lhc rf feedback

Dec. 15, 2005 SM18 tests Aug-Sept 05 1

LHC RF FeedbackSM18 tests, Aug-Sept 2005

Reported by P. Baudrenghien

Donat StellfeldJohn MolendijkPhilippe BaudrenghienPierre MaesenUrs Wehrle


Cavity Servo Controller.Simplified Block Diagram

Technology:DSP

CPLD or FPGA (40 or 80 MHz)Analog RF

Signals: Digital:

Analog:

Digital I/Q pair:

Analog I/Q pair:

X

DigitalIQ

Demod

SUM

TunerProcessor

X

DigitalIQ

Demod

Dir.Coupler

Single-CellSuperconducting

Cavity

Fwd

Rev

X

DigitalIQ

Demod

DAC

Digital RF feedback (FPGA or DSP)

1 kHz

60 dB

From long.Damper

Voltagefct

I0

Q0

dpdV

Set PointGeneration

DA

C

PhaseEqualizer

ADCDAC

DIFF

Vcav

DIFF

SUM

Analog RF feedback

1 kHz

20 dB40 dB

1-Turn FeedforwardWideband

PUDAC ADC

An

alo

g IQ

Dem

od

ula

tor

Ic fwd

X

DigitalIQ

Demod

Ic rev

Master F RF

TUNER LOOP

SETPOINT

RF FEEDBACK

Dual Var GainIF Ampifier

An

alo

g IQ

Mo

du

lato

r

RFPhase Shifter

RF MODULATOR300 kW Klystron

Circ

Ig fwd

An

alo

g IQ

De

mo

du

lato

rQ

I

ANALOGDEMOD

KlystronPolar Loop(1 kHz BW)

AD

C

PhaseShift

Gain CntrlSUM

BasebandNetworkAnalyzer

DA

C

noise

X

DigitalIQ

Demod

X

DigitalIQ

Demod

1-Turn Feedback

Tuner Control

Ic fwd

CONDITIONING DDS SWITCH/PROTECTION

SWITCH


Open LoopSynthesizer

SPre-

driverDriver Klystron

Cavity

Attn

Coupler

NLP5504

way

Demod

Fdbk

Fdbk

Mod

NLP550

Network Analyzer

Hol

CW @ 400.8 MHz

Freq sweep

I

Q

S

Beam loading test

circ

7/8" cable200 ns

7/8" cable200 ns

56 kV, 7.8 A

Q 20000 -> 180000

DC coupled Analog Fdbk (Digital Fdbk OFF)


RF feedback Theory

• RF Feedback theory [1],[2]– Minimal cavity impedance (with

feedback) scales linearly with T (600 ns)

– Achieved for a gain value proportional to Q

– Achievable fdbk BW inversely proportional to T

TQ

RR 0min

2

T

3.1=ωΔ

assumed single-cell

Tω

Q≈G

oopt

690 kz 2-sided BW

43.2 k

Q Gopt20000 13.560000 39.8

180000 119.4


Effect of coupling

cavitycirc

Pin Pout

1

• Transmission Loss Pout/Pin [3]:

• At resonance we get:

• So Vout/Vin is proportional to sqrt[QL]

21

0L

0

220

221

21

in

out

β+β+1

Q=Q

ω

ω-ω=δ

δQ4+)β+β+1(

ββ4=

P

P=)ω(T

20

L

1

20 β

Q

Q4=

β

β4=)ω(T


RF Feedback Module (eda-586.v2)Notch to damp the resonance of the second klystron cavity (404.8-405.45 MHz) + phase advance to increase closed loop BW.

400.8 MHz

10 MHz

klystronloop amplifier


O.L. vs. Notch position

• Left: notch well adjusted• Right: Notch at min and max positions (~800 kHz range)


Stability. Open Loop:What we expected…

• Nyquist Plot using measurement of real klystron• Q=20000, real klystron, loop delay 450 ns (excluding klystron)• Low Level: Notch plus Phase Advance

2 4 6 8 10 12

-6

-4

-2

2

4

6

Nyquist : RF fbk

-1 -0.75 -0.5 -0.25 0.25 0.5 0.75 1

-1

-0.75

-0.5

-0.25

0.25

0.5

0.75

1Nyquist : RF fbk

10 dB gain marginOpen Loop gain = 13


O.L. Stability vs. CW power

• Positive Fdbk -> unstable point = (+1,0)• 50 kW vs 200 kW• Gain drops by 2 dB• Phase does not change

