Automatic Frequency Response of Frequency-Modulated Generators Using the Bessel Null Method

This paper describes a Bessel null technique to measure the frequency response of a frequency-modulated rf carrier and a program to automate frequency response measurements of signal generators with output frequencies from 0.450 to 2000 MHz. The measurements obtained using this technique are more accurate than those obtained by a highly trained technician using a manual system. Automated measurement of this process is desirable since the manual method is subject to the following problems: (1) excessive time, (2) error in finding the null, and (3) lack of assurance that the null is the first Bessel null. Automated measurements can be performed using a system controller, a spectrum analyzer, a function generator, and a voltmeter (all of which are compatible and control1ab1e remotely). The nonlinear relationship between modulating signal amplitude and the center frequency amplitude of the carrier is a major obstacle to automated measurement. This problem was solved by obtaining an approximate formula for this nonlinear curve. Assurance that the null found is the first Bessel null is provided by the analysis of the frequency response of the test signal generator as displayed on the spectrum analyzer.