1. Introduction
Ultra-low phase noise is a critical requirement for radar test equipment, especially in aerospace and defense applications. Radar systems often operate using pulse signals, where the pulse width and pulse repetition frequency (PRF) directly influence the radar's range and velocity resolution. For instance, early warning radars typically require long pulse widths and low PRF signals, while pulsed Doppler (PD) radars demand narrow pulse widths and high PRF signals. Accurately measuring the phase noise of these pulse signals under varying conditions has become increasingly important.
Traditionally, testing phase noise in pulse signals was a complex and costly process, requiring synchronization between the reference pulse source and the device under test. Moreover, the ability to measure at different pulse widths and PRFs was limited by the number of available PRF filters. However, this challenge is now easily overcome with the R&S FSWP analyzer equipped with the R&S FSWP-K4 option. This advanced tool simplifies the process—allowing engineers to perform all necessary measurements with just one click. It automatically captures the signal, calculates key parameters like PRF and pulse width, and constructs digital PRF filters. The system then demodulates the signal, displays both phase and amplitude noise, and automatically handles maximum offset frequency ranges and calibration, eliminating the need for manual parameter setup.
Engineers can also define custom pulse gate parameters to avoid interference from transient edge effects, which could otherwise reduce measurement accuracy. Additionally, the cross-correlation technique can be used to improve sensitivity when measuring signals with lower power, compensating for the loss caused by pulse modulation.
Equation 1 below illustrates the expected improvement in dynamic range:
ΔL = 5·log(n) [1]
Where ΔL represents the increase in phase noise sensitivity in dB, and n is the number of cross-correlations. For example, if 10 cross-correlations are performed, the sensitivity increases by approximately 5 dB.
2. Theoretical Analysis
A common method for generating pulse-modulated signals involves amplitude modulation of a continuous carrier wave with a pulse waveform. Before modulation, several standard terms related to pulse signals are defined. Figure 1 shows the waveform of a typical pulse signal, while Table 1 outlines its main parameters.
[Image: Pulse waveform]
Figure 1: Pulse waveform
[Image: Standard terms for pulse signals]
Table 1: Key parameters of pulse signals
Understanding both the time-domain and frequency-domain characteristics of a pulse signal is essential. From the principles of amplitude modulation, we know that multiplying the carrier and the modulating signal in the time domain results in convolution in the frequency domain. After pulse modulation, the frequency spectrum of the signal becomes a discrete spectrum, spaced at intervals equal to the PRF. The shape of the spectrum follows a sinx/x function, with the zero crossings located at the reciprocal of the pulse width.
[Image: Power spectrum after pulse modulation]
Figure 2: Power spectrum after pulse modulation
This theoretical understanding helps in designing and analyzing pulse radar systems, ensuring accurate phase noise measurements across various pulse configurations.
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Description
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