31. B.A. Munk,P. Munk, and J. Pryor, On designing Jaumann and circuit analog absorbers (CA absorbers) for oblique angle of incidence, IEEE Trans Antennas Propag 55 (186), 186–193. 32. A.K. Zadeh and A. Karlsson, Capacitive circuit method for fast and efficient design of wideband radar absorbers, IEEE Trans Antennas Propag 57 (2307), 2307–2314. 33. A. Itou,O. Hashimoto,H. Yokokawa, and K. Sumi, A fundamental study of a thin wave absorber using FSS technology, Electron Commun Jpn 87 (2004), 77–86 34. A. Itou,H. Ebara,H. Nakajima,K. Wada, and O. Hashimoto, An experimental study of a wave absorber using a frequency-selective surface, Microwave Opt Technol Lett 28 (321), 321–323. 35. G.I. Kiani,A.R. Weily, and K.P. Esselle, A novel absorb/transmit Fss for secure indoor wireless networks with reduced multipath fading, IEEE Microwave Wireless Compon Lett 16 (378), 378–380. 36. N. Engheta and R.W. Ziolkowski, Metamaterials: Physics and engineering explorations, Wiley-IEEE Press, Hoboken/Piscataway, NJ, 2006. 37. M. Xu,T.H. Hubing,J. Chen,T.P. Van Doren,J.L. Drewniak, and R.E. DuBroff, Power-bus decoupling with embedded capacitance in printed circuit board design, IEEE Trans Electromagn Compat 45 (22), 22–30. C 2011 Wiley Periodicals, Inc. V
FIBER LENGTH COMPENSATED TRANSMISSION OF 2998.01 MHZ RF SIGNAL WITH FEMTOSECOND PRECISION Jurij Tratnik,1 Leon Pavlovic,1 Bostjan Batagelj,1 Primoz Lemut,2 Patrik Ritosa,3 Mario Ferianis,4 and Matjaz Vidmar1 1 University of Ljubljana, Faculty of Electrical Engineering, Trzaska c. 25, 1000 Ljubljana, Slovenia; Corresponding author:
[email protected] 2 Instrumentation Technologies d.d., Velika pot 22, 5250 Solkan, Slovenia 3 Telekom Slovenije d.d., Cigaletova 15, 1000 Ljubljana, Slovenia 4 Sincrotrone Trieste S.C.p.A., Trieste, Italy Received 5 October 2010 ABSTRACT: An electro-optical RF-clock-distribution system for 2998.01 MHz is presented. An added timing jitter of 5 fsRMS and a clock drift of 9.5 fsRMS over 24 h have been measured. The RF signal is distributed through a transmitter–receiver topology as intensity modulation on a CW laser at 1550 nm. Temperature-induced fiber group-delay variations are compensated by adjusting the wavelength of the laser and making use of the fiber chromatic dispersion. The system uses standard, commercially-available telecommunication optical C 2011 Wiley Periodicals, Inc. Microwave Opt Technol components. V Lett 53:1553–1555, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26087
evolving fiber-optic solutions for the timing distribution and the RF synchronization use interferometric schemes for the stabilization [3–5] and correction [6] of fiber links that transport the clock signal or/and use mode-locked pulsed lasers [7]. A previously proposed electro-optical scheme in 2001 [8] somewhat similar to our setup used a single optical fiber and a directly modulated DFB laser. Because of the low RF clock frequency (below 1 GHz) used, that solution did not achieve better jitter results than classic coaxial cable. Our proposed electro-optical synchronization system includes a transmitter (Tx), located at the place of the low-jitter master oscillator and a receiver (Rx), located at the remote location. Both units are connected with a single-mode optical-fiber pair in a loop-back to measure and correct fiber group-delay variations. 2. OPERATING PRINCIPLE
The block diagram of the transmitter and the receiver is shown in Figure 1. The transmitter includes two main compensation blocks and a laser source, whereas the receiver includes a third, identical compensation block, and a clock-filtering flywheel cavity. The source of the optical signal is a commercially available DFB laser at 1550 nm (7 dBm), with an integrated electro-optical modulator (EOM) and thermo-electric cooler/heater (TEC). The EOM modulates the optical carrier with the external master RF-clock signal at 2998.01 MHz. The modulated signal is then propagated to the Rx unit, where part of the signal is decoupled and demodulated on the photodiode PD3. As the signal to noise ratio (SNR) at the output of the photodiode is very low for the intended application, a flywheel is used to filter the microwave clock signal. A 500 kHz-wide, high-Q-cavity band-pass filter at the 2998.01 MHz clock frequency is used as