50 kW CW 200 kW CW

Q=20000O.L. gain = 20(26 dB)


O.L. stability vs. Q

• All meas, 50 kW CW• To keep same gain margin, LL

gain varies as SQRT(Q)• No significant phase change

Q=20000Q=60000, LL gain x 1.7

Q=180000, LL gain x 3

O.L. gain =20

O.L. gain =60

O.L. gain =180


O.L. stability vs. detuning

• Q=60000, 50 kW CW• No change in gain neither phase• Detuning has no effect on stability

Cavity detuned by 10 kHzCavity on tune


O.L. stability vs. Klystron HV

MHz400@reesdeg840=

UUΔ

φΔ RF

• Q=60000,• Change HV 56 kV to 54 kV• Thomson TH2089 measurements:

• But Cathode current does not change RF phase. But changes gain.


Closed Loop

• Closes OK on first trial

• Tracks Iref,Qref OK

• But when large step in Iref, remains stuck with strong pure CW

Synthesizer

SPre-


Cavity

Attn

Coupler

NLP5504

way

DemodMod

NLP550

Hol/(1-Hol)

CW @ 400.8 MHz

I

Q

S

Beam loading test

NetworkAnalyzer

-20 dB

-10 dB

Iref

Qref


Overdriving the Modulator

• AD8345 wants IF levels max +-0.3 V offset by 0.7 V DC

• Driven by AD8138 single-ended to differential• Adding clamping diodes HSMS-2820 (RF

Schottky diodes) on input of AD8138 cured problem


Closed Loop. LL output noise

• Pre-driver -20 dB out TP• Noise varies linearly with Low Level gain -> it is a

Measurement noise: Analog Demodulator noise• High HF gain due to Phase Advance• Probably no effect on beam… but…

All spectra with phase advance, Q20000, 130 kW, 1MVaccO.L. gain 20 O.L. gain 10

O.L. gain 5


Phase Advance and LL output noise

• Pre-driver -20 dB out TP• Phase Advance network boosts noise by 5 linear at +-1 MHz offset• Probably no effect on beam… but…

With phase advance No phase advance

Both spectra loop closed, O.L. gain 20 linear, Q20000, 130 kW, 1MVacc


Closed Loop:What we expected…

• Mathematica using measurement of real klystron• Q=20000, real klystron, loop delay 450 ns (excluding klystron)• Low Level: Notch, no Phase Advance. Gain set for 10 dB margin

2-sided -3 dB BW = 600 kHz

-2106

-1106

1106

2106

Freq .Hz

-25

-20

-15

-10

-5

Gain

-210 6 -110 6 110 6 210 6Freq .Hz

-150

-100

-50

50

100

150

Phase

linear phase response in <1 MHz band


C.L. vs. LL gain

• No Phase Advance Network• Q=20000, 1MVacc, 135 kW• O.L. gain = 20 linear for 10 dB gain margin

O.L. gain 20 O.L. gain 28

Group delay compensated


C.L. vs. CW power

• No Phase Advance Network• Q=20000• O.L. gain = 20 linear for 10 dB gain margin

50 kW CW (0.62 MVacc)

135 kW CW (1 MVacc)


C.L. vs. Q

• No Phase Advance Network

• O.L. gain = proportional to Q (keep 10 dB gain margin)

• Low Level gain = proportional to Sqrt[Q]

• Significant change in phase distortion

Q200001 MVacc (135 kW)

Q1800002 MVacc (60 kW)


C.L. with/without Phase Advance

• Q=20000, O.L. gain set to keep 10 dB gain margin, 135 kW CW -> 1 MVacc• Phase Advance

– increases gain outside 3 dB BW

– reduces non-linear phase distortion

No Phase Advance With Phase Advance


Impedance ReductionSynthesizer

SPre-


Cavity

Attn

Coupler

NLP5504

way

DemodMod

NLP550

1/(1-Hol)

