This feature that some radios have seem to have various names:
- band scope
- spectrum scope
- water fall
I understand the first three names, but not panadapter. Where does it come from?
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Marcel Wallace founded Panoramic Radio Corp. and details his "Panoramic radio receiving system" in patent US2279151A (1938). Page 2, column 1, line 19 of the patent gives us a clue as to how the physical "panoramic" concept made its way into what we now consider to be a purely electrical phenomenon:
...provide a panoramic radio compass system having visual indicating means operative in correlation to the radio compass system...
In "One-Chip Panoramic Adpapter" Shortwave Magazine for January 1995 pp. 18-25, Bill Wilson makes this attribution:
To the best of my knowledge, the panoramic receiver was invented in the early thirties by a Marcel Wallace ... Dr. Wallace later devised the panoramic adapter which could be connected to a conventional superhet to permit a visual spectrum display, and, along with the Hallicrafters company, coined the term 'Panadapter'.
Figure 4 from Wilson's article shows that the "adapter" picked off its input before the IF signal was filtered by the receiver's high-selectivity stages. Comprising its own RF amp, mixer, oscillator, filter and detector, the panadapter is essentially a second receiver, but one who's output is meant to be displayed visually rather than decoded, audibly or otherwise.