the flywheel in our system. Because any frequency drifts of the cavity (ageing, thermal) are converted to unwanted phase shifts, the cavity is thermoelectrically fine-tuned by a phase-locked loop (PLL). To compensate fiber group-delay variations, most of the incoming optical signal is fed back to the Tx unit using a second, identical optical fiber, where the signal is demodulated on the photodiode PD1 and compared with the reference signal. The phase-error signal controls the integrated TEC and, in this way, tunes the laser wavelength. Exploiting the fiber’s inherent chromatic dispersion, link-length (RF-signal group-delay) variations are compensated stabilizing the RF-signal phase throughout the forward and backward optical links. The control bandwidth of our system is only a few Hz, but, nevertheless, this is fast enough to compensate any perturbations on the fiber placed in the accelerator tunnel.
Key words: electro-optical synchronization system; clock transfer; phase noise; long-term phase stability; femtoseconds
1. INTRODUCTION
Precise timing and synchronization systems [1, 2] are needed to operate the fourth generation light sources based on linear accelerators driven free electron lasers (FELs). Traditionally, coaxial cables have been used for radio frequency (RF) and microwave clock distribution. Optical-fiber systems have two main drawbacks for this application: (1) a relatively high temperature coefficient of the fiber refraction index and (2) a relatively low signal-to-noise ratio resulting in a high timing jitter. Some
DOI 10.1002/mop
Figure 1 Block diagram of the electro-optical clock-distribution system. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 7, July 2011
1553
confirmed by measurements) that the PMD is lower than 10 fs and can be neglected in the 360-m long fiber. To achieve such a low total PMD, G.652 category optical fibers with a specified PMD of less than 0.02 ps/(km)1/2 were selected.
3. MEASUREMENT RESULTS
Figure 2 Measured RMS jitter of the Rx-output signal is 13.4 fs (100 Hz to 10 MHz) - dark curve, and the RMS jitter of the reference signal is 12.4 fs (100 Hz to 10 MHz) - bright curve. The calculated RMS added jitter of the clock-distribution system is 5 fs (100 Hz to 10 MHz)
The integrated EOM is subjected to large temperature variations due to the thermoelectric laser tuning on the same chip. Part of the modulated optical signal is also fed to the photodiode PD2 and compared with the reference signal to correct any phase changes in the laser, EOM and corresponding RF driver with an electrical phase shifter (EPS). A variable gain amplifier (VGA) is used to maintain a constant modulation depth in the EOM. Adaptive EOM biasing is used to control the second harmonic of the RF signal. All photodiodes are constantly fed with equal optical powers (0 dBm) by monitoring the DC photo current on the PD2 and properly adjusting the laser bias current over the entire wavelength tuning range so that any illumination intensity to RF phase conversion can be avoided. All RF components except the laser module, which is independently heated or cooled, are kept in precisely temperaturecontrolled chambers to an accuracy of 60.01 C. Temperaturestabilized chambers minimize thermal drift to achieve long-term stability. Most important, three identical control blocks with three identical photodiodes PD1,2,3, identical amplifiers, and identical phase comparators are used so that any temperature, long-term ageing, RF signal amplitude, modulator harmonics, power-supply, and other variations effectively cancel out in the final output-signal phase. With a 5-nm laser-wavelength tuning range (50 C of temperture difference of the laser module), 66 of RF phase compensation at 2998.01 MHz can be achieved in a 720-m fiber-loop length [9]. Therefore, at system initialization, a mechanical phase shifter (MPS) in the Tx unit is used for a rough phase setting so that the laser is put in the middle of its tuning range. At every subsequent power-up of the transmission system, the phase of the RF signal does not have to be reset. During operation, manageable temperature changes in the optical path are around 2.8 C and can be calculated as DT¼(cDDk)/(kn þ nkt), where DT is the temperature change, c is the speed of light, D ¼ 17 ps/nm km is the chromatic dispersion coefficient, Dk is the wavelength-tuning range, n is the refractive index of the fiber, kn ¼ 8 106/K is the temperature coefficient of the refractive index and kt ¼ 7.5 107/K is the temperature expansion coefficient of the glass-fiber length. The proposed electro-optical system is, however, not compensating the polarization-mode dispersion (PMD) effects. For both optical lines (transmission and return), it was assumed (and
