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LCLS Band RF System

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  • pg 1
									                          LCLS X-Band RF System
The LCLS beam is run off crest on the RF in order to set up a particle position verses energy
correlation. The correlation is used to compress the bunch when it is run through a chicane. The
RF waveform is a sine shape and the correlation set up by running the bunch off crest has a large
second order component, it’s not very linear. The X-band RF station, with higher frequency, has
a much larger curvature in RF voltage verses time. The beam is run on the decelerating crest in
order to remove most of the second order, nonlinear, part of the correlation. The required 22MV
of peak accelerating gradient will require a klystron power output of 24MW. More information
can be found in the CDR Chapter 7, by Paul Emma. Results of the beam energy vs. position
correlation and how the X-Band station takes out the curvature are shown in figure 1.




Figure 1. X-Band Station to Linearize Energy Position Correlation - P. Emma

                                Accelerator Structure
The accelerating structure is the H60VG3N-C structure from NLC. The structure is a 60cm long
5π/6 which has been tested at NLCTA to 65MV/m. LCLS operation requires 32MV/m with a
peak input power of 21MW. The structure has a dual feed at the input and output. The output of
the structure goes to two independent RF loads one of which has a WR90 directional coupler in
between the structure and load. This is used to measure the output power and RF phase of the
structure. The input feeds come from each arm of a magic-T. There is a WR90 direction coupler
at the input to the magic-T to measure the input power and RF phase to the structure.
Water flow through the accelerator is about 0.3GPM. The average power into the structure is
only 126W at 30pps 200nS. The structure's temperature coefficient is 36 degX/degC and the
structure is tuned for 45degC. The nominal water settings at 30Hz is 111degF for accelerator
water in the gallery. The water flow required to raise 1.1degC with 126W of power input is
126W/(4.2J/(cc-degC)x 1.1degC)= 30cc/sec = 1.8l/min = 0.47GPM
                              High Power Waveguide
The XL4 klystron is powered by modulator 21-2 in the klystron gallery. There is about 6ft of
WR90 at the output of the klystron before it reaches a window assemble. The window assembly
uses 2 mode converters to go from WR90 to circular waveguide where the window is mounted
and then back to WR90. The WR90 is routed above penetration 21-3 where a mode converter
changes to WC293. This over-moded circular waveguide runs 35feet through penetration 21-3
into the tunnel. In the tunnel a mode converter changes from WC293 to WR90. The WR90 is
routed over to the accelerator structure. A diagram of the system is shown in figure X.




Figure X, X-Band waveguide layout.

                          XL4 Klystron and modulator
The XL4 klystron was designed to output over 50MW of X-Band power. The beam power to
achieve this is 410kV at 350A, 144MW. XL4 klystrons have been run reliably at NLCTA for
thousands of hours at over 50MW of output at a 1.6uS pulse width and 60Hz. LCLS requires
24MW at 200nS and 120Hz. The tube is expected to run reliably at these levels.
The standard 5045 15:1 pulse transformer was changed to a 17:1 transformer to achieve higher
voltages at the tube from a standard 5045 modulator. The 5045 modulator PFN was redesigned
for a short pulse. The pulse shape from the modulator is Gaussian with the 200nS RF pulse at
the top. The modulator has an SCR front end control and uses a dequing circuit to regulate the
PFN voltage.
As of August 2008, the klystron has been in operation at 30Hz, 22MW at 200nS for the last 2
commissioning runs. The klystron beam is at 372kV at 250A, 93MW.

XL4 Klystron Magnet Power Supplies
There are three coils in an XL4 klystron, as shown in figure X, a upper and lower focus coil and
a bucking coil. The upper and lower focus coils each take 2 power supplies in parallel to get up
over 300 amps. The current for each circuit is run through shunts which are connected to meters
with interlocking capability. Windows are set for each circuit. The modulator is shut off
through the MKSU if the current goes outside the window. There are Klixons and water circuits
that will shut off the magnet power supplies if they trip.




