Sideband stabilization in the presence of LO phase noise: analysis and system demonstrator Friederike Brendel, Thomas Zwick, Julien Poette, Beatrice Cabon To cite this version: Friederike Brendel, Thomas Zwick, Julien Poette, Beatrice Cabon. Sideband stabilization in the presence of LO phase noise: analysis and system demonstrator. International Journal of Microwave and Wireless Technologies, Cambridge University Press/European Microwave Association 2014, 6 (02), pp.129-137. 10.1017/S1759078713001025. hal-01954322 HAL Id: hal-01954322 https://hal.archives-ouvertes.fr/hal-01954322 Submitted on 12 Feb 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. INTERNATIONAL JOURNAL OF MICROWAVE AND WIRELESS TECHNOLOGIES, VOL. X, NO. X, X 2013 1 Sideband Stabilization in the Presence of LO Phase Noise: Analysis and System Demonstrator Friederike Brendel, Thomas Zwick, Julien Poëtte and Béatrice Cabon Abstract—We present a technique allowing the stabilization 50 and tuning of a modulation sideband in the presence of high carrier frequency jitter and increased carrier phase noise. This 40 technique is of particular interest in communication systems 30 σ = 15° where oscillators providing the carrier signal cannot be stabilized rms by a conventional phase-locked loop, such as systems relying on σ = 10° low-cost optical LO generation techniques. The results obtained 20 rms in simulation are validated by measurements carried out on a σ = 5° modular system demonstrator. 10 rms σ Error vector magnitude in % = 1° Index Terms—frequency jitter, phase noise, phase-locked loop, rms 0 radio-over-fiber systems, mode-locked lasers. 0 10 20 30 40 50 60 70 80 Signal to noise ratio in dB Fig. 1: Impact of phase noise on EVM. Dashed line: ideal I. INTRODUCTION case, no phase noise. HE trend in modern radio communication systems has T been moving towards carrier signals at millimeter wave (mm-wave) frequencies. Signals at such high frequencies can loops and is therefore problematic in the design of high data be generated through the frequency multiplication of a lower rate communication systems. frequency signal, or by optical heterodyning, where a signal A communication system has to adhere to a well-defined at an arbitrary frequency can be generated by detecting the spectral mask. Limited long-term stability in frequency, fre- interference of two or more phase-locked optical laser modes, quency jitter, or imprecise selection of the band center fre- separated by the desired carrier frequency, by means of a quency can cause leakage into adjacent channels. At the photo-detector (PD). An electrical double sideband (DSB) receiver, requirements on the carrier recovery mechanisms spectrum is created at the PD if the laser pump current must be tightened. Furthermore, increased phase noise will is varied with a modulation signal. Any instability present spoil the signal quality even at a stable center frequency. Error in the heterodyne carrier signal translates into an instability vector magnitude is a common figure of merit for transmission present in the desired output signal of the system, namely the quality defined in most communication standards (see e.g. information-carrying sidebands, which is why the transmission [3]). Georgiadis has investigated the relationship between the quality is strongly dependent on the purity of the carrier. It signal-to-noise ratio (SNR), the root-mean-square (rms) phase thus ultimately depends on how well the phases of the optical error srms and the EVM for the case where the peak symbol modes can be locked together. power equals the average symbol power [4]: r We have previously demonstrated the impact of phase noise 1 EVM = + 2 − 2cos(s ): (1) in an mm-wave communication system where the carrier rms SNR rms signal is provided by heterodyning the optical modes of a As can be seen from figure 1, s 6= 0 introduces a constant free-running passively mode-locked laser diode (MLLD) [1]. rms error floor even for very high SNR values (the dashed line Such laser diodes are potentially low-cost devices which are corresponds to s = 0). It is thus paramount for any com- easy to integrate, but they exhibit rather high frequency jitter rms munication system to minimize s by stabilizing the transmit and/or phase noise (-55 dBc/Hz at 10 kHz offset from the rms (Tx) signal in both frequency and phase. carrier, and a carrier variation of ≈600 kHz) [2]. Furthermore, Typically, phase-locked loops (PLLs) are employed to sta- the precision of frequency adjustment in the manufacturing bilize the inherently unstable signal