Abstract
This paper describes a comparative nonlinear analysis of low-noise amplifiers (LNAs) under different stimuli for use in astronomical applications. Wide-band Gaussian-noise input signals, together with the high values of gain required, make that figures of merit, such as the 1 dB compression (1 dBc) point of amplifiers, become crucial in the design process of radiometric receivers in order to guarantee the linearity in their nominal operation. The typical method to obtain the 1 dBc point is by using single-tone excitation signals to get the nonlinear amplitude to amplitude (AM-AM) characteristic but, as will be shown in the paper, in radiometers, the nature of the wide-band Gaussian-noise excitation signals makes the amplifiers present higher nonlinearity than when using single tone excitation signals. Therefore, in order to analyze the suitability of the LNA's nominal operation, the 1 dBc point has to be obtained, but using realistic excitation signals. In this work, an analytical study of compression effects in amplifiers due to excitation signals composed of several tones is reported. Moreover, LNA nonlinear characteristics, as AM-AM, total distortion, and power to distortion ratio, have been obtained by simulation and measurement with wide-band Gaussian-noise excitation signals. This kind of signal can be considered as a limit case of a multitone signal, when the number of tones is very high. The work is illustrated by means of the extraction of realistic nonlinear characteristics, through simulation and measurement, of a 31 GHz back-end module LNA used in the radiometer of the QUIJOTE (Q U I JOint TEnerife) CMB experiment.
Highlights
In the design process of radiometric receivers,[1] linearity is one of the main goals to be achieved.[2,3,4,5] In these receivers there are two main sources of nonlinear behavior: detectors and low-noise amplifiersLNAs
This is the case of radiometric receivers where the excitation signal comes from the sky in the form of a wide-band Gaussian-noise signal to be filtered and amplified in the band of interest
As will be shown in this work, the different nature of this kind of excitation signal, with respect to the typical one-tone test signal, makes the receiver LNAs present a more nonlinear behavior than it would be expected for the single-tone nonlinear characteristics.[12]
Summary
In the design process of radiometric receivers,[1] linearity is one of the main goals to be achieved.[2,3,4,5] In these receivers there are two main sources of nonlinear behavior: detectors and low-noise amplifiersLNAs. Linearity of astronomical detectors is considered an important issue.[6,7,8,9,10,11] In the particular case of LNAs, linearity is used to be defined by taking into account a figure of merit such as the 1 dBc point and setting an appropriate input power back-off level This point is part of the nonlinear amplitude to amplitudeAM-AMcharacteristic, typically estimated by means of single-tone excitation signals. Nonlinear characteristics with wide-band Gaussian-noise excitation signals are obtained by simulation and measurement This kind of signal can be considered as a limit case of a multitone signal with a very high number of tones.
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