Re: A "single conversion" question


"Michael Black" <et472@xxxxxxxxxxxxxxxxxxx> wrote in message
news:dlbpu9$9ln$1@xxxxxxxxxxxxxxxxx
>
> (jfeng@xxxxxxxxxxx) writes:
> > m II wrote:
> >> Perhaps he meant to say 'any sort of selectivity' ?
> >
> > I re-read his posting, and I think he meant amplification. In context,
> > he was referring to the earliest vacuum tube days. The frequency
> > response of those tubes was limited. If I recall correctly, it was
> > limited by the physically large size and the spacing between the
> > filament, the grid, and the plate.
> >
>
> Howard Armstrong received the patent for the superhet, US patent number
> 1,342,885 in 1920. He wanted to receive what were astoundingly high
> frequencies at the time, like in the 2 or 3MHz range.

The story I remember is, during World War One, it was feared the Germans had
developed a way to communicate at 100 meters. Armstrong wanted to intercept
those communications, if they existed.

>
> At the time he cooked it up, even at the time the patent was issued,
> there was no commercial radio broadcasting. The spectrum above
> what is now the AM broadcast band was deemed useless (which is
> why amateurs were relegated to "200 meters and down" after WWI.
>
> I don't recall the schematic in Armstrong's patent, but if you look
> in the history books, you find early schematics that use a chain
> of RC coupled tubes for the IF strip, no selectivity.

It's worth mentioning that there's a practical limit as to how much gain can
be obtained at any one frequency, and that practical limit was much lower
back in the earliest days. The superhet split it's gain at supersonic and
sonic frequencies, and could have much more gain without breaking out into
uncontrolled oscillation than a simple audio frequency amplifier. The tubes
of that era were just about useless as amplifiers at 3 MHz.

After Armstrong's invention, better triodes combined with better circuits
such as the Neutrodyne, as well as the screen grid tubes, put the TRF back
in the game into the early 30s, or so.




>
> Amplification has always lagged after frequency use. During WWII,
> radar development was limited because they had problems getting
> receiving tubes to work in the microwave frequencies, so they went
> to diode mixers. It's pretty much always been easier to convert
> to a lower frequency for amplification.
>
> Michael
>

Frank Dresser


.

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