Implementation of the Low Level RF System at PLS Storage Ring
Inha Yu, Hong J. Park, M. H. Chun, M. Kwon, and I. S. Ko
Pohang Accelerator Laboratory, POSTECH
Pohang, 790-784 KOREA
Abstract
The RF system in the storage ring of the Pohang Light
Source (PLS) consists of four independent RF station andits total RF power of 240 kW (500 MHz) can support thestorage ring operation up to 400 mA at 2 GeV. Continuous
variation of power levels from the klystron amplifier isdemanded to accommodate the signal phase and amplitudechanges to each RF cavity during beam stacking anddecaying. The signal amplitude control, the forward path
phase control, and the tuner control are used to achieve thisdemand and, thus, to keep the RF cavity on tune. We have
improved beam operation properties by improving circuitsin the low level RF system. We can now understand themain causes of beam instabilities better in variousconditions.
1 Introduction
Low level RF system at PLS storage ring consists of 3
independent feedback control circuits such as forward path
phase control, automatic gain control, and tuner control
circuit. These circuits will ensure the required stability
condition for the PLS RF cavities operation. Fig. 1 shows asimplified block diagram of the low level RF system of thestorage ring RF system.
The RF power for four cavities is provided by four
independent RF stations with a 60kW klystron amplifiersystem per each station. The output power from each RFstation is fed via high power circulator to drive the RFcavity. In the 2.0 GeV operation, the dissipation of theRF cavity is 20kW for 400 kV of accelerating voltage witha coupling factor of 1.8. The RF cavity dissipation should
remain constant through the entire range of operating beam
current zero to 400mA.
When the beam is demanding more power from the RFcavities, the automatic gain control loop should adjust thedrive signal of the RF system providing more power for thebeam and keeping the RF cavities voltage constant. As theRF signal must go through the low level RF system
including three automatic control circuits and finally theklystron amplifier system, the phase of RF signal will varywith the RF output power level. Since the phase variation
may give some effects on beam stacking, the phase control
circuit has to correct it precisely to keep the phase constantthroughout the whole range of the operating power. Inorder to keep the four RF cavities on-tune during theoperation, each RF cavity tuner control loop controls a
stepping motor to move the plunger in and out of the RFcavity to keep it on tune.

RF OUPUT(15 dBm)

COUPLED SIGNAL

RF INPUT(0 dBm)

FEEDER 1

CAVITY 1

COAXIAL
SWITCH(SPDT)RF IN

CAVITY 2

FEEDER 2

RF IN

RF OUT

LOCAL OSCILLATOR
LO
500.082MHz-10dBm

PHASE SHIFTER 2

RF IN(REF)

RF IN(CPL)DOWN CONVERTER 1OUT 1

OUT 12 MHz

2 MHz

2 MHz

REF

CPLCPLPHASE
DETECTOR 1

DC Volts relative to RF Signals
Amplitude

RF IN(REF)

DOWN
CONVERTER 2

RF IN(CPL)OUT 2

JUMPER(FP)500.082MHz-11dBmRF SIGNAL DRIVER

RF IN

REFPHASE
DETECTOR 2
TUNER CONTROLLERPHASESHIFTER 1

DIRECTIONAL COUPLER 1

PHASE SHIFTER 3

2 MHz

Bias(-5 to +5V or 0 to10V)

500.082MHzRF OUT

OUT 2

RF SIGNAL CONTROLLER

500.082MHz
16dBm Max.

TRIP SIGNALSBAND PASS FILTER

500.082MHz
15dBm Max.

DIRECTIONAL COUPLER 2RF OUT MON.
(-5 dBm)

Fig. 1: A simplified block diagram of the low level RF system of the PLS storage ringto TUNERMIXER
RFREFERENCE
SIGNALLOW PHASE SHIFT
LIMIT AMP.

5MHz

IF

TTL DISCRIMNATOR

2MHz

COMPARATOR

AMP
LPF

LPF
2MHz

LO
2MHz

500.082MHz

2MHz

2MHz

2MHz

U

R

2MHz

PHASE
DETECTORLOCAL SIGNAL

V

498.082MHz

MIXER
RF

COUPLED SIGNALLOW PHASE SHIFT
LIMIT AMP.

5MHz

IF

DC SIGNAL(-2.5 to +2.5V)
PHASE(DEG.)

