An S-band solid state radio frequency power amplifier used at Shanghai soft X-ray FEL facility

SYNCHROTRON RADIATION TECHNOLOGY AND APPLICATIONS

An S-band solid state radio frequency power amplifier used at Shanghai soft X-ray FEL facility

Hong-Li Ding，

Ming-Hua Zhao，

Cheng-Cheng Xiao，

Shao-Peng Zhong，

Jun-Qiang Zhang

Nuclear Science and Techniques

Vol.27, No.6

Article number 146

Published in print 20 Dec 2016

Available online 31 Oct 2016

DOI：10.1007/s41365-016-0144-0

41200

In this paper we present the general design methods and parameter measurements of a 1 kW solid-state radio frequency (RF) power amplifier at 2856 MHz, for the Soft X-ray Free Electron Laser (SXFEL) facility. Three-stage amplification with a 4-way combination is used. An RF switch module is integrated in the solid-state RF power amplifier to convert the continuous wave (CW) signal into pulse signal, with adjustable pulse width. The power gain is measured at 57.7 dB at 60 dBm output. The RF phase noise, which is measured by the low level RF system (LLRF) system, is less than 0.015 degree (RMS), while the pulse frontier jitter is less than 5 ns.

S-bandSolid-state power amplifierPhase stability

1 Introduction

Free Electron Laser (FEL), a new form of light source, is generated by using a high-quality relativistic electron beam as the lasing medium. With periodic variation in the magnetic field of the undulator, electromagnetic radiations are amplified by stimulated emission, producing a very bright beam of light [1, 2]. To achieve high brightness of FEL, it is important to stabilize the magnitude and phase of RF power source, with short pulses and good coherency [3].

With the development of RF semiconductor technology such as laterally diffused metal oxide semiconductor (LDMOS), in the recent years, solid-state RF power amplifiers (SSRFPA) for accelerators have been increasingly used as the leading end of klystron amplifiers [4-6]. Over vacuum tubes, SSRFPAs are advantageous in their availability, reliability, graceful degradation and ease of maintenance [7-9]. Also, an SSRFPA operates at a lower voltage, at which X-rays will not be generated [10]. A number of FEL labs have designed kilowatt S-band SSRFPAs [11, 12]. An SSRFPA with a peak power of 1 kW is used to drive the linac klystron of Beijing Electron Positron Collider, China. It uses a three-step amplification process, followed by a 4-way combination to amplify a 10 dB input signal into a 60 dB output signal [13]. The High Power Microwave Engineering Laboratory at Kwangwoon University, South Korea uses a 1.5 kW SSRFPA as a radar transmitter. The power amplifier operates at 2.7–2.9 GHz with eight stages of amplification, and the peak output power at 1.61 kW [14]. To drive a 2 MW klystron of a linac, the Information and Communication Technology Institute at Isfahan University of Technology, Iran developed a 2 kW multi-stage S-band SSRFPA, with 16 amplifier modules of 180 W each, producing a peak power of 2 kW, and adjustable pulse width of 2–10 µs [15].

A compact Soft X-ray Free Electron Laser test facility is now underway at Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Science [16, 17]. Fig. 1 shows schematically an RF station of SXFEL. The reference RF signal is modulated by a vector modulator in the MTCA4.0 LLRF system, amplified by a solid state amplifier and a klystron, and finally fed into the two S-band accelerating structures. The SXFEL device requests highly in the RF signal’s phase stability [18-20]. At S-band, the total phase noise should be less than 0.09°. The phase noise can be divided into the individual components by assuming that the jitter contribution is uncorrelated and equally distributed amongst the subsystems.

Fig. 1

RF system layout of SXFEL.

