A Technique for the Rapid Analysis of Whistlers

This paper discusses the design of a new type of sound spectrograph, intended for analyzing whistlers. This instrument is of the single-channel scanning type, but its basic action is to scan the frequency-time plane in frequency at a fixed time rather than, as in existing instruments, in time at a fixed frequency. The principle is as follows. First, a very short section of the signal to be analyzed (roughly equal in duration to the reciprocal of the bandwidth of the analyzing filter) is stored in the instrument electronically. The stored signal is then read out repeatedly many times faster than its original speed, and this repeated waveform is analyzed by a variable-tuned filter which sweeps once very rapidly through the expanded frequency band which the signal now occupies. The varying output from the filter, representing the variations of the amplitude of the signal with frequency at one particular time, is recorded as one line of a scan across a continuous strip display. Finally, the stored sample of signal is erased, replaced with the next sample, and the whole process repeated. An audio frequency spectrograph of this type appears to combine speed of operation with fine resolution in frequency, rendering it useful for the rapid analysis of whistlers.