Lhc rf feedback
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LHC RF Feedback. Donat Stellfeld John Molendijk Philippe Baudrenghien Pierre Maesen Urs Wehrle. SM18 tests, Aug-Sept 2005. Reported by P. Baudrenghien. Open Loop. 56 kV, 7.8 A. Q 20000 -> 180000. DC coupled Analog Fdbk (Digital Fdbk OFF). RF feedback Theory.

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LHC RF Feedback

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Lhc rf feedback

LHC RF Feedback

Donat Stellfeld

John Molendijk

Philippe Baudrenghien

Pierre Maesen

Urs Wehrle

SM18 tests, Aug-Sept 2005

Reported by P. Baudrenghien

SM18 tests Aug-Sept 05


Lhc rf feedback

SM18 tests Aug-Sept 05


Open loop

Open Loop

56 kV, 7.8 A

Q 20000 -> 180000

DC coupled Analog Fdbk (Digital Fdbk OFF)

SM18 tests Aug-Sept 05


Rf feedback theory

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

43.2 kW

assumed single-cell

690 kHz

2-sided BW

SM18 tests Aug-Sept 05


Effect of coupling

Effect of coupling

  • Transmission Loss Pout/Pin [3]:

  • At resonance we get:

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

SM18 tests Aug-Sept 05


Rf feedback module eda 586 v2

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.

loop amplifier

klystron

10 MHz

400.8 MHz

SM18 tests Aug-Sept 05


O l vs notch position

O.L. vs. Notch position

  • Left: notch well adjusted

  • Right: Notch at min and max positions (~800 kHz range)

SM18 tests Aug-Sept 05


Stability open loop what we expected

Stability. Open Loop:What we expected…

10 dB gain margin

Open Loop gain = 13

  • Nyquist Plot using measurement of real klystron

  • Q=20000, real klystron, loop delay 450 ns (excluding klystron)

  • Low Level: Notch plus Phase Advance

SM18 tests Aug-Sept 05


O l stability vs cw power

O.L. Stability vs. CW power

200 kW CW

50 kW CW

Q=20000

O.L. gain = 20

(26 dB)

  • Positive Fdbk -> unstable point = (+1,0)

  • 50 kW vs 200 kW

  • Gain drops by 2 dB

  • Phase does not change

SM18 tests Aug-Sept 05


O l stability vs q

O.L. stability vs. Q

Q=60000, LL gain x 1.7

Q=20000

O.L. gain =60

O.L. gain =20

  • All meas, 50 kW CW

  • To keep same gain margin, LL gain varies as SQRT(Q)

  • No significant phase change

Q=180000, LL gain x 3

O.L. gain =180

SM18 tests Aug-Sept 05


O l stability vs detuning

O.L. stability vs. detuning

Cavity on tune

Cavity detuned by 10 kHz

  • Q=60000, 50 kW CW

  • No change in gain neither phase

  • Detuning has no effect on stability

SM18 tests Aug-Sept 05


O l stability vs klystron hv

O.L. stability vs. Klystron HV

  • Q=60000,

  • Change HV 56 kV to 54 kV

  • Thomson TH2089 measurements:

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

SM18 tests Aug-Sept 05


Closed loop

Closed Loop

  • Closes OK on first trial

  • Tracks Iref,Qref OK

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

SM18 tests Aug-Sept 05


Overdriving the modulator

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

SM18 tests Aug-Sept 05


Closed loop ll output noise

Closed Loop. LL output noise

All spectra with phase advance, Q20000, 130 kW, 1MVacc

O.L. gain 20

O.L. gain 10

O.L. gain 5

  • 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…

SM18 tests Aug-Sept 05


Phase advance and ll output noise

Phase Advance and LL output noise

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

With phase advance

No phase advance

  • Pre-driver -20 dB out TP

  • Phase Advance network boosts noise by 5 linear at +-1 MHz offset

  • Probably no effect on beam… but…

SM18 tests Aug-Sept 05


Closed loop what we expected

Closed Loop:What we expected…

2-sided -3 dB BW = 600 kHz

linear phase response in <1 MHz band

  • 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

SM18 tests Aug-Sept 05


C l vs ll gain

C.L. vs. LL gain

O.L. gain 20

O.L. gain 28

Group delay compensated

  • No Phase Advance Network

  • Q=20000, 1MVacc, 135 kW

  • O.L. gain = 20 linear for 10 dB gain margin

SM18 tests Aug-Sept 05


C l vs cw power

C.L. vs. CW power

135 kW CW (1 MVacc)

50 kW CW (0.62 MVacc)