Figure X, Klystron Magnet Power Supplies

XL4 Klystron Water Circuits
Flow rates for the three klystron water circuits follow:
       1. Klystron Body, 2GPM, trip at 1GPM
       2. Klystron Collector, 8 to 10GPM, trip at 6GPM
       3. Klystron Tank and Magnet, 8 to 10GPM, trip at 6GPM
The water circuits interlock the modulator through the MKSU. The klystron tank and magnet
water circuit shuts off the magnet power supplies if tripped.
                              Station Vacuum Interlocks
The new station gauge interlocks and an ion pump PLC interlock is run into the interlocking
summing chassis. The output of interlock summing chassis connects to the MKSU station gauge
vacuum interlock. The interlocking summing chassis takes inputs from the vacuum gauges
before and after the X-Band window assemble. The PLC monitors ion pump power supplies and
is connected to the interlocking summing chassis to be able to turn off the modulator if ion pump
currents are above a set threshold. The EPICS L1X vacuum panel show a diagram in figure X.
The black bar between W200 and W220 is the X-Band window assembly.




Figure X, EPICS L1X Vacuum panel.
Drive Amplifier
Saturated drive levels for different XL4s are listed in table 1. LDF2-50, 3/8 inch Heliax, is used
to go from the drive amplifier to the tube input. Ten feet of LDF2-50 Heliax has a loss of 1.5dB.
The drive power coupler, KRYTAR 1824, has an insertion loss of 0.7dB. An HP X362A Low
Pass Filter, loss less than 1dB, is at the input of the klystron. Total loss from the drive amplifier
to the klystron is about 3.2dB. 400W of power at the klystron requires 840W of power out of the
drive amplifier. If lower loss is required the Heliax can be replaced with WR90 waveguide. At
11.424GHz 10ft of copper WR90 has a loss of about 0.04dB. This would reduce the drive
amplifier power requirement from 840W to 600W.

XL4 Tube Number                            Saturated Drive Power
2C                                         362
3A                                         400
5D                                         145
6A                                         400
7B                                         800
8A                                         400
9A                                         400
12A                                        400
13A                                        400
Table 1. Saturated Drive Powers for XL4 Tubes. Data taken from tube folders.
                                                                                 LLRF Control System
A diagram for the X-Band RF system is shown in figure X.
                                                        25.5MHz from
                          RF HUT                        2830.5MHz LO Gen
                                                        LKG-2821-J5
                                                                                                                         PAD
                                                                                                                  380-208-60
                                                                                                                                                                               TUNNEL
                                                                                   J5
                                                                                                                  LKG-2827     J26 AccOut
2856MHz from                                                    380-208-51                               +2dBmJ24          CH1
                                                                                                                 LO
RF Reference         380-208-50                                 X-Band                                                         J22 AccIn
LKG-2730-J10        4 X Multiplier                              LO Generator                                 J16           CH0 +2dBm
                                                                                J8                         16dBm CLK
                    6dBm In                                     11398.5MHz 9dBm
               J3                  16dBm    J6                -2dBm
                    11424MHz                                                                                             CH3
                                   6dBm                 J7                       9dBm
                    LKG-2836       16dBm    J4                   LKG-2830                J9                         +2dBm    J27          Monitor
                                     J5                                                                                                                   J15
   380-208-21                                                                                                                             Panel
                              J12                                                                                                                         J14
   CLK Distribution                                                                                                                       LKG-2608
   LKG-2718                   J7



