Six-Port-Based Architecture for Phase Noise Measurement in the UWB Band

Six-Port-Based Architecture for Phase Noise Measurement in the UWB Band J. M. Ávila-Ruiz, A. Moscoso-Mártir, E. Durán-Valdeiglesias, L. Moreno-Pozas, J. de-Oliva-Rubio, and I. Molina-Fernández Departamento de Ingeniería de Comunicaciones, ETSI Telecomunicación, Universidad de Málaga, 29071 Málaga, Spain Received 31 July 2013; Revised 29 November 2013; Accepted 4 December 2013; Published 2 January 2014 Academic Editor: Mohamed Helaoui Copyright © 2014 J. M. Ávila-Ruiz et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract A six-port-based frequency discriminator for phase noise measurement is proposed. This circuit makes use of a delay line discriminator configuration, thus not requiring reference oscillator. Furthermore, the use of a six-port network allows an extremely simple and completely passive solution well suited for low power oscillator measurement. A detailed study of the architecture is performed including the system noise sources. Besides, a prototype of the proposed six-port based delay line frequency discriminator is evaluated. Phase noise measurements of a commercial RF VCO are performed and compared with the results obtained with commercial metrology equipment with good agreement. 1. Introduction The high data rate transmission requirements of current radiofrequency standards demand efficient and robust modulation techniques. One of these modulation techniques is Orthogonal Frequency Division Multiplexing (OFDM), which is widely used in wireless communications due to its good performance against noise and multipath effects. On the other hand, OFDM receivers are very sensitive to local oscillator phase noise, which produces adjacent channel crosstalk due to reciprocal mixing [1, 2]. This adjacent channel crosstalk increases bit error rate (BER) [3, 4]. Phase noise effects in radiofrequency communications are the reason why feedback systems, such as phase locked loops (PLL), have been used for phase noise reduction for 80 years [5, 6]. Moreover, a great amount of new on chip low noise oscillators designs are carried out every year [7, 8]. An outstanding issue, as important as the design of low phase noise oscillators, is the accurate measurement of their phase noise. Modern oscillators with low phase noise level require metrology systems with even more demanding specifications as well as measurement methods to compensate their limitations [2, 9]. Several methods have been defined for phase noise measurement, depending on the characteristics of the system to be measured. As a main distinction, these methods can be first classified depending on whether they require a reference oscillator or not. This differentiation is important, since reference oscillator phase noise will increase the noise floor of the measurement system. In order to increase the instrument sensitivity, ultralow phase noise reference oscillators are needed. Therefore, high performance systems that rely on reference oscillators are more expensive than other instrument systems. The group of phase noise measurement techniques that make use of a reference oscillator comprises the direct spectrum measurement and the PLL techniques. The direct spectrum measurement is the most straightforward method (Figure 1(a)). The spectrum of the oscillator under test, from now on the device under test (DUT), can be directly measured with a spectrum analyzer or previously down-converted to an intermediate frequency (IF) using a reference oscillator and a mixer. The PLL method makes use of a mixer as a phase detector (Figure 1(b)). Then, the output of the phase detector is low-pass-filtered to eliminate the higher frequency terms, retaining only the baseband component. The phase noise of the DUT is extracted from the spectrum of the low-pass filtered output. In this method, a PLL is needed to lock the reference oscillator to the oscillator under test. Figure 1: Phase noise measurement methods with reference oscillator. (a) Direct downconversion method and (b) PLL method. The need of a reference oscillator is removed by the second group of phase noise measurement methods, which make use of a frequency discriminator. The most used one is the delay line frequency discriminator. A typical phase noise measurement system based on a delay line discriminator is shown in Figure 2. This method makes use of a mixer-based phase detector, as in the aforementioned PLL method. However, in this architecture, the RF signal from the oscillator under test is compared in the phase detector with a delayed version of itself. A phase shifter must be included in order to keep the quadrature condition between both signals at the phase detector inputs [10]. Even though this method gets rid of the reference oscillator, it has an important drawback: the complex circuitry needed to control the phase shifter and keep the system into the quadrature condition. Figure 2: Delay line frequency discriminator method. Recently, a new architecture for phase noise measurement based on delay line discriminator has been proposed [11]. In this circuit, a dual phase detector architecture and a 90-degree hybrid allow the detection of the phase noise from the in-phase and quadrature signal components without quadrature adjustment. Even though this architecture needs neither reference oscillator nor quadrature adjustment circuitry, it has an important disadvantage: its input-referred phase noise floor considerably increases at lower input power levels [11]. A good solution to improve the phase noise measurement of this new architecture is the utilization of a six-port demodulator (see Figure 3). Six-port demodulators can work with low input power levels and their calibration has been widely studied [12]. The six-port architecture has been previously implemented in [13], for W-band phase noise measurements, but using a reference oscillator, obtaining good accuracy in phase noise measurement of one-port and two-port systems. Figure 3: Equivalence between [11] mixer architecture and classical six-port. The architecture proposed in this work makes use of the idea of dual phase noise detection based on a delay line discriminator [11] and the six-port architecture used in [13] in order to implement a compact phase noise measurement system. This architecture uses neither reference oscillator nor a phase shifter for quadrature adjustments. Moreover, the existence of powerful calibration strategies for six-port demodulators [12] and their good performance for low input power levels make the use of this structure advantageous against traditional mixers. The rest of this paper is organized as follows. In Section 2, the six-port-based delay line discriminator architecture is described. In Section 3, the noise contributions and their effects are detailed. In Sectio (...truncated)