (CavFw) ∆A (Cell2-Cell4) Tuning FF PLG1 PLG2

φ > TM -- ON OFF ↑ ↑

φ < -TM -- ON OFF ↓ ↓

-TM < φ < TM ∆A > FFM OFF ON ↑ ↓

-TM < φ < TM ∆A < -FFM OFF ON ↓ ↑

-TM < φ < TM -FFM < ∆A < FFM OFF OFF Stop Stop

TM: Tuning Margin FFM: FF Margin

Booster Field Flatness (FF)� Parameters of Booster Field Flatness (FF)

Field Flatness Margin or deadband

Gain of Amplitude of Cell 2 and Cell 4 to compensate different attenuations between channels

� Field Flatness Power tests

FF ONFF OFF

FF OffAmp Cell2 > Amp Cell 4

Temp Cell2 > Temp Cell4

FF On

Amp Cell2 = Amp Cell 4Temp Cell2 = Temp Cell4

RF Diagnostics

RF Diagnostics

�CavityCavity PowerForward and Reversed Cavity Power

� Waveguide SystemFw and Rv Circulator InputFw and Rv Circulator OutputFw and RV Load Power

�Transmitter SignalsFw Transmitter1 Input PowerFw Transmitter2 Input PowerFw and Rv IOT-01 PowerFw and Rv IOT-02 Power

Other RF signals Digital IQ Demodulated

RF DiagnosticsFast Diagnostics

� Signals acquired at 5MHz rate

� Signals stored in a circular buffer of 128MBytes (equivalent to 0.5s of operation)

� Used for post mortem analysis and transient studies

Booster Ramping @ 3Hz

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-4

433

433.5

434

434.5

435

435.5

436

436.5

437

437.5Cav Amplitude

t(s)

mV

Cav Volt 80MHz

Cav Volt 1MHzReference

Amplitude RMS Errors:

δVrms = 0.50mV/435mV = 0.11% @ 80MHz

δVrms = 0.18mV/435mV = 0.03% @ 1MHz

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x 10-4

44.75

44.8

44.85

44.9

44.95

45

45.05

45.1

45.15

45.2

45.25Ang Cav atan

t(s)

mV

Cav Phase 80MHz

Cav Phase 1MHz

Phase RMS Errors:

δPhrms = 0.05º @ 80MHz

δPhrms = 0.02º @ 1MHz

RF Diagnostics

0 0.5 1 1.5 2 2.5 3 3.5 4

x 10-3

100

110

120

130

140

150Beam Phase

t(s)

(º)

Beam Phase (º)

0 0.5 1 1.5 2 2.5 3 3.5 4

x 10-3

0

2

4

6

8

10

12Cav Dis - RvCav - Beam Power

t(s)

kW

Cav DisBeamPowerRvCav

Behavior of 06B after a trip in 10B and no beam dump (61mA)

Post Mortem Analysis Example

�Power to beam increases

�Beam phsae gets reduced

�Frequency oscillations ~ 6kHz (synchrotron freq)

�Stabilization time ~ 3ms (longitudinal damping time)

RF Diagnostics

Slow diagnostics� Archiving and Taurus Trend

� Signals acquired at 1Hz rate

� Historical purposes

� Slow trend analysis

Test Points

� BNC Test points of main RF signals

900 1000 1100 1200 1300 1400 1500 1600 1700

0

50

100

150

200

250

t(s)

Pow

er (

a.u.

)P

hase

(de

gree

s)

Dephase Cavity&ForwardCavity Reflected Power

Cavtiy Voltage Forward Cavtiy Voltage RF Drive

Booster Ramping @ 3Hz

RF Drive

Conditioning

Conditioning

Cavity conditioning� Process to “clean” the cavity before putting full power

Multipacting�Secondary electron emission in resonance with RF

�Depending on geometry of cavity components

�Depending on RF power level

�Depending on RF frequency

�Depending on surface roughness

Effects�Vacuum pressure increase

�Reflected power

�Arcs

e-

RF

Cavity

Manual ConditioningRF Drive square modulated

�Duty Cycle of pulses adjustable (1-100%)

�Amplitude adjustable

�Time between pulses = 100ms (10Hz)

� Tuning Loop only enable at top of the pulses

Drawbacks

�Operator needed to adjust amplitude and duty cycle

�Vacuum levels not considered by LLRF �frequent interlocks

Automated Conditioning

�Amplitude and duty cycle increase depending on vacuum levels

�Amplitude increase rate (slope): adjusted by operator

�Vacuum signal connected to LLRF

� Vacuum < Limit Down � Voltage Amplitude Increases/Decreases

� Vacuum > Limit Up � Voltage Amplitude remains constant until vacuum is below limit down

Voltage Increase rate set to 0.03mV/s Voltage Decrease rate set to 1mV/s

8kW

1kW

600W

Pow

er (

kW)

Power Down

0 50 100 1500

2

4

6x 10

-7

Vac

uum

(m

bar)

Vacuum

Cavity Reference

Cavity Voltage0

2

5

x 10-7 Power Up

Vac

(m

bar)

0 50 100 150 200 250 300 35045

50

55

60

65

Pow

er (

a.u.

)

Cavity Reference

Cavity Voltage

Vacuum

Vac Limit Up

Vac Limit Down

Automatic Startup

RF Power

Tuning pulses

Plungers direction

After an RF trip, LLRF goes to Standby State:�Low RF Drive�Tuning Disable�Amplitude and Phase loops opened

After resetting the interlock and switching on transmitter� Detection of RF presence in cavity� Cavity tuning before increasing power� Amplitude and Phase Loops closed at low power� Smooth power increase� Message: Ready for beam operation

Automatic Startup without beam

RF Plant GUI

RF Plant GUI

Transmitters Information Cavity Interlocks from EPS

Fast Interlocks

(FIM)

EPS Interlocks of RF Auxiliaries

LLRF Settings

Operation Modes

Thank you

Francis Perez

ilsf - 3-llrf - francis...

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