CW @ 400.8 MHz

I

Q

S

Beam loading test

NetworkAnalyzer

-20 dB

-10 dB

Iref

Qref

• First calibrate with Feedback Off• Then measure response


Impedance Reduction:What we expected…

• Mathematica using measurement of real klystron• Q=20000, real klystron, loop delay 450 ns (excluding klystron)• Low Level: Notch, no Phase Advance. Gain set to 13 (linear) for 10

dB margin

-110 6 -500000 500000 110 6Freq .Hz

-40

-30

-20

-10

ZeffRdB Reduction by 13 linear at the tune

Reduction in a 300 kHz band (2-sided)

Increase


Impedance Reduction vs. LL gain

• Q=60000, zero CW. No Phase Advance • 10 dB margin is the “best”• Modulus of Z is reduced in a +- 150 kHz band

Gain set to 10 dB margin

Gain set to 7 dB margin


Impedance Reduction vs. CW power

• Q=60000. No Phase Advance• Marginal effect of CW power.

Zero CW

2 MVacc (190 kW)


Klystron ripples reduction

• Using vector voltmeter 8508A Analog Out on high Z (1 kHz BW)• Q=20000, 1 MVacc, 130 kW, with phase advance, O.L. gain = 20 linear• Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\PhaseNoise.xls

Phase noise Vacc: 10 mV/dg

-0.003

-0.0025

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0.001

-2.50E-02

-2.00E-02

-1.50E-02

-1.00E-02

-5.00E-03

0.00E+00

5.00E-03

1.00E-02

1.50E-02

2.00E-02

2.50E-02

time (s)vo

ltag

e (V

)

Series1

Cavity field phase ripples at 50 Hz and 600 HzFdbk OFF. ~3 degrees pkpkFdbk ON. ~ 0.2 degree pkpk Enlargment Fdbk ON. ~0.2 degrees pkpk

Phase noise Vacc: 10 mV/dg

-0.02

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

-2.50E-02

-2.00E-02

-1.50E-02

-1.00E-02

-5.00E-03

0.00E+00

5.00E-03

1.00E-02

1.50E-02

2.00E-02

2.50E-02

time (s)

volt

age

(V)

FBoff

FBon



• Using HP423A X-tal detector plus 100 kHx LPF on high Z Analog Out on high Z • Q=20000, 1 MVacc, 130 kW, with phase advance, O.L. gain = 20 linear• Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\AmplNoise.xls

Cavity field amplitude ripples at 50 Hz and 600 HzFdbk OFF. ~25 kVacc pkpk Fdbk ON. ~15 kVacc pkpk

Amplitude noise Vacc: 10 kVacc / mV

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

-6.00E-02 -4.00E-02 -2.00E-02 0.00E+00 2.00E-02 4.00E-02 6.00E-02

time (s)

volt

age

(V)

Series1

Amplitude noise Vacc: 10 kVacc/mV

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

-6.00E-02 -4.00E-02 -2.00E-02 0.00E+00 2.00E-02 4.00E-02 6.00E-02

Time (s)

Vo

ltag

e (V

)

Series1

Left with 50 Hz measurement noise ?



-3

-2

-1

0

1

2

3

-3.00E-02

-2.00E-02

-1.00E-02

0.00E+00

1.00E-02

2.00E-02

3.00E-02

Time (s)

Ph

as

e (

de

gre

e)

FDBK OPEN

gain 3

gain 10

gain 40

• Using ZLW-1W mixer plus SLP-100 onto 50 ohm• Q=60000, 2 MVacc, 170 kW, no phase advance• Data in..\..\Modules\RFfeedback\Tests\SM18\Week37\PhaseNoise.xls

Cavity field phase ripples at 50 Hz and 600 Hz

Gain Phase NoiseO.L. 4 degrees pkpk

3 1.4 dg pkpk10 0.4 dg pkpk20 0.2 dg pkpk40 0.2 dg pkpk


Cavity field noise

• Using Independent Analog I/Q demodulator plus 20 MHz LPF (1 mV-> 6.6 kVacc)• Q=60000, 2 MVacc, no phase advance, O.L. gain = 40• Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\IndepNoiseMeas.xls

AC coupled Vcav I and Q vs time

Vcav Q DC 20 MHz BW

0

0.05

0.1

0.15

0.2

0.25

-0.25 -0.2 -0.15 -0.1 -0.05 0

Vcav Q DC 20 MHz BW

Vcav on I/Q plot

Noise ~ 13 kVacc pkpk around 2 MVacc (0.4 degrees pkpk, 0.65 % pkpk)

