Transcript WE01_NSLS2RF_measurements

RF Measurements:
Techniques and Data
CWRF 2010
Jim Rose
With help from Hengjie Ma and in collaboration with
Mark deJong, Song Hu and Jonathon Stampe of CLS
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Original talk was to be
“Experience with the RF Systems at NSLS-II
But as you can see we are not quite ready………
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Outline
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Motivations
Traditional RF phase measurements on a seeded FEL
RF system and on the Canadian Light Source
• Measurements with Spectrum/dynamic spectrum analyzer
• Mixer measurements with digital scope and post processing
on the PC
Digital cavity controller based measurements
Summary
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Motivations
The NSLS-2 program has requirements of <0.15 degree phase
jitter; need to characterize potential RF sources.
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• Solid state and IOT have better phase jitter performance than the
klystron with its large cathode voltage-to-RF phase transfer
function. Since the NSLS-2 is a virtual machine, we went to CLS to
measure the klystron amplifier to quantify the phase jitter
Commitment: The other mother of invention.
• ~9 years ago, while interviewing to change departments at BNL I
was asked if I could measure jitter between a 100fs cathode
driver/seed laser and the RF system to characterize and improve
system- said yes even though I was not exactly sure how. This
became the finger-pointing circuit to resolve arguments between
RF and laser stability (we always won after this)
I would like to begin by describing this latter circuit as an example of
the accuracy of the mixer based phase measurement
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Seeded-FEL Laser and S-Band LINAC RF system
Photo-cathode
laser
Seed
laser
traveling wave tank
photo-cathode
gun
Laser/RF Phase
Measurement
Amplifier
+ tripler
10
Hz
Bandpass
filter
Klystron
I/Q modulator
81.6 MHz CW
I/Q demodulator
LP filter
preamp
Ti:sapphire
laser
(not implemented)
2856 MHz
synthesizer
81.6MHz xtal
DSA
Laser and RF synthesizer independently locked to Oscillator
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Laser Room
Tsunami Laser
Oscillator
GaAs photo-diode
EOT Inc.
ET-4000
TTE 315P-2856M-A
Cage code 07766
F1237
+12 V
Hi-Q Bessel BP
81.6 MHz Comb
MiniCircuits
ZJL-3G
ARRA 4194-10
HD21678
HD Communications Inc.
From LLRF
Front
Panel
Anaren 74125
To
Pre-amp,
DSA
Rear panel
To LLRF
Scope to monitor zero-crossing
SRS785 Dynamic
Signal Analyzer
SR560 Pre_amp
Gain ~5-20
LP 30k-300k
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Phase Jitter Measured between Laser
Oscillator and RF Drive
Each plot consists of histograms of 8 second duration of a mixer-based phase
error measurement. Left plot is for laser oscillator and RF synthesizer driven
from same crystal, on right laser output pulses are filtered to drive RF directly.In
each plot the outside histograms are data taken by programming+/- 3 degree
and +/- 2 degree steps, respectively, in the RF path for calibration.