                                                                                                                         MKSU       KF-21-2B28
                                             +2dBm    J24
                                                                                                                    J13            J9              J12
                                                                                 J23 KLYS BEAM
                                                   LO       PAD          CH2                                        Klystron       Klystron   Reflected
                                    16dBm    J16     380-208-60     J27                                             Drive          Forward    Power
                                                 CLK            CH3                                                 Control
                                                     KF-21-2B15                                                                    X-Band Coupler
                                                                    J26                                     J8
                                             J22 CH0 +2dBm      CH1                                                                Chassis
                                                                                        ?dB                         KF-21-2B33
                                                                                                                                   380-208-56
                                                                                                         FP J3
                                                                                                                                    Monitor
                                                                                                            J7 Filter
                                                                                                10dB                                Couplers
                                                                                                                 Attenuator
                                                                                                                                    Diode detectors
               13dBm   J13                                                                     -20dB                J5             J10                J11
                                                      MIXER                                    Monitor
                    CLK                                          TWT Amplifier
        IN
             PAC    OUT                               LO RF
                                                                       714-107
 J15                      J14
                                                         IF




-2dBm   380-208-40        13dBm                                       KF21-2B39
                                           ?dB
        KF21-2B36                                    10dB
                                                                                                    KLYSTRON


                                                                  Klystron Station 21-2




                                                                                                                                                                   RF IN 1




                                                                                                                                                                                             RF OUT 2
Figure X, X-Band system diagram

X-Band 4X Multiplier
The X-Band reference RF is generated in the RF Hut. The X-Band 4X Multiplier chassis, figure
X, uses 2856MHz from the RF reference system and multiplies it by 4 to get 11424MHz. The
11424MHz outputs of the multiplier chassis feeds the X-Band LO Generator, the PAC at linac
station 21-2, and is monitored by channel 3 of the X-Band PAD in the RF Hut.

                                                                                                                                                                             2856MHz Mon FP N J1

                                                   4 X Multiplier                                                                        Midwest Microwave
2856MHz IN                                         Marki AQA-1933K                            10dB                    0dBm               CPL-5215-10-SMA-79
 RP N J3                                                                                                                           20dBm 10dB Coupler                              11.4GHz RP N J5
                                             +5dBm In                 +10dBm Out
                10dB Coupler                                                                                                                                                    11.4GHz RP SMA J6
                                                                                                                                                                Pulsar
              Pulsar                                 +5VDC 200mA                                    11424MHz BPF            CIAO CA1112-343
                                                                                                                                                        PS2-19-450/8S
              CS20-05-436/10                                                                                                Gain = +20dB
                                                                                                                            P1dB = +27dBm
                                                                                                                            NF = 5dB                         11.4GHz Mon RP SMA J4
                                                                                                                            IP3 = 35dBm     Pulsar           11.4GHz Mon FP N J2
                                                                                                                            +15VDC 600mA    PS2-19-450/8S



Figure X, X-Band 4X Multiplier Chassis SD-380-208-50-C0

X-Band LO Generator
The X-Band LO Generator uses the 11424MHz from the multiplier and the 25.5MHz from the S-
Band LO Generator chassis to Single Side Band, SSB, generate 11398.5MHz LO frequency for
the Phase and Amplitude Detectors, PADs. A diagram for the X-Band LO Generator Chassis is
shown in Figure X.
                                                                                                  REAR PANEL N      J7
                                                                                                   25.5MHz
                                                                                       2
 +13dBm out of 2830.5LO chassis                                                S       1
                                                                                                   25.5MHz
      REAR PANEL N                                        ZFSC-2-2             ZFSC-2-2           REAR PANEL N      J6
                                                                         2                             +7dBm
 J5   25.5MHz In
                                                 +23dBm    S
                             F5 SLP-30                               1       +20dBm




                                            CAP     PI NETWORK
                             +11dBm                                      +16dBm
                                                                                   90         0

                                                                                   0
                                                                                   JSPQ-80

                         DC Block



                                                                                                           +V    15V4
                                                                                                   1
                                                                                                   2
      J4                                                                                           3            + - 15V 4mA
                                                                                                           -V
REAR PANEL N                                                                                                     -15V
11.424GHz In
  17dBm                                                                       25.5MHz
                                                 DIPLEXER
                                                                              Balancing
                                                                              Hybrid