-0.003

-0.0025

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

-1.50E-02

-1.00E-02

-5.00E-03

0.00E+00

5.00E-03

1.00E-02

1.50E-02

Vcav I AC

Vcav Q AC


Beam Loading Test50 kV step in quadrature with 1 MVacc

• 400 MHz rectangular burst on Beam Loading Test input. (see page 22)• Measure Voltage error signal, that is RF feedback I/Q inputs• After transient, beam loading perturbation reduced by O.L. gain• Q=20000, 1 MVacc in I, 130 kW, with phase advance, O.L. gain = 20• Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\BeamLoading.xls

50 kV step in 1 s (+ 600 ns delay)

1 s

Beam loading transient

-0.18

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

-5.00E-06 0.00E+00 5.00E-06 1.00E-05 1.50E-05

time (s)

Vca

v

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

Det

Pw

r Vcav I

Vcav Q

Ic,fwd Det

Beam loading transient

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

-2.00E-06

-1.00E-06

0.00E+00

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

time (s)

Vca

v

Vcav Q

1 s falltime

600 ns delay


Beam Loading Test50 kV step in phase with 1 MVacc

• Very asymmetric• After transient, beam loading perturbation reduced by O.L. gain• Q=20000, 1 MVacc in I, 130 kW, with phase advance, O.L. gain = 20• Data in..\..\Modules\RFfeedback\Tests\SM18\Week35\BeamLoading.xls

50 kV step in 1 s (+ 600 ns delay)

Transient beam loading

-0.18

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

-1.50E-05 -1.00E-05 -5.00E-06 0.00E+00 5.00E-06

Time (s)

V e

rro

r

-0.25

-0.2

-0.15

-0.1

-0.05

0

Det

Pw

r Vcav I

Vcav Q

Ic,fwd Det

Vcav I

-0.165

-0.16

-0.155

-0.15

-0.145

-0.14

-0.135

-1.50E-05 -1.00E-05 -5.00E-06 0.00E+00 5.00E-06

V e

rro

r

Vcav I

5 s falltime

1 s risetime


-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

-2.00E-04

-1.50E-04

-1.00E-04

-5.00E-05

0.00E+00

5.00E-05 1.00E-04

Vcav I

VcavQ

Step Response70 kV step in quadrature with 1 MVacc

• Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20• Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls

VcavQ

-0.006

-0.004

-0.002

0

0.002

0.004

0.006

0.008

-8.00E-06 -6.00E-06 -4.00E-06 -2.00E-06 0.00E+00 2.00E-06

VcavQ

70 kV step in 1 s -> limit saturation (with 1 MVacc CW)

1 s risetime


-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

-2.00E-04

-1.50E-04

-1.00E-04

-5.00E-05

0.00E+00

5.00E-05 1.00E-04

VcavI

VcavQ

Step ResponseVcavI

-0.148

-0.146

-0.144

-0.142

-0.14

-0.138

-0.136

-0.134

-8.00E-06 -6.00E-06 -4.00E-06 -2.00E-06 0.00E+00 2.00E-06

VcavI

70 kV step in phase with 1 MVacc


saturation when step adds to 1MVacc CW

1 s risetime

VcavI

-0.15

-0.148

-0.146

-0.144

-0.142

-0.14

-0.138

-0.136

-0.134

-0.132

-1.05E-04 -1.03E-04 -1.01E-04 -9.90E-05 -9.70E-05 -9.50E-05

VcavI

5 s falltime


-0.2

-0.19

-0.18

-0.17

-0.16

-0.15

-0.14

-0.13

-0.12

-0.11

-0.1

-1.50E-04 -1.00E-04 -5.00E-05 0.00E+00 5.00E-05

0

0.05

0.1

0.15

0.2

0.25

0.3

VcavI

"-Ic,fwd det"

Step Response70 kV step in phase with 1 MVacc


saturation at 280 kW when step adds to 1MVacc CW

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

-8.00E-06 -6.00E-06 -4.00E-06 -2.00E-06 0.00E+00 2.00E-06

-0.148

-0.146

-0.144

-0.142

-0.14

-0.138

-0.136

-0.134

-0.132

"-Ic,fwd det"