1554
Several measurements of the RMS added jitter and long-term stability were made on the proposed clock-distribution system with different optical fiber lengths. A 2998.01 MHz ultra-low jitter generator, developed at University of Ljubljana, was used as a master-clock oscillator (reference signal), and the output of the system was connected to a signal source analyser Agilent E5052A. RMS added jitter jittadd is calculated as jittadd ¼ (jitt2meas jitt2gen )1/2, where jittmeas is the measured RMS jitter of the complete transfer chain, and jittgen is the master-oscillator jitter. Single-mode optical fibers adopted for the distribution of the master reference on FERMI@Elettra, Italy, have been used during the field tests. The length of the optical link was 360 m (one way) including two optical rack-mount enclosures and several standard patch cords. The RMS added jitter of the clock-distribution system is 38 fs (10 Hz to 10 MHz) and 5 fs (100 Hz to 10 MHz), respectively. The latter is calculated from measurement results shown in Figure 2. The long-term phase stability of the proposed system was measured with a setup shown in Figure 3(a). The master-RF-oscillator signal was compared to the signal transfered over the 360-m long compensated optical link with an idependent phase detector built on purpose in different technology (Gilbert-cell AD8302) from that used in the link electronics (doubly balanced diode mixers). The independent phase detector was installed in its own, independent, thermally stabilzed enclosure. The detected phase difference on the phase detector was measured with a Datron 1281 multimeter (integration time was 5 sec) and sampled every minute with a computer acquisitioning system. With such system, we obtained a 9.5 fs RMS time drift in a 24h period and a 13.4 fs RMS time drift in a 38-h period, respectively. During that time the range of thermal perturbations in the tunnel was 0.4 C (between 26.6 and 27.0 C), and the temperature of the laser varied by 7 C. The relative time difference
Figure 3 (a) The long-term phase-stability measurement system and (b) relative time difference (long-term stability) between the master RF oscillator and the 360-m fiber link is 9.5 fs in a 24-h period and 13.4 fs in a 38-h period, respectively. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com]
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 7, July 2011
DOI 10.1002/mop
and the curve of the laser temperature tracking are shown in Figure 3(b). 4. CONCLUSIONS
We have shown that a CW-clock transfer is possible over several 100-meters long links using affordable and commercially available telecom-grade optical and RF components achieving an extremely low-added timing jitter and high long-term stability. Group-delay variations of the RF signal in the presented clock-distribution system are compensated by the laser-wavelength tuning and the exploitation of the chromatic dispersion of the optical fibers in the forward and backward direction. Besides our fully compensated and filtered clock-distribution scheme, we believe that our outstanding results are also due to the excellentquality, mass-produced electrical and optical components that became available recently: (1) microwave and optoelectronic components: monolithic gain blocks with stable gain, monolithic mixers with excellent balance and offset used as phase detectors, integrated PIN-FET modules with repeatable performance and (2) mass-produced optical fiber to very tight specifications including PMD. Although PMD effects were not compensated in our system, we experimentally confirmed that all fiber tolerances including PMD are tight enough to allow almost perfect tracking in an inexpensive fiber pair, a much cheaper solution than using a single fiber together with fiber couplers, circulators and/or Faraday mirrors.
