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Synchrodyne · 19th Mar 2018, 6:28 am

Armstrong evidently “borrowed” the upconversion approach from communications receiver practice. As I recall, one reason for doing it this way was to facilitate the use of varicap tuning for AM as well as for FM. At the time it was designed, I think that varicap diodes with a wide enough tuning frequency range (>3 to 1) to cover the MW band with the normal IF (455 kHz or thereabouts) were not readily available. But the combination of an aperiodic RF amplifier (of the feedback type) with a 3.1 MHz IF put the required local oscillator range (about 1.5 to 1 for LW + MW combined) within varicap reach, the oscillator being the only tuned element in the AM front end.

Cheers,

19th Mar 2018, 6:28 am	#8
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: Armstrong 625 receiver Armstrong evidently “borrowed” the upconversion approach from communications receiver practice. As I recall, one reason for doing it this way was to facilitate the use of varicap tuning for AM as well as for FM. At the time it was designed, I think that varicap diodes with a wide enough tuning frequency range (>3 to 1) to cover the MW band with the normal IF (455 kHz or thereabouts) were not readily available. But the combination of an aperiodic RF amplifier (of the feedback type) with a 3.1 MHz IF put the required local oscillator range (about 1.5 to 1 for LW + MW combined) within varicap reach, the oscillator being the only tuned element in the AM front end. Cheers,