δ_{φ} = \sqrt{{(δ_{φ})}^{2}_{MO} + {(δ_{φ})}^{2}_{CLK ＆ LO} + {(δ_{φ})}^{2}_{LLRF} + {(δ_{φ})}^{2}_{AMP} + {(δ_{φ})}^{2}_{KLY}},

(1)

where, $δ_{φ}$ is the total phase noise; and the subscriptions of MO, CLK&LO, LLRF, AMP and KLY denote master oscillator, clock and local oscillator, low level RF system, solid-state power amplifier, and klystron, respectively. The phase noise requirements for the RF system in the SXFEL device are given in Table 1. The phase noise distributed to the solid-state power amplifier should be less than 0.015°. According to the requirements above we developed a three-stage SSRFPA working at 2856 MHz with an output power of 1 kW. This amplifier can be used to drive the klystron of SXFEL accelerator.

The 2856 MHz phase noise requirements of RF equipment

Parameter	Value (RMS)
Phase noise requirement	87.5 fs/0.09°
Master oscillator	40 fs/0.041°
Clock &LO	40 fs/0.041°
Low level RF system	20 fs/0.021°
Solid state amplifier	15 fs/0.015°
Klystron	61 fs/0.063°

In this paper, we report the SSRFPA structure, functions of each module, and the experiment results.

2 Description of the SSRFPA

2.1. General description

The 1 kW SSRFPA consists of a RF switch module, a pulse control module, a pre-amplifier module, a mid-amplifier module, a 4-way power divider, 4 final-amplifier modules and a 4-way power combiner, as shown schematically in Fig. 2. The RF switch is used to convert the input CW signal into a pulse signal. The pulse control module is used to adjust the pulse width. An RF signal passes through the pre-amplifier and mid-amplifier, and is then split into 4 paths by the power splitter to drive the final-amplifier individually. Lastly, it goes through the power combiner as an output. The entire amplifier is able to amplify a 0 dBm input signal into a 59.55 dBm output signal, with an amplification gain of 59 dB.

Fig. 2

Schematics of the 1 kW power amplifier.

2.2 The final-amplifier module

The final-amplifier module is the core component of the SSRFPA. As shown in Fig. 3(a), the transistor is MRF8P29300RH from Freescale Semiconductor Inc., TX, USA, for its high output power and good stability. It has two sub-transistors, working in the AB class mode [21].

Fig. 3

Schematics (a) and photograph (b) of the amplifier module.

The design of matching network has been optimized using the Advanced Design system (ADS) software, and adjusted according to the actual results. Due to the transistor’s push-pull amplifier design, it is essential to include an unbalanced- to-balanced signal converter structure a.k.a. balun in the matching network. Micro-strip lines T1-T4 and T13-T16 thus act as baluns. The two isolators ISO1 and ISO2 are at the input and the output of the power amplifier, respectively, to isolate the reflected current at the input and output, thus protecting the transistor [22]. The actual module is shown in Fig. 3(b).

The mid-amplifier module is identical to the final-amplifier module in structure and performance.

2.3. Other modules

The output pulse of the RF switch module has an outstanding leading and trailing edges.

The pulse control module is used to render the input time-to-live (TTL) signal adjustable. At the input end, an anti-electromagnetic interference device is used to increase the signal’s stability.

A schematic diagram of the pre-amplifier module is shown in Fig. 4.It consists of four cascaded power transistors, with three isolators in the middle protecting the circuit. An adjustable attenuator is added at the circuit input to adjust the amplification gain. At the DC input, a DC power protection network protects the transistor from breakdown. The power combiner and divider is made of three 90° electrical bridges, each of which uses a 1/4 wavelength micro-strip line to delay the signal phase by 90°.

Fig. 4

Schematic diagram of the pre-amplifier module.

3 Results and Discussion

3.1. The module performance tests

A parameter measurement platform (Fig. 5) was established, with a signal generator, a DC supply, a waveform generator, a power meter, a spectrum analyzer (SPA), a vector network analyzer (VNA) and a power attenuator. The test devices included a pulse switch module, a pre-amplifier module and a mid-amplifier module.

Fig. 5

Parameter measurement platform of the amplifier.

The signal generator (E8247C, frequency range: 250 kHz–20 GHz, power level: 15 dBm), the power meter (E4416A), the spectrum analyzer (E4440A, 3 Hz–26.5 GHz), and the VNA (E5071B, 300 kHz–8.5 GHz) are manufactured by Agilent Technologies, CA, USA. The waveform generator, TATFA20B, is from Atten Instruments.

