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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
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DEVELOPMENT OF THE RF PHASE SHIFTER WITH FEMTOSECONDTIME DELAY RESOLUTION FOR THE PAL-XFEL LASER SYSTEM*D.C. Shin†, G.J. Kim, H. -S. Kang, G. Mun, C.-K. Min
Pohang Accelerator Laboratory, Pohang, South Korea
Abstract
We introduce the RF Phase Shifter (RPS) developed in
the Pohang Accelerator Laboratory X-ray Free-ElectronLaser (PAL-XFEL) to control the timing of optical laser
system. This equipment is designed to finely adjust the timing of laser pulses with femtosecond scale by manipulating
the phase of the RF reference using a couple of Direct Digital Synthesizer (DDS) devices. Furthermore, it is designedwith low phase noise and low phase drift features in orderto minimize the impact on the system in an open-loop operation.
Currently these units are installed at the Injection site,
Hard X-ray and Soft X-ray Beamline. They are implemented for the feedback control of the photocathode gun
phase at the Injector and for the use in pump-probe experiments at the Beamlines. This paper describes the design,
fabrication, and experimental results of the RPS, as well as
its usage status at PAL-XFEL.

INTRODUCTION
Optical Laser systems have been established in the Injector and Beamlines at PAL-XFEL for generating a highly
stable electron beam and ultrafast X-ray sciences.
Time delay is a critical function in laser systems. In theInjector section of PAL-XFEL, laser time delay is utilized
to compensate for drift of the electron beam. In the Beamline, it is employed to control the time delay of the pumplaser in pump-probe experiments. The time delay of lasers
can typically be adjusted by altering the physical length ofthe laser path using mirrors and delay stages. The PALXFEL laser system has employed this method since itsearly operational stages [1].
However, the approach involving delay stages can leadto degradation in laser quality due to vibration and shift inlaser focus. Additionally, the difficulty in adjusting the timedelay at pulse-to-pulse speed (60 Hz) results in the wasteof the probe laser in pump-probe experiments.

Figure 1: RPS and delay stage in the injector laser.
____________________________________________* Work supported by Ministry of Science and ICT of Korea† dcshin@postech.ac.kr

To address these aspects, we believed that a digital timedelay control device would be more effective than mechanical time delay methods. As shown in Fig. 1, we considered
installing an RF phase shifter between the REF and PLL tomodify the phase. By adjusting the phase in this manner,
we expected that a corresponding time delay proportional
to the phase change would naturally occur during the Lock
Tracking process of the PLL. Additionally, we fabricated a
prototype device and confirmed its operation to be as expected.
We manufactured and installed a total of six devices inboth the Injector laser and Beamline laser systems. In thefollowing section, we will introduce the fabrication process
and outcomes of the RF Phase Shifter.

DESIGN OF THE RPSKey Specifications
Considering the requirements from the laser team andthe limitations of electronic devices, we have defined thespecifications as follows: Phase Control Resolution: 0.0055 degrees
 Control Range: (-2^30 ~ 2^30)×resolution
 Linearity (0~360 degrees): ≤ ±0.05 degrees
 Update Rate: ≥ 10 kHz
 Stability degrees/day: ≤ 0.1 degrees
 Phase Noise: ≤ -150 dBc@100 kHz~1 MHz

Determination of Phase Shifting Methods
We explored various phase shifting methods to develop
a unit that satisfies the specifications at a reasonable cost.
Initially, we considered Digital/Voltage Controlled Phase
Shifter components due to their affordability and straightforward control mechanisms. However, after reviewing thedatasheets, we determined that they would not meet the required resolution and linearity specifications [2, 3].
The vector modulation approach allows for high resolution implementation, but due to hardware issues such asphase unbalance in Hybrid and Combiner, as well as I/Q
DC offset, the linearity of the output phase deteriorates.
Therefore, meticulous calibration efforts are required tomeet the linearity specifications. However, even with suchcalibration, we had determined that achieving the less than
±0.05 degrees linearity is very challenging [4].
Next, we examined the DDS device. The DDS enablesprecise frequency/phase control and fast response characteristics by converting digital values obtained through the'Phase Accumulator' and 'Amplitude/Sine Converter' into
RF signals via the DAC (Digital-to-Analog Converter) [5].