Side Note: shot to shot jitter from 40 MW klystron is ~150fs FWHM with 0.05% regulated charging PS
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Don’t fix the jitter, join it:
Laser-Driven and LINAC RF system
Photo-cathode
laser
Seed
laser
traveling wave tank
photo-cathode
gun
Laser/RF Phase
Measurement
Bandpass
filter
Klystron
I/Q modulator
I/Q demodulator
Ti:sapphire/YaG
laser
Worked well for this small class of experiments,
81.6MHz xtal
with no downstream synchronization
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DSA
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Characterization of the CLS klystron transmitter
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Part of a broader collaboration between CLS and NSLS-II on
RF systems
NSLS-II must meet the <0.15 deg phase jitter specification:
factor of ~3 beyond existing LS machines (I think)- past
experience is no guarantee of future success. Decision to use
the CLS system to determine whether klystron system can
meet specification
Thanks to Mark deJong, Jonathon Stampe, Hengjie Ma,
Nathan Towne and Song Hu for contributing to this work
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Spectrum/Dynamic Signal Analyzer
Data taken with Agilent 89441A DSA
Transmitter forward power-Phase
Cavity Field pickup- Phase
Due to machine returning to normal operations measurements had to be taken
closed loop, Cavity field response with 20 dB of gain in feedback masks effect
of transmitter noise-transmitter forward power shows switching noise
•Pro/Con of DSA:
•Only one RF channel
•High resolution, easy to configure •Few points (3201 max in89441A)
•Need to configure and calibrate TP
•High dynamic range
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Traditional detector/mixer based
RF measurements- Using Scope as digitizer
Simple system with components costing a few hundred $ (even less € ) plus
digital scope allows high resolution four channel correlated measurements
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PSM modulated HV power supply switching lines and
klystron RF phase
High Voltage, klystron data set 3
120
Klystron HV
100
X: 343.8
Y: 85.21
80
FFT
Switching frequency of
100kHz over 86 switch
modules creates subharmonics of ~ 1.2kHz
This is a concern for
NSLS2 since the
synchrotron frequency is
between 3-4 kHz
X: 2297
Y: 49.45
X: 3484
Y: 40.18
60
X: 9297
Y: 34.5
40
Phase, klystron
data set 3
X: 1203
X: 1.045e+004
Y: 23.63
X: 1.164e+004
Y: 15.49
X: 8141
Y: 20.82
Y: 24.18
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30
0
25
0
0.2
0.4
Klystron RF out phase
0.8
1
Frequency
1.2
1.4
1.6
1.8
2
4
x 10
FFT
20
0.6
15
X: 3484
Y: 10.05
X: 4703
Y: 9.387
X: 8141
Y: 5.154
X: 2297
Y: 2.692
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Although the harmonics
are <-75dBc, they can be
suppressed further by
filtering with modest effort
X: 9297
Y: 9.176
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X: 1.045e+004
Y: 5.933
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0
2000
4000
6000
8000
10000
Frequency
12000
14000
12
16000
18000
20000
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CLS PSM HV supply + klystron Transmitter.
Phase, Cavity, set 5
-85
-80
-90
-85
Phase (rad)
-75
-95
-100
-105
-90
-95
-100
-110
-105
-115
-110
-120
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4
6
8
10
12
14
16
18
20
0
2
4
6
Frequency (kHz)
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10
12
14
16
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Frequency (kHz)
Amplitude, Cavity, set 5
-55
-60
Magnitude(dB)
Intensity (dB)
High Voltage, Cavity, set 5
-80
-65
-70
-75
-80
-85
-90
0
2
4
6
8
10
12
13 (kHz)
Frequency
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Digital RF Controller for data measurement
and analysis
Example of NSLS-II controller GUI in MATLAB- 8 fast ADC
channels with display, +history, + signal processing.
Built in AM, FM modulation
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Digital RF controllers as DSA
Built in sweep features allows
AM, PM and FM modulation of
Reference input via feed-forward
Transfer functions can
be measured easily
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Klystron Power sweep
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Matlab processing allows power spectrum
measurements online - 300kW-open loop into load
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Digital controller as SA measuring Analog
controller closed loop response
Cavity field open and closed
loop response: closed loop
noise reduced-
At cost of increased
closed loop amplifier
power
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Open loop response
Digital Controller Cavity Voltage 2.4MV klystron power 72kW, no beam
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Closed loop response
Digital Controller Cavity Voltage 2.4MV klystron power 72kW, no beam
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Summary-Measurements
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Traditional spectrum analyzer measurements can be
complemented by mixer based measurements
extended to four channels for correlation between PS
HV and RF
These measurements can be built into digital RF
controllers and displayed and put into history buffers
for post-mortem
Digital controller based measurements are flexible,
always online. No need to hook up equipment,
calibration easier to keep correct
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Summary- Switching PS/Klystron phase noise
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Open loop measurements of klystron at full power
shows switching harmonics at {switching
frequency/#modules}- for CLS case these are within
the synchrotron frequency band of the NSLS-II
Closed loop measurements show this noise to be
reduced greater than 20db: within requirements. The
20 dB loop gain is not a fundamental limit, the
firmware gain needs to be increased- we reached the
end of the ‘knob’
The PSM PS klystron transmitter can easily meet the
NSLS-II requirements
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