The 4-port VNA is used to measure the S parameter, gain and phase characteristics of the test module. Its power and frequency scanning options are adjusted to suit the test module’s capability. The power meter is used to measure the amplifier module’s output power. The SPA is used to measure the phase noise. An input signal, from in a signal source, passes through the module under test. It is attenuated by a 40 dB attenuator and transmitted to the power meter and SPA. The signal generator generates a pulse signal of fixed width to provide the pulse control module with the original waveform.

Fig.6. shows the change in spectrum curve after signal amplification. A phase noise degradation of 0.57 dB can be seen in the single side band 50 kHz. The pulse transition response rate is higher than other digital circuit switch of the same cost. Also, the anti-electromagnetic interference device at the pulse switch’s input end lowers external interferences effectively.

Fig. 6

The change of the spectrum curve.

3.2. Performance of the SSRFPA

The assembled S-band solid-state power amplifier is shown in Fig. 7(a). Its input power, output power and power gain are shown in Fig. 7(b). It can be seen that the maximum power output was 59.9 dBm, with a power gain of 57.4 dB.

Fig. 7

The S-band solid-state power amplifier (a) and its output power and power gain (b).

The low level RF system to measure the phase stability developed at SINAP consists of the CLK & LO, MTCA4.0 (with the down converter DWC8VM1 and digitizer SIS8300L), and solid state amplifier (see Fig. 1).

The LLRF captures the 2856 MHz signal at the amplifier’s input and output ends separately, and then converts the frequency to 25.5 MHz. So, its samplings is through a 102 MHz clock. The data flow after sampling is: I., Q., –I. and −Q., where I. and Q. represent the in-phase and quadrature, respectively. After demodulating IQ, the I and Q signals can be obtained. The signals are calculated by an FPGA board using a CORDIC algorithm to finally obtain the phase jitter values at the amplifier’s input and output ends. An interface of the LLRF measurement system in Fig. 8 displays the phase and amplitude of signals from the amplifier’s input and output ends. The data collected are processed by Matlab.

Fig. 8

LLRF measurement system interface.

Fig. 9(a) shows the phase jitter values at the amplifier’s input and output ends, and their differences as well. Root mean square (RMS) jitter is an important parameter to describe the phase jitter. The time point of the n^th zero-crossing leading edge is referred to as t_n The n^th period is then defined as T_n= t_n+1 − t_n. The time difference varies with n as a result of noise in the circuit. This causes a deviation of ΔT_n= T_n− T_mean from the mean period T_mean. The quantity ΔT_n is an indication of jitter. RMS jitter can be written as:

Fig. 9

Measured phase jitter of RF signal (a) and Pulse envelope of output signal (b).

Δ T_{RMS} = \lim_{N \to \infty} \sqrt{\frac{1}{N} \sum_{n = 1}^{N} {(Δ T_{n})}^{2}} .

(2)

RMS jitter describes magnitude of the signal fluctuations [23]. Using Matlab, RMS phase jitter of the extracted samples is at 0.014°, while the system’s requirement is <0.015°. It ensured the stability of the amplitude and phase of FEL’s input RF signal, and is especially meaningful to the highly-bright and stable FEL. In Fig. 9(b), a single pulse envelope at output port shows that its frontier jitter is <5 ns. After calculations, the intra pulse flat top’s RMS jitter is 0.16%.

4 Conclusion

An S-band solid-state RF amplifier has been developed successfully. The measurememt results show that the output power is over 1 kW, the intra-pulse flat top is less than 0.2% RMS, and the RF phase stability is less than 0.015°. Low phase noise and commendable flatness in the pulse envelope are extremely meaningful to improve the stability and efficiency of FELs. All these are helpful for FELs to attain table performances.

References

P. Schmüser, M. Dohlus, J. Rossbach.

Ultraviolet and soft X-ray free-electron lasers: introduction to physical principles, experimental results, technological challenges

. Springer Tracts Mod Phys. 3-9 (2009). doi: 10.1007/978-3-540-79572-8