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ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 2 1Figure 2 shows the block diagram of the AD9914 DDS
that we applied. The AD9914 has a 16-bit phase offset
value, providing a phase resolution of approximately
0.0056 degrees (360/2^16). Although the DAC's resolution
is limited to 12 bits, the phase truncation process that maintains the higher bits ensures that all 16 bits of phase information are preserved [5].

Figure 2: AD9914 block diagram [6].

Frequency Plan of the System
When performing frequency up-conversion and downconversion, the phase of the output signal generally mirrors
the changes in the phase of the input signal. Therefore, we
had planned to utilize the DDS output as an Intermediate
Frequency (IF) signal and feed it into the mixer in the process of converting the RF 2856 MHz REF input frequency.
Then we can control the output phase by manipulating thephase of this DDS signal.
The design concept is shown in Fig. 3.

Figure 3: RPS frequency plan.
On the other hand, in order to minimize the generationof spurious signals during the conversion process, we initially set the DDS output frequency to a few hundred MHz.
However, based on the information from the datasheet ofAD9914, we confirmed that higher output frequencies leadto deteriorated residual phase noise performance [6].
Therefore, we made a trade-off between spurious reduction
and phase noise performance. Consequently, we determined the DDS output frequency to be 89.25 MHz.

FABRICATION OF THE RPSThe following block diagram shows the internal structure of the fabricated unit.

05 Longitudinal Diagnostics and Synchronization

Figure 4: RPS block diagram.
The Power of the input REF signal is sufficiently large
to drive the mixer, so it is directly routed to the mixer’s LO
port after passing through the coupler without an additional
amplifier. The output from the coupler serves as the masterclock for the DDS, and an ADCLK925 1:2 clock buffer is
employed to supply the signal to two DDS chip.
The frequency of the down-converted signal at the firstmixer is 2766.75 MHz, and this signal is then passed
through a filter and amplifier before being fed into the LO
port of the second mixer. The IF signal produced by thesecond DDS and the 2766.75 MHz signal are mixed together at the up-conversion mixer and it generates an RF
signal at 2856 MHz. The phase control in this process isachieved by adjusting the phase offset value of the DDS
dedicated to up-conversion. An FPGA was utilized for
DDS timing control, and a Raspberry Pi, acting as a Single
Board Computer, was employed for EPICS IOC processing.
In Fig. 4, the blue region represents the temperature-controlled area. To minimize drift caused by external temperature variations, this area is surrounded by thick insulationmaterial, and external chiller along with specially designed
heat dissipation plates are used to control the temperature.
For precise temperature monitoring, temperature sensors
are placed at four locations within the unit. Additionally,power detectors and phase detectors are mounted inside theRF modules for continuous monitoring of input/output
power and phase. Figure 5 shows the fabricated unit.

Figure 5: The fabricated unit.
The control board containing the DDSs was exposed toroom temperature due to the longer wavelength of the IFband, which results in reduced sensitivity to temperaturechanges.
Meanwhile, the RF circuitry is placed in an area withtemperature control. It is tightly integrated within a metal
TUP021
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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Content from this work may be used under the terms of the CC-BY-4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - T U P 0 2 1housing which is further affixed to a metal heat dissipation
plate, enhancing thermal conductivity between the heatdissipation plate and the RF circuitry. Furthermore, as thecooling fluid flows through the heat dissipation plate fromthe chiller, heat exchange takes place. Therefore, to ensure
a high level of temperature stability, it is important for theinsulation performance to be excellent, and the temperatureof the cooling fluid should remain consistent.