  • No Phase Advance Network

  • Q=20000

  • O.L. gain = 20 linear for 10 dB gain margin

SM18 tests Aug-Sept 05


C l vs q

C.L. vs. Q

Q20000

1 MVacc (135 kW)

Q180000

2 MVacc (60 kW)

  • 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

SM18 tests Aug-Sept 05


C l with without phase advance

C.L. with/without Phase Advance

No Phase Advance

With 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

SM18 tests Aug-Sept 05


Impedance reduction

Impedance Reduction

  • First calibrate with Feedback Off

  • Then measure response

SM18 tests Aug-Sept 05


Impedance reduction what we expected

Impedance Reduction:What we expected…

Reduction by 13 linear at the tune

Reduction in a 300 kHz band (2-sided)

Increase

  • 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

SM18 tests Aug-Sept 05


Impedance reduction vs ll gain

Impedance Reduction vs. LL gain

Gain set to 7 dB margin

Gain set to 10 dB margin

  • Q=60000, zero CW. No Phase Advance

  • 10 dB margin is the “best”

  • Modulus of Z is reduced in a +- 150 kHz band

SM18 tests Aug-Sept 05


Impedance reduction vs cw power

Impedance Reduction vs. CW power

Zero CW

2 MVacc (190 kW)

  • Q=60000. No Phase Advance

  • Marginal effect of CW power.

SM18 tests Aug-Sept 05


Klystron ripples reduction

Klystron ripples reduction

Cavity field phase ripples at 50 Hz and 600 Hz

Fdbk OFF. ~3 degrees pkpk

Fdbk ON. ~ 0.2 degree pkpk

Enlargment Fdbk ON. ~0.2 degrees pkpk

  • 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

SM18 tests Aug-Sept 05


Klystron ripples reduction1

Klystron ripples reduction

Cavity field amplitude ripples at 50 Hz and 600 Hz

Fdbk OFF. ~25 kVacc pkpk

Fdbk ON. ~15 kVacc pkpk

Left with 50 Hz measurement noise ?

  • 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

SM18 tests Aug-Sept 05


Klystron ripples reduction2

Klystron ripples reduction

Cavity field phase ripples at 50 Hz and 600 Hz

  • 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

SM18 tests Aug-Sept 05


Cavity field noise

Cavity field noise

AC coupled Vcav I and Q vs time

Vcav on I/Q plot

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

  • 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

SM18 tests Aug-Sept 05


Beam loading test

Beam Loading Test

50 kV step in quadrature with 1 MVacc

1 ms falltime

600 ns delay

1 ms

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

  • 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

SM18 tests Aug-Sept 05


Beam loading test1

Beam Loading Test

5 ms falltime

50 kV step in phase with 1 MVacc

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

1 ms risetime

  • 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

SM18 tests Aug-Sept 05


Step response

Step Response

70 kV step in quadrature with 1 MVacc

1 ms risetime

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

  • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20

  • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls

SM18 tests Aug-Sept 05


Step response1

Step Response

70 kV step in phase with 1 MVacc

1 ms risetime

5 ms falltime

saturation when step adds to 1MVacc CW

  • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20

  • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls

SM18 tests Aug-Sept 05


Step response2

Step Response

70 kV step in phase with 1 MVacc

saturation at 280 kW when step adds to 1MVacc CW

  • Q=20000, 1 MVacc in I, 130 kW, no phase advance, O.L. gain = 20

  • Data in..\..\Modules\RFfeedback\Tests\SM18\Week39\RefStepVcav.xls

SM18 tests Aug-Sept 05


Huge step response

Huge Step Response

+- 0.5 MV MV step in quadrature with 1 MVacc

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

  • 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

SM18 tests Aug-Sept 05


Huge step response1

Huge Step Response

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

From 0.5 MV to 1.5 MV in 25 ms

From 1.5 MV to 0.5 MV in <10 ms

  • 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

SM18 tests Aug-Sept 05


Why we have been lucky

Why we have been lucky…

Klystron used in SM18 tests

Typical klystron curve

Courtesy of Olivier Brunner

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

SM18 tests Aug-Sept 05


Overdriving the klystron ramp

Overdriving the klystron (ramp)

Klystron input clamped

Klystron over-driven

Courtesy of Janne Holma

SM18 tests Aug-Sept 05


Overdriving the klystron steps

Overdriving the klystron (steps)

Klystron input clamped

Klystron over-driven

Courtesy of Janne Holma

SM18 tests Aug-Sept 05


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)

SM18 tests Aug-Sept 05


Conclusions good1

With 1 MVacc, Q=20000, step 70 kV in quadrature in 1 ms (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 (good)

SM18 tests Aug-Sept 05


Conclusions bad

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?

SM18 tests Aug-Sept 05


References

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

SM18 tests Aug-Sept 05


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