VcavI

0

0.05

0.1

0.15

0.2

0.25

0.3

-1.05E-04 -1.03E-04 -1.01E-04 -9.90E-05 -9.70E-05 -9.50E-05

-0.15

-0.148

-0.146

-0.144

-0.142

-0.14

-0.138

-0.136

-0.134

-0.132

"-Ic,fwd det"

VcavI


Huge Step Response+- 0.5 MV MV step in quadrature with 1 MVacc

• Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20• During heavily saturated transients, measurements from input coupler do not make sense!!!• Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls

1 MV step in 10 s -> severe saturation (with 1 MVacc CW)

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

-2.00E-04 -1.50E-04 -1.00E-04 -5.00E-05 0.00E+00 5.00E-05

Vca

v

0

0.05

0.1

0.15

0.2

0.25

0.3

Det

Pw

r Vcav I

Vcav Q

-Ig,det

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

-1.50E-04 -1.00E-04 -5.00E-05 0.00E+00 5.00E-05

Ic, fwd det

Ic,rev det


Huge Step Response1 MV step in phase with 1 MVacc (from 0.5 MV to 1.5 MV)

• Q=20000, I from 0.5 MV to 1.5 MV, no phase advance, O.L. gain = 20• During heavily saturated transients, measurements from input coupler do not make sense• Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls

From 0.5 MV to 1.5 MV in 25 sFrom 1.5 MV to 0.5 MV in <10 s

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

-1.50E-04 -1.00E-04 -5.00E-05 0.00E+00 5.00E-05

Vca

v

-0.5

-0.45

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0D

et P

wr VcavI

Vcav Q

Ic,fwd det


Why we have been lucky…K020 - saturation curve

0

50

100

150

200

250

300

350

0 20 40 60 80 100 120 140 160

drive power (W)

forw

ard

po

we

r (k

W)

• During SM18 tests, the klystron could never go above saturation peak…

Typical klystron curveKlystron used in SM18 tests

Courtesy of Olivier Brunner


Overdriving the klystron (ramp)

Courtesy of Janne Holma

Klystron input clamped Klystron over-driven


Overdriving the klystron (steps)

Courtesy of Janne Holma

Klystron input clamped Klystron over-driven


Conclusions (good)• If the LL gain scales with sqrt[Q] we have

– Closed loop BW 550 kHz, independent of Q and CW power -> Apparent Q < 1000

– Apparent cavity impedance ~45 kohm, independent of Q– Reduction of apparent cavity impedance modulus in a 300

kHz band, independent of Q• Phase advance is not worth it…rid of it in eda586.v3• 1% ripple in HV causes 8.4 degrees phase shift @400.8 MHz.

That is acceptable for stability. (At least in the absence of 1-T fdbk).

• Close Loop phase characteristic OK for 1-T feedback in a 1 MHz band (2-sided)

• Reduction of klystron phase ripples by O.L. gain (as expected)


Conclusions (good)• With 1 MVacc, Q=20000, step 70 kV in quadrature in 1 s (OK for

long damper at injection?)• No problem when klystron saturates on transients• Rare trips: Main Coupler Vacuum (always cured by additional

conditioning) and Klystron Body Overheat when running in saturation for long time.

• RF feedback and Tuner Loop fully compatible


Conclusions (bad)

• Non-linear phase distortion changes with Q• Without phase advance the phase distortion is bigger• Close Loop phase characteristic NOT OK for 1-T feedback in a >1

MHz band (2-sided). Foresee Phase Equalizer in 1-T feedback?• Reduction of klystron amplitude ripple by less than loop gain

(measurement noise at 50 Hz?)• Measurements from the cavity input coupler do not make sense

during heavily saturated transients• What if the klystron is allowed to go over saturation peak?


References

[1] Control of cavities with high beam loading, D. Boussard, IEEE Tr. On N.S. Vol NS 32 No.5, p. 1852, 1985

[2] Low Level RF Systems for Synchrotrons, Part 2, P. Baudrenghien, CERN SL-Note-2001-028 HRF

[3] Microwave Measurements, Edward L. Ginzton, McGraw-Hill Book Company Inc., New-York, 1957, pp 404-405

lhc rf feedback

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