PERFORMANCE AND USAGE
Performance
Phase Noise Figure 6 displays the measurement results of the input and output phase noise characteristics ofthe RPS, displaying that the desired phase noise performance of 150 dBc or lower is achieved in the target frequency range exceeding 100 kHz.

0.9 V. This device features an output characteristic of10 mV per degree [7].
Resolution and Linearity To demonstrate the resolution performance of the RPS, we incrementally increased
the phase of the RPS by 0.0055 degrees using LLRF fortesting. Figure 9 shows the results, clearly indicating increments of 0.0055 degrees. For reference, the phase measurement performance of LLRF is rms 0.01 degrees.
Figure 10 shows the results of the linearity measurements for the RPS. The RPS phase was incremented in 10
degrees steps from 0 to 350 degrees, and the phase measurements obtained through LLRF are displayed on the left
vertical axis, with the relative error indicated on the rightvertical axis. It can be observed that the relative error remains within ±0.05 degrees.

Figure 9: RPS resolution (0.0055 degrees/step).
Figure 6: Phase noise, yellow: REF, blue: RPS output.
Long Term Stability The measurement of long-term
stability utilized the Phase Detector from i-tech Corp., aswell as the LLRF deployed at PAL-XFEL.

Figure 10: RPS linearity (10 degrees/step).

Usage of PAL-XFEL
PFS units have been installed in the Hard/Soft X-ray
Beamline laser room and the Injector laser room. The installation images are shown in Fig. 11.

Figure 7: Long-term stability by i-tech phase detector.

Figure 8: Long-term stability by LLRF.
As shown in Figs. 7 and 8, both sets of results demonstrated a stability of 0.1 degrees per week. To minimize errors, the output voltage of the Phase Detector was set toFigure 11: RPSs in the beamline and injector.
In the Injector laser, the RPS is utilized to correct phasedrift between laser and electron beam. As depicted inFig. 12, the RPS adjusts the laser delay time by reading theTUP021
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05 Longitudinal Diagnostics and Synchronization

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position value of the BPM located downstream of the Gunand making adjustments based on the error.
In the beamline, the RPS is currently used partially forpump-probe experiments. It appears that many users still
prefer the time delay control method using the conventional motion stage. However, we plan to actively explain
the advantages of the device to users in order to increase
opportunities for its usage.

REFERENCES[1] M. Kim, C.K. Min, and I. Eom, “Laser systems for time-resolved experiments at the Pohang accelerator Laboratory Xray Free-Electron Laser beamlines”, J. Synchrotron Rad., vol.
26, pp. 868-873, 2019.
doi: 10.1107/S1600577519003515[2] Analog Device, “HMC647ALP6E Digital Phase Shifter
Datasheet”,
https://www.analog.com[3] Mini-Circuits, “JSPHS-1000 Voltage Controlled Phase
Shifter Datasheet”,
https://minicircuits.com[4] P. Tosovsky, D. Valuch, “Improvement of RF Vector Modulator Performance by Feed-forward Based Calibration”, Radioengineering, vol. 19, no. 4, pp. 627-632, 2010.
[5] Analog Device, “A Technical Tutorial on Digital Signal Synthesis”,
https://www.analog.com

Figure 12: Feedback using RPS in the injector.

SUMMARY
We have developed a cost-effective phase controller witha manufacturing cost of less than $10,000, and through experiments, we have confirmed its satisfactory performance.
As mentioned earlier, this unit is currently operated as aGun laser feedback system for PAL-XFEL, and we anticipate its gradual expansion for pump-probe experiments inthe beamline.

[6] Analog Device, “AD9914 Direct Digital Synthesizer
Datasheet”,
https://www.analog.com[7] Instrumentation Technologies, “Libera Phase Detector UserManual”,
https://www.i-tech.si

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12th Int. Beam Instrum. Conf.
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