                              10dB          10dB Marki
                                                 IQ0714LXP                                                              FRONT PANEL N   J1
                              0dBm                                                                                        11.3985GHz
                                                 +10 to +13dBm
                                                                                                                        REAR PANEL N
                                           IF




                                         LO RF                 -6dBm                                                      11.3985GHz    J9
                     13dBm
 PULSAR
                                                                                                                    REAR PANEL N        J8
 CS10-12-435/1                           LO RF                                15V3      PULSAR                            11.3985GHz
                                           IF




                                                                             CAIO       CS10-12-435/1
                                                                             CA1112-441          PULSAR
                     11.424GHz to 11.3985GHz SSB IQ Modulator
                                                                                                       PS2-19-450/8S

                             DIODE
                             DETECTOR
                                                     DC 0 to ? Volt           J3           CIAO CA1112-441
 PULSAR                                            FRONT PANEL BNC                         Gain = +30dB
 CS10-12-435/1                                     In Diode Det                            P1dB = +30dBm
                                                                                           NF = 3.4dB
                                     FRONT PANEL N                                         +15VDC 900mA
                                      11.424GHz       J2

Figure X. X-Band LO Generator Chassis

X-Band Coupler Chassis
The X-Band Coupler Chassis is used to interface control of the drive power to the MKSU. It is
also used to couple down high power RF signals for monitoring. Detector diodes change the
klystron forward power, reflected power, and drive power into video signals which are connected
to the MKSU for monitoring by the control system. The coupler chassis and connections are
shown in figure X.
                                                                                                                               DRIVE                                                FORWARD                                REFLECTED
          PAC                                                                           MKSU                                   FP N To PAD CH1                                      FP N test                               FP N test
                OUT
                                                                                                           IPA
  Coupler           13dBm
                                                                                                                           J13 J3                                                            J2                                  J1
                                                                                                           Connector
  to PAD                                                                                                                                    X-Band                                                                                    -20dB
  CH0                                  RP BNC                                                                                                                                                       -20dB
                                       Drive                                                                                          -20dB Couplers
                                                                               LP Filter 10kHz
                               6dBm                       J5                                                                                Chassis
 KRYTAR                                                        0-28mA          Attenuator 17dB              0-200mA
 Model 1824                       10dB
 -20dB
                                                               16dBm                                        33dBm                           380-208-56                                           -20dB
       ?dB                             FP BNC
                                       Test                                                        VideoAmp                           -20dB
                                                          J4
  M8-0412NZ
  MIXER 17dBm                          BNC to                                                      239-025
                                       MKSU               J6                                                                                                                                   -20dB                 -20dB
                RF LO




                        IF


       0dBm
                                                          J7                                                                                      J8               J9                        J10                                 J11                          J12
                             10dB                                                                                                             N to
                                                                                                                                                               BNC to                                                                               BNC to
                                                                                                                                              PAD
                                                                                                                                                               MKSU                                                                                 MKSU
                                                                                                                                              CH3
      IN 10dBm MAX
                                                                         -20dB
                                                                                         HP X362A LPF

       TWT                   OUT                                                                                    KLYSTRON                                                                                                                To Accelerator
                                                                   KRYTAR                                                                                                                      -53dB
         714-107                                                   Coupler                                          XL4
                                                                   Model 1824
                                                                   -20dB


Figure X, X-Band Coupler Chassis 380-208-56 and connections.