MODIFIED LINC TRANSMITTER USING EFFICIENT PARALLEL AMPLIFICATION SCHEME Rui Liu,1,2 Dominique Schreurs,2 Walter De Raedt,1 and Robert Mertens1 1 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium; Corresponding author:
[email protected] 2 ESAT-TELEMIC, K.U.Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium Received 5 October 2010 ABSTRACT: A modified linear amplification with nonlinear component (LINC) transmitter is presented using a novel power combination scheme. By using multiple LINC sections in parallel with a compact adaptive power combining network, the output power of power amplifiers can be boosted, together with obtaining higher efficiency over a large power back-off region. Compared to existing designs, no gate bias adaptation is needed and efficiency can be further improved. C 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1555– V 1558, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26086 Key words: RF power amplifier; parallel amplification; LINC; Chireix outphasing amplifier; adaptive matching 1. INTRODUCTION
ACKNOWLEDGMENTS
This work was supported by Instrumentation Technologies d.d., Slovenia, Sincrotrone Trieste S.C.p.A., Italy, and Ministry of Higher Education, Science and Technology of the Republic of Slovenia under the research program P2-0246. The authors are grateful to V. Poucki and M. Predonzani for help with long-term measurements and data acquisition.
REFERENCES 1. E. Allaria, C.J. Bocchetta, D. Bulfone, F. Cargnello, D. Cocco, M. Cornacchia, and D. Wang, FERMI@Elettra: A seeded FEL facility for EUV and soft X-rays, FEL 2006, MOPPH054 (2006) 166–169. 2. M. Ferianis, State of the art in high-stability timing, phase reference distribution and synchronization systems, PAC09, WE3GRI02 (2009). 3. J.M. Byrd, L. Doolittle, A. Ratti, and R. Wilcox, Timing distribution in accelerators via stabilized optical fiber links, LINAC 2006, THP007 (2006) 577–579. 4. J.W. Staples, R. Wilcox, and J.M. Byrd, Demonstration of femtosecond-phase stabilization in 2 km optical fiber, PAC07, MOPAS028 (2007) 494–496. 5. J.W. Staples, J. Byrd, L. Doolittle, G. Huang, and R. Wilcox, A femtosecond-level fiber-optics timing distribution system using frequency-offset interferometry, LINAC08, THP118 (2008) 1063– 1065. 6. R. Wilcox, J.M. Byrd, L. Doolittle, G. Huang, and J.W. Staples, Stable transmission of radio frequency signals on fiber links using interferometric delay sensing, Opt Lett 34 (2009), 3050–3052. 7. A. Winter, J. Becker, F. Loehl, K. Rehlich, S. Simrock, and P. Tege, An integrated optical timing and RF reference distribution system for large-scale linear accelerator, LINAC 2006, THP003 (2006) 565–567. 8. J. Frisch, D. Bernstein, D. Brown, and E. Cisneros, A high stability, low noise RF distribution system, PAC 2001 (2001), 816–818. 9. E. Udd, Fiber optic sensors, Wiley, Hoboken, NJ, 2006. C 2011 Wiley Periodicals, Inc. V
DOI 10.1002/mop
Modern wireless systems adopt nonconstant envelope signals with a high peak to average power ratio (PAPR) to achieve the increasing demand for spectrum efficiency. These force the power amplifier (PA) to work at a large power back-off region usually associated with a low efficiency. Moreover, the low breakdown voltage and high knee voltage of current deep-submicron CMOS processes bring a big challenge in PA implementation. These obstacles directly limit the available voltage swing over a given load, thereby limiting the output power. To fulfill the output power requirement and balance the trade-off between linearity and efficiency of CMOS PAs, some researchers have proposed the parallel amplification concept with efficient power control capability [1, 2], as shown in Figure 1. While the drawbacks are that the efficiency can only be improved at limited back-off regions and that by adapting gate voltage with time varying input envelope noise and distortion may be introduced in reality. Nevertheless, linear amplification using nonlinear components (LINC), also named as Chireix outphasing architecture, is one of the most promising techniques that can simultaneously provide high efficiency and high linearity [3–6].
Figure 1 Conventional parallel amplification with discrete power control. (a) System diagram. (b) Efficiency versus power back-off of an ideal operation
MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 53, No. 7, July 2011
1555