of a VCO [5]. The PLL process is limited to a few hundred MHz to a few GHz. Unlike principle relies on the fact that there exists a monotonous a voltage-controlled oscillator (VCO), the MLLD does not give relationship between the physical quantity influencing the access to a physical quantity which can be used to fine-tune oscillation (e.g. the capacitance in an LC tank in a VCO) and the oscillation - which hinders the construction of stabilization an electrical signal controlling this physical quantity (e.g. the F. Brendel was and T. Zwick is with the Institut für Hochfrequenztechnik voltage). The PLL principle has been extended to include laser und Elektronik at the Karlsruhe Institute of Technology, D-76131 Karlsruhe, oscillators and opto-electronic components in optical PLLs Germany email: [email protected]. [6]. If, however, the design of the oscillator does not allow J. Poëtte and B. Cabon are with the Institut de Microélectronique, Electro- magnétisme et Photonique, F-38016 Grenoble cedex 1, France. access to such a physical quantity (as in the case of various Manuscript received x, 2012; revised x, 2013. laser-based techniques, where the oscillation cannot be fine- INTERNATIONAL JOURNAL OF MICROWAVE AND WIRELESS TECHNOLOGIES, VOL. X, NO. X, X 2013 2 tuned by varying the laser pump current), the construction of a = STAGE 3= LO conventional PLL is not feasible. Other techniques commonly ys employed in the context of laser physics like optical active f nMϕ mode-locking [7] or optical feedback loops ([2], [8]) tend to B LO LO π3K MM System be complex and allow at most a stabilization of frequency - OUT f nMϕ’ but no tuning capability. IF IF In this work, we present a full analysis of a novel sta- ys bilization and tuning architecture allowing the stable trans- fRF= fLOg3=IF mission of a sideband signal in the presence of long-term ϕRF= ϕLOg3=ϕ’IF C carrier instability (jitter or drift) and increased phase noise; MM its key principle was presented in [9]. Here, we show a complete system demonstrator encompassing full heterodyne AMP3 AMP up-conversion, and we enclose a thorough Laplace analysis K of the resulting noise suppression. The paper is organized as RECOVERY follows: In section II, the stabilization technique is introduced. BPFK CAM We discuss the capture ranges of the proposed architecture in section III. The loop response to phase noise is evaluated in f nMϕ’ f nMϕ Km LO LO terms of the phase power spectral density. Simulations and A IF IF measurements on the test hardware according to section IV OUT KV = are compared in section V. The results are discussed in section AMPY fLOg3=IF KP yLOg3=IFs3 VI, and we will conclude the work in section VII. ϕ’LOg3=ϕIF n LF 3PFD BPFY D fref ϕref fref3n ϕref3 II. STABILIZATION PRINCIPLE ref 3 M The proposed architecture is depicted in figure 2. The bold MLMRCHITECTURE letters A-D are intended for an easy comparison between the block diagram and the hardware implementation shown in figure 3. For a discussion of the inclusion of an optical sub- Fig. 2: Transmitter architecture: Stabilization principle. Letter system, refer to [9]. The key ideas are the following: A-D correspond to points A-D in figure 3. • The complete stabilization structure incorporates a PLL modified by a loop mixer and a band-pass filter, as well as an up-conversion stage where the DSB spectrum is the loop for the reference phase jref as input signal: created and a carrier-recovery stage on the Tx’s side. KV • The modified PLL acts on the mixing product of the car- GF(s) = KP · ZLPF(s) · ; (2a) rier signal provided by the up-conversion stage (mixing s 1 of LO and IF oscillators). G (s) = ; (2b) R N • This mixing product, i.e. the upper (USB) or lower (LSB) sideband in the DSB spectrum, is the desired where KP is the phase-detector gain, ZLPF is the loop filter output signal of the system. The mixing product carries transfer function, KV is the sensitivity of the VCO, and N is the contributions from both the LO and IF oscillators; the frequency divider value [5]. Here, these functions must regarding the phase noise present in the mixing signal, be modified to include additional components: the loop mixer it is correlated to both sources. The PLL can thus be and the band-pass filter BPF1, as well as an amplifier AMP1 constructed either to act on the LO - which, as we have necessary in order to provide sufficient RF power to the PLL. stated before might not be possible in the case of optical 1) Effect of a mixer: Regarding the phase or the frequency LO generation - or on the IF oscillator.