X-Band PAC
The X-Band PAC chassis uses the same control board as the other PAC chassis. The control
board puts out 2 preset waveforms on a trigger pulse to drive I and Q of the IQ Modulator. The
diagram of the PAC board is in figure X.
                                                                                                                                                                                        TRIGGER     EXTERNAL
                                                                                                                                                                   CLOCK                Monitor TTL TRIGGER  SSSB         SSSB
                                                                                                                                                                   119MHz               FP BNC      120Hz
                                                                                                                                                                                                    60nS NIM Gate Monitor Chassis
                                                                                                                                                                   RP N                                      FP BNC       RP 15 Pin D
                                                                                                                 RF OUTPUT                                                                          RP BNC
                             RF INPUT                                                                RF OUTPUT   To TWT Amp
                             Monitor                                                                 Monitor     RP N            I MONITOR Q MONITOR
                             FP N                                                                    FP N        +13dBm Out      FP BNC    FP BNC


                                                                                                                                       H9        H10                               J2          H7       J3    SSSB       H6               P5
                                                        RF Module                                                                                                                                             Trig
                                                                                                                                                                                                              TTL
                                                                                VLFX-80 Low Pass                                                                                                              17 to 30uS
                                                                        10dB

                                       -10dB Couplers
                                       Pulsar
                                       CS10-12-435/1                                                                                                                       16bit DATA
                                                                                                        -10dB                                                                                                     16 bit
                                                                   3




                                                                                                                                                                                                                                                       ETHERNET
11.424GHz Ref                                                          I&Q MODULATOR           Amp
                                                                   I                                                                                         I                                                    DATA
                   Amp                                                              1                                                                                          CLK         XILINX                          CONTROL /
RP N                                         -10dB
                                                        2 RF                   LO                                               J5                        MAX5875
-2dBm In                                                                                                                                                  2 X 16 bit DAC                   SPARTAN 3                       Arcturus uC5282
                               -10dB
                                                                   Q
                                                                                                                                                          119MHz Clock
                                                                                                                                                                           16bit DATA      FPGA                            Microcontroller Module
                                                                                                                                                          (1MHz to 200MHz)                                         CS/
                                                     IQ0714MXP                                                                                                                                                             with 10/100 Ethernet
                                                                   4




                                                                       10dB VLFX-80 Low Pass                                                                 Q                                                     CLK
                                                     13 to 16dBm                                                                                                               CLK
                                                                                                                                                                                                                                                    ETHERNET
                                                                                                                                                                                                                                                    RAW




                                                                                                                                 J4

                                                                                                                                            AD8099 Diff Amp


                                                                                                                                                                                Control                 Control
                                                                CIAO CA1112-343
                                                                                                                                                                                                                                                     COM




                                                                Gain = +20dB
                                                                P1dB = +27dBm
                                                                                                                 Temperature
                                                                NF = 5dB                                         Monitor
                                                                IP3 = 35dBm                                                                                                                  Temperature
                                                                                                                                                         SLOW ADCs                          t Monitor
                                                                +15VDC 600mA                                                    H12                      PAC Temp
                                                                                                                       t
                                                                                                                                                         IQ Temp
                                                                                                                                                         SSSB Temp
                                                                                                                                                         SSSB P-FWD
                                          RF INPUT                                                                                                       SSSB P-RFL
                                          Monitor
                                                                                                                                                         SSSB PWR
                                                                                                                                                         +5V
                                                                                                                                                                                  Control Board
                                          Diode
                                                                                                                                                         -12V
                                          FP BNC




Figure X, X-Band PAC Chassis FS-380-208-40-C0

X-Band PAD
The PAD Chassis down mixes the 11424MHz with 11398.5MHz to a 25.5MHz IF frequency on
3 channels, 0, 1, and 3. Channel 2 is connected to a coupler and an input transformer before
being digitized. The coupler, Minicircuits ZFDC-10-6-S+, 0.005MHz to 20MHz limits the
bandwidth of the signal before it reached the digitizer board. On the digitizer board, the standard
input transformer, Minicircuits TC4-1T, 0.5 to 300MHz, is replaced by Minicircuits TT1-6-
KK81, 0.004MHz to 300MHz to give a total bandwidth of 0.005MHz to 20MHz. This signal is
used to measure the beam voltage to the klystron from the MKSU.
                                                                                                                         CHAN 0
                                                                                                                         25.5MHz IF
                                                                                                                         FP BNC - J1

