MEASURING A PERIODICALLY MODULATED SIGNAL USING SPECTRAL STITCHING
Publication Date: 2017-May-04
The IP.com Prior Art Database
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A mixer-based receiver typically down-converts a radio-frequency (RF) signal to a predefined lower frequency signal known as an intermediate frequency (IF) signal. Down-converting the RF signal to the IF signal allows the use of circuitry that can operate at lower frequencies and also provides a predefined IF bandwidth that can be used to accommodate a certain range of RF signals coupled into the mixer-based receiver. The use of such a mixer-based receiver has proved quite successful in a wide variety of applications. However, certain types of RF signals can present unique challenges to a conventional mixer-based receiver. More particularly, a conventional mixer-based receiver having a fixed, predefined IF bandwidth may be unable to accommodate a periodically modulated RF signal having certain characteristics. These characteristics can include: energy centered around integer multiples of a fundamental carrier frequency (fc); a difference between a maximum and a minimum frequency contained in each harmonic spectrum exceeding an IF bandwidth of the mixer-based receiver; and a difference between a maximum frequency in a given harmonic spectrum and a minimum frequency of an adjacent harmonic spectrum exceeding the IF bandwidth of the mixer-based receiver. Fig. 1 below illustrates the characteristics of such an RF signal.
Fig. 1: RF input signal with respect to IF bandwidth of a mixer-based receiver
In one practical application where an RF signal such as the one shown in Fig. 1 may be encountered, is a power amplifier that is provided with a continuously aggregated 5-carrier LTE-A signal with a bandwidth of 100 MHz and a carrier frequency (fc) of 1.8 GHz. As a result of spectral re-growth, the bandwidth of the fundamental power amplifier output signal can easily exceed 300 MHz (corresponding to the 1st harmonic spectrum shown in Fig. 1). Moreover, in the case of a broadband power amplifier having a non-linear response characteristic, signal energy in the power amplifier output signal will be centered around not only at the fundamental frequency (fc) but around the harmonic carriers (2fc, 3fc, … nfc) as well. Understandably, a measurement instrument, such as a spectrum analyzer having a mixer-based receiver and an IF bandwidth of about 40 MHz, will be unable to handle such a power amplifier output signal without the application of certain specialized measurement techniques. In one such specialized measurement technique that is used conventionally, a local oscillator (LO) frequency in the spectrum analyzer is changed to measure each of a number of segments of the wideband spectrum that corresponds to the power amplifier output signal. A spectrum stitching technique is then applied to the individual segments to reconstruct the overall spectrum. The major disadvantage in this conventional measurement approach is that each LO setting and data acquisition step (da...