D2MHz

TTL DISCRIMNATOR

2MHz

COMPARATOR

AMP
LPF

SUMMINGAMPLIFIER02MHzDC OUT(V)

LPF

LO
500.082MHz

2MHz

2MHz

2MHz

2MHz

Fig. 2: Phase detection circuit diagram & output waveform of each stage
2 Implementation of the low level RF system at pls
storage ring
The intermediate frequency (IF) output signal of themixer is filtered and amplified and fed back to theelectronic phase shifter to compensate for the phase error
in the RF signal path.
Fig. 2 shows the phase detection circuit diagram andoutput waveform of each stage. In late 1996, an upgrade
was pursued to improve dynamic range of phase detection
by separating the low frequency signal from the highfrequency signal to isolate interference between them andby strengthening and shielding the high frequency circuitwith a down converter.
The master oscillator signal goes through an electronic
phase shifter to a phase detector as the station phasereference signal. A directional coupler in the feeder of the
RF cavity picks up a portion of the RF signal and feeds itback to the RF port of the phase detector.
The intermediate frequency (IF) output signal of themixer is filtered and amplified and fed back to theelectronic phase shifter to compensate for the phase error
in the RF signal path.
This phase detector has shown a satisfactory dynamic
range in the input range of -40dBm to 0dBm as shown inFig. 3. We have designed and produced them for reliability
using improved EPLD (Erased programmable logic device)in the automatic gain control and the tuner control circuits,
compared with the old version of low level RF system. As
a result, we have less possible errors in normal operations.
The new system produces better reliability and the fast
response time. Table 1 shows the performance comparisonbetween EP910 and EPM7064LC.
During the test without beam, the amount of phasevariation of the system is more than 30 degrees between
operating power level of 6kW and 60kW. With the forward
path phase loop, the phase variation is reduced to less than
± 0.3 degree. With an automatic gain controlfeedback
loop, the amplitude variation of RF power into the RF

cavity is measured to be less than ± 0.5% (0.039dB). At
the initial phase, the operating power for the four cavities is
80 kW without beam. With beam current of 200mA at
2.0GeV, the total RF power required is about 120kW.
With the forward path phase control loop the phasevariations reduced to be less than 0.3 degrees.
Table1: EPLD (EP910 and EPM7064LC) performancec
omparison
FeatureAvailable Gates
Usable Gates
Macrocells
Max. User I/O
FmaxIcc(mA)Standby Icc(mA)
TPD
Package

EP910
Over 900
750
242441.7MHz
20.0
0.130nS
CerDIP 40pins

EPM7064
2500
1250
6468
125MHz12nS
PLCC84pins

3 The purity of RF signal
By comparing with result of theoretical calculation, weadjust the forward path phase of each RF stations to theoptimum point. Fig. 4 shows a spectrum of RF signal
measured at the RF cavity input port without beam. Fig. 5
shows a spectrum of RF signal with beam of 140mA. In
the injection stage, as the current reaches a threshold level,
the revolution frequency (1.06855MHz) appears as a sideband periodically back to the RF port of the phase detector.
4 Station phase tuning
Information of the synchrotron oscillation frequency isused for tuning the station phase. The synchrotron
oscillation frequency is given as
1 α h ω 02 e V c o s (φ 0 ) 1/ 2fs =()2π2 πEMIXER
REFERENCE
SIGNALRF

COUPLED SIGNAL: -40 dBm

LOW PHASE SHIFT
LIMIT AMP.

5MHzIF2MHzAMP
LPF

LPF

LO

LOCAL SIGNAL

COUPLED SIGNAL: -20 dBm,

COUPLED SIGNAL
COUPLED SIGNAL: 0 dBm

Fig. 3: Characteristics of phase detection and amplification of phase detector
where the momentum compaction α is 0.001809, theharmonic number h is 468, the revolution angularfrequency ω 0 is 2π × 1.06855MHz , and the RF voltage
eV is 1.6 MV. The synchronous phase angle
determined bycosφ 0 = [1 − (φ0isU 0 2 12) ]eV

where the synchrotron radiation loss per turn at 100 mA
U 0 is 225 keV, and the electron energy E is 2 GeV,
producing the synchrotron oscillation frequency of 11.4
kHz. We adjusted the forward phase of four RF stations
to a correct value, then we could obtained maximum
synchrotron frequency as the theoretical one. Fig. 6 shows
the spectrum of the measured synchrotron oscillation
frequency.

5 Summary
A new low level RF system has been implemented forthe PLS storage ring RF system in late 1996. Each of thethree feedback circuits shows a good performance. Withthe electron beam, however, some minor problems havebeen noticed such as slow drift of the phase reference andreduced dynamic range. With an extensive study andmeasurements with beam, we anticipate a better system tobe ready very soon.
Acknowledgments
This work is supported by the Ministry of Science andTechnology in Korea and Pohang Iron & Steel Company.
References[1]

PLS Conceptual Design Report, PAL (1992).

Fig. 4: A spectrum of RF signal from the RF cavity input
without beam

Fig. 6: The spectrum of the synchrotron oscillation
frequency

Fig. 5: A spectrum of RF signal from the cavity input
with beam (beam current 140mA)