                                                                                                                                                                         4 X 16 bit ADC                            Control Board
                                                                                                                                                                         102MHz Clock
                        CHAN 0                                                                                               TEST                                        LTC2208
                        RF INPUT           MIXER M80412NZ
                                                                                                IF Board                                                                 Transformer Coupled Inputs
                        RP N                                                                                                                                                                    16bit DATA
                                                                                                                                                                                                                                         16 bit




                                       LO RF
                                               IF
                                                                                           IF
                                                                                                                                                                            Chan. 0
                                                                                                                                                                                                              FIFO                       DATA
                                                                                                                                    OUT
                                                    10dB
                                                                                                                                                                                                  WCLK        64k words
                                                                                                                                                                                                                                                  CONTROL /




                                                                                                                                                                                                                                                                              J15
                                                                                                                                                                                                                                                                              COM
                                                           VLFX-80 Minicircuits                         FILTER 25.5MHz BP
                                                                                                                                           CHAN 1
                                                                                                                                                                                                16bit DATA
                                                                                                                                                                                                                                          CS/
                                                                                                                                                                                                                                                  Arcturus uC5282
                                                                                                                       714-114-50-R3       25.5MHz IF                                                         FIFO                        CLK     Microcontroller Module
                                                                                                                                                                            Chan. 1
                                                                                                +12VDC                                     FP BNC - J3                                            WCLK        64k words
                                                      CHAN 1                                                                                                                                                                                      with 10/100 Ethernet
                                                      RF INPUT




                                                                                                                                                                                                                                                                              J14
                                                                                                                                                                                                                                                                              ETH1
            CIAO CA1112-343                                                                                                                                                                     16bit DATA
            Gain = +20dB
                                                      RP N                                                                                                                                                    FIFO
                                                                                                                                                                            Chan. 2
            P1dB = +27dBm
                                                                                                                                                 TEST                                             WCLK        64k words
            NF = 5dB
            IP3 = 35dBm            1                                  MIXER M80412NZ
                                                                                                                        IF Board
            +15VDC 600mA                                                                                                                                                                        16bit DATA
                                   2                                                                                                                             10dBm                                        FIFO
                                                                  LO RF



                              3                                                                                   IF                                                        Chan. 3
LO INPUT                           4
                                                                          IF
                                                                                                                                                        OUT                                       WCLK        64k words
11.4GHz +2dBm                      5
                                                                                  10dB   VLFX-80 Minicircuits                FILTER 25.5MHz BP
                                                                                                                                                                                                                Control




                                                                                                                                                                                                                                                                              RAW J13
                                                                                                                                                                                                                                                                              ETH2
RP N          +15VDC                                                                                                                      714-114-50-R3

                                                                                                                        +12VDC                                                                                 CPLD
                                                                                                                                                                                            5VDC                                                              QSPI
            LO1                                                                                                                                                   CLK IN          CLK OUT   0.8A x 2 Analog                       TRIG
            TEST PORT                                                                                                                                                                                           5VDC
            FP N - J9                                                                                                                                         CLOCK IN      CLOCK Mon                                                                                 20 pin ribbon
                                                                                                    CHAN 2                                                                                                      0.5A Digital   TRIG In        TRIG Mon
                                                                                                    Test                                                      102MHz        102MHz                                             120Hz          FP BNC - J12
                                                                                                    FP BNC - J5                                               RP N          FP N - J11                                         RP BNC

                                                                                                                                                                                                                                                        QSPI
                                                                                  CHAN 2
                                                                                  RF INPUT
                                                                                  RP N
                                                                                                         ZFDC-10-6-S+                                                                                                                                  24Bit
                                                                                                         0.005 to 20MHz
                                                                                                         11.3dB coupling                  CHAN 3                                                                                                       Analog
                                                                                                                                          Test
                                                                                                                                          FP BNC - J7                                                                                                  Input
                                                             CHAN 3
                                                             RF INPUT
                                                                                                                                                                                                                                                       Board
                                                             RP N
                                                                                                                                                 TEST                                                                                             ANALOG IN ANALOG IN

