A plotter of intermodulation distortion

For use mainly in audio systems, the instrument plots on a CRT screen or chart recorder the amplitude of the difference-frequency tone vs the lower excitation frequency in CCIF intermodulation distortion measurements. The unit furnishes two swept equal-amplitude tones with a selected audio difference frequency to excite the tested system. Readout is on a modified audio-frequency spectrum analyzer which remains tuned to the difference frequency. The automatic excursion provides considerable test time economy in quality measurements of such devices as loud-speakers which exhibit wide variation in intermodulation with excitation frequency. Point-by-point techniques, in addition to being tedious, run the risk of missing critical conditions. With slight modification, the unit is also capable of plotting the lower third-order distortion component at (2f
1
-f
2
) vs excitation frequency. A swept spectrum analyzer and means for single-line plots of frequency response curves are also integral components of the equipment. Switching facilities permit alternate displays on successive scans of any two of the modes of operation. For example, acoustic output and IM distortion vs frequency of loud-speakers may be plotted alternately for rapid comparisons.