1LLRF Pre-readiness review (26th May, 2009)20/04/23

LLRF performance and its limitation based on KEK's

experiments

Shin Michizono (KEK)

KEK’s LLRF performance Noise analysis Improvements by averaging (or filtering) Algorithm 8/9pi mode Summary

LLRF Pre-readiness review (26th May, 2009) 2

RF Stability @KEK-STF

4 vector sum control 0.007%rms 0.018deg.rms

0.01%rms 0.02deg.rms in case of single cavity feedback

The performance is similar between 4-vectorsum and single cavity. We analyzed the limitation of the performance

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LLRF Pre-readiness review (26th May, 2009) 3

Possible noise sources

[LLRF hardware/software dependent]1. Digital noise at ADC/FPGA-> Clean 10 MHz input-> Feedback algorithm 2. Phase noise at LO signal (phase jitter)-> Phase noise measurement3. Clock jitter at ADC-> Phase noise measurement4. Harmonics at downconverter-> Spectrum measurement[Cavity dependent]• 8/9pi, 7/9pi modes elimination

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Clean 10 MHz or downconverted signals were measured at our digital system. ADC data acquisition and FFT analysis

Bench test of cPCI system

　 Amplitude [%]

Phase [mdeg.]

STF 4 vector sum 7.00E-03 20single cav. 1.00E-03 20

clean 10 MHz 7.00E-03 5Mixer 10 MHz 1.50E-02 12

FFT spectrum at cPCI(input: clean 10 MHz)-> No spurious observed

Phase error is better at clean 10 MHz input.(noise source would be LO or Mixer ）

20/04/23 LLRF Pre-readiness review (26th May, 2009) 5

Digital filter (averaging)

Digital low-pass filter is quite effective to improve the signal noise ratio.

(M) (M)

(M) (M)

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Phase noise measurement

　 [mdeg.]10MHz 0.58

Mixer 10MHz 11.85 40MHz CLK 1.81*

MO1300MHz 16.09 LO1310MHz 18.11

*jitter in 10 MHz

Jitter (10 Hz-1 MHz) is calculated. Higher jitter of LO would be the dominant factor of the phase error.

20/04/23 LLRF Pre-readiness review (26th May, 2009) 7

without LPF-56dBc (2nd), -66dBc(3rd) @0dBm

Harmonics of downconverter

RFvsIF(No LPF)

y = 1.9647x - 49.718

y = 1.0014x + 7.2433

y = 2.9266x - 58.91

-160

-140

-120

-100

-80

-60

-40

-20

0

20

-35 -30 -25 -20 -15 -10 -5 0 5 10RF input [dBm]

Mix

er o

utpu

t [d

Bm

]

IF(10MHz)2nd harmonic3rd harmonic

(2nd harmonic)線形 (IF(10MHz))線形 (3rd harmonic)線形

RFvsIF(LPF=15MHz)

y = 1.7327x - 71.54

y = 1.0007x + 3.1165

-140

-120

-100

-80

-60

-40

-20

0

20

-35 -30 -25 -20 -15 -10 -5 0 5 10RF input [dBm]

Mix

er o

utpu

t [d

Bm

]

IF(10MHz)2nd harmonic3rd harmonic

(2nd harmonic)線形 (IF(10MHz))線形

with LPF-73dBc (2nd), <-75dBc(3rd) @0dBm

Due to non-linearity, 2nd and 3rd should be supperessed <-60dBc.

1.3GHz

1.3GHz+250kHz

+1.3GHz (Leakage from LO generator)

0kHz (ADC offset ,

DC comp. by leakage from LO),

250kHz (IF),

500kHz (2nd harmonics of IF),

…

FPGA algorithm (example:250 kHz IF)

20/04/23 8LLRF Pre-readiness review (26th May, 2009)

These errors can be eliminated by the definition of I/Q comp.

I’ = ( I - (-I) )/2

Q’ = ( Q - (-Q) )/2

(J-PARC and STF adopted this procedure.)

Feedback-instability due to 8/9 and 7/9 modes

L.O.

Klystron

400kW AMP.

IQ Modulator M.O. ADC

DA

C

I

QLPF (fc=0.4MHz)

FPGA

9cell cavity

remove LPF

< Digital Delay >< Digital Delay >A digital delay system (0.0246 s/clock) was implemented in the FPGA in order to observe the relation between feedback loop delay and instability.

LPF was removed in order to allow other passband modes pass through.

FB loop delay was changed using digital delay.Additional delay:1 clock ~ 120 clock(3 s)

9LLRF Pre-readiness review (26th May, 2009)

Intensities of 8/9, 7/9 and 6/9 Modes

Vec.sum

cav4

cav3

cav2

cav1In the case of P-Gain~55

Additional Delay Time (s)

Stable region is extremely narrow.

Stable positions are different from each cavity because of different mode frequencies.

by superposition

In the case of vectorsum operation,FB control is unstable for all delay time.

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unstable

stable

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KEK’s cavity performance was verified. Phase error (0.02deg. compared with 0.07% in amplitude) comes from the rf signal

jitter. The largest part of noises comes from LO jitter. Due to the synchronized noises between 4 cavity pickups, phase errors are almost

same even after 4 cavity vector sum. Averaging (digital filter) improves such phase jitter.

Harmonics of the mixer should be considered when high stability operation. 8/9pi, 7/9pi instabilities are enhanced at high gain operation.

Summary

20/04/23 LLRF Pre-readiness review (26th May, 2009) 12

Cavity fields and their frequency spectra

IF ( mode)

7/9 6/9

8/9

Stable Case : Digital Delay =1clock (0.025s)

Unstable Case : Digital Delay =5clock (0.12s)

Unstable Case : Digital Delay=23clock (0.57s)

cavity

Examples of the results for different delay

In unstable case, 8/9, 7/9, and 6/9 modes were observed.

20/04/23 13LLRF Pre-readiness review (26th May, 2009)