                                                                           MIXER M80412NZ
                                                                                                                        IF Board
                                                                                                                                                                                                                                                  PAD Usage
                                                                                                                                                                                                                                                  Ch0 - LO Power
                                                                     LO RF




                                                                             IF
                                                                                                                 IF
                                                                                                                                                        OUT                                                                                       Ch1 - RF Head Temp
                                                                                  10dB                                                                                                                                                            Ch2 - ADC BRD Temp
                                                                                          VLFX-80 Minicircuits               FILTER 25.5MHz BP
                                                                                                                                                                                                                                                  Ch3 - +12VDC
                                                                                                                                          714-114-50-R3                                                                                           Ch4 to Ch7 external temps

                                                                                                                        +12VDC


Figure X, X-Band PAD Chassis FS-380-208-60-C0

The PAD chassis in the RF Hut measures the input and output RF for the accelerator structure on
channel 0 and channel 1 respectively. Channel 2 is not used and Channel 3 measures the
reference RF from the multipler. The PAD chassis at the klystron station measures the PAC
output on channel 0, the drive amplifier output on channel 1, the klystron beam voltage on
channel 2, and the klystron output on channel 3. The RF Hut X-Band PAD panel is shown in
figure X. Since the fill time of the structure is only 100nS and the RF pulse width is only 200nS
the window to look at the structure is only 12 points, or 118nS at 102MSPS.
Figure X, RF Hut X-Band PAD Panel
PAC Calibration, Set-up, and Operation
There are 2 EPICS panels used for PAC control. One is for operation (destination), PAC L1X,
and one for calibration, PAC CAL L1X. The two panels are shown in Figure X.




Figure X PAC Destination (Operation) Panel and PAC Calibration Panel.

Calibration:

To enter calibration mode press the "Calib Rqst" button on the PAC CAL L1X panel.

In calibration mode there are 2 waveform selection which can be made. Both load a cosine
waveform into I. One of the calibration wavforms loads a sine waveform into Q while the other
load a sin+pi waveform into Q. These waveforms will make a Single Side Band SSB modulator
out of the IQ modulator driven by the PAC. The two selections of waveforms make either an
upper side band modulator or a lower side band modulator.

The FPGA "Trigger mode", bit 0 and 1 of the Control Register (2000h) is set to "Internal
Trigger", 01 for calibration mode. No external trigger is required for calibration.

There is also an internal and external clock mode. The mode used depends on if the PAC is a
Slow PAC, SPAC, or normal PAC. The internal clock is used for SPACs and the external clock
is used for normal PACs. This uses bit 0 of the Clock Select reg (200Eh). Bit 0 set is internal
clock, bit 0 cleared is external clock. The software selects the correct clock mode.

Once calibration mode is entered the "SLOW update of I waveform in FPGA" should show a
cosine wave and the "SLOW update of Q waveform in FPGA" should show a sine wave. The
"Amplitude of Calib WF" is a gain factor applied to both waveforms. The default value for this
is 16384.

To calibrate the PAC a spectrum analyzer is connected to the output of the unit. The front panel
RF Out Mon J2, figure X, is a good place to connect the analyzer. The center frequency of the
spectrum analyzer is set to the frequency passing through the PAC. The span should be set to
about 500kHz. This will enable viewing of upper and lower sidebands when the PAC turns into
a SSB modulator.

Offset Calibration:
To calibrate the offset the I Gain and Q Gain are both set to 0. This sets the output of the DACs
to zero. To remove offsets in the system the fundamental frequency needs to be suppressed by
adjusting I Offset and Q Offset. Start by moving in 100 count changes and go back and forth
between I and Q to minimize the fundamental. Fine adjustments are made by changing the
offsets by 10. The fundamental should be suppressed by 60dB by adjusting the offsets.

