Testing, aligning, and troubleshooting a conventional amplitude-modulation (AM)/frequency-modulation (FM) radio requires a good grasp of DC and AC electronic basics and RF behavior. A thorough knowledge of signal generators and oscilloscopes also is needed. In "Arbitrary/Function Generator Offers Versatile and Convenient Solutions for AM/FM Radio Test and Alignment," Tektronix (Beaverton, OR) shows how these tools support FM radio measurements spanning 88 to 108 MHz.
In its examples, this 12-page application note uses a digital storage oscilloscope as the analysis tool. An arbitrary/function generator (AFG) serves as the test input signal source. The radio is assumed to be a monaural unit switchable to either the AM or FM band.
A superheterodyne radio converts incoming signal frequencies to a lower frequency via a frequency mixer. The receiver mixes a signal from the built-in local oscillator (LO) with the incoming signals from the antenna. By adding and subtracting the two different signals, two new signals are produced: one at the frequency sum and the other at the frequency difference of the original two signals.
The resulting signals are said to be at an intermediate frequency (IF), which continues through the radio circuit. There, it is filtered, amplified, and demodulated to produce the final audio output. Through application examples, the note details some key test steps for the superheterodyne receiver. It strives to show how the setting up or troubleshooting of an FM radio can be simplified with a versatile signal source, built-in modulation, and a multichannel signal source.
Tektronix, Inc., 14200 SW Karl Braun Dr., P.O. Box 500, Beaverton, OR 97077; (800) 835-9433, Internet: www.tektronix.com