Gain Calibration:
Once the offsets are calibrated enter values of 32000 in I Gain and Q Gain. This should change
the PAC into a SSB modulator. The fundamental should decrease and one side band come up to
about the fundamental power level. The opposite side band will be down by at least 10dB to
start. Lower either I Gain or Q Gain initially by increments of 1000 and then by increments of
100 to reduce the level of the lower sideband. The ratio of the two side bands gives an estimate
of the linearity of the system. Since there is not phase correction on the modulator, the ratio of
the two sidebands will likely be between 25dB and 40dB based on the accuracy of the 90 degree
phase difference between I and Q in the modulator.

Once the I Offset, Q Offset, I Gain and Q Gain values are set, the calibration of the modulator is
complete. The Calib Done button can now be pressed. The PAC is now running and phase and
amplitude can be changes by entering the I Adjust and Q Adjust values on the PAC L1X panel.
The I Adjust is the diagonal elements of a rotation matrix and Q Adjust are the off diagonal
elements of the matrix.




Figure X, X-Band PAC Chassis
Feedback Operation
The VME L1X feedback panel is shown in figure X.




Figure X, VME L1X feedback panel.

The initial I and Q readings from the PAD are rotated by Phase Offset corrections and displayed
at the top of the panel. The Phase Offset corrections are entered on the "Adjust scale factors &
offsets" panel. The Phase Offset corrections are set so that a corrected reading of zero is the
phase at which the beam has maximum energy gain through the accelerator structure. The
Voltage Scale Factor is set so that the energy gain of an electron passing through the accelerator
structure at zero phase is displayed. These parameters are set by the operators using beam based
measurements.
Either or both of the 2 channels, L1X In and L1X Out, can be used in the phase or Amplitude
feedback. Weighting factors as to the fraction of each input which is used in the phase and
amplitude feedbacks are set independently with the CH0 Weighting Factor and CH1 Weighting
Factor. These weighting factors are typically set for equal weighting of each channel. The
weighted average is then passed on to the phase and amplitude feedbacks.
The Phase feedback looks at the difference between the entered "Desired" phase and the "Wt
average" phase calculated from the PAD readings. For the feedback to calculate a correction,
three conditions must be met, the amplitude must be larger than the Minimum Amplitude, the
difference, error, between the Desired and Wt averge must be larger than the Minimum
Correction, and the Local Phase FB button must be in an On state. If those three conditions are
met, the absolute value of the error signal is clamped by the Maximum Correction. The clamped
error is then multiplied by the Smoothing factor and subtracted from the Previous set point to
generate a new set point. The new phase set point is then used along with the new amplitude set
point to calculate I Adjust and Q Adjust which get sent to the PAC.
The Amplitude feedback divides the Desired value by the Wt average. As with the phase
feedback, there are three conditions which allow the feedback correction to be calculated, the
Local Amplitude FB button must be in an on state, the fractional error must be larger than the
Minimum Correction, and the Wt average must be larger than the Minimum Amplitude. If the
three conditions are met, the fractional error is clamped by the Maximum Correction multiplied
by the smoothing factor and used to scale the previous set point to get a new set point. The new
set point is then limited to a range between lower and upper Ampl Setpt limits.

Feedback setup
The lower and upper amplitude set point limits are set so the feedback does not move far from
the desired operating point. The upper set point limit is set to about 3% above the operational
voltage level. The PACs are vary non-linear due to saturation in the drive amplifiers and
klystrons. The lower limit needs to be set high so the power out does not go below the Minimum
Amplitude required for feedback operation and should be about 3% below the operating point.
The Minimum Amplitude is set to prohibit feedback response to dropouts and should be set to
about 5% below the operating point.

The smoothing factors are currently set to 0.4 so the feedbacks will correct fast enough for
software running routines which require large phase and/or amplitude movements.

								
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