On Tue, 13 Nov 2012 23:12:53 -0700, Greg Goss <
go...@gossg.org>
wrote:
>Robert Carnegie <
rja.ca...@excite.com> wrote:
>
>>I suppose they'd have to know that "heterodyne" /is/ a word.
>>It comes up in science fiction written before a certain point
>>in time... I'm assuming that it came up at Philcon, maybe
>>even from the lips of our Bren.
>
>When I was a lad, all the radios had "superheterodyne" stamped in with
>the logos somewhere. In the pre-digital era, it was a technique that
>made it easier to deal with high frequencies, and thus produced a
>better radio. I'm not sure what the "super" meant in that context.
It was just a glamour placed on heterodyne. Armstrong invented
heterodyning; he named it that.
It didn't make it easier to deal with high frequencies. It made
it cheaper. The alternative was that each amplification stage
would require individual tuning, a technique called Tuned Radio
Frequency (TRF), using a large multi-section ganged capacitor, or
several fiddly individual ones. With heterodyning, a two-section
capacitor could be used. One section tuned the first
amplification stage, and the second tuned the local oscillator.
The local oscillator, which was usually either 142.5 or 455 kHz
above or below the desired tuned station, was mixed with the
incoming signal. This resulted in four signals out of the first
stage: the original signal, the local signal, the sum of the
two, and their difference. This was then fed to the
intermediate, or IF stage or stages, which could be fixed-tuned
at the difference frequency of 142.5 or 455, and didn't have to
be tuned with a change of station. The more tuned stages,
whether RF or IF, the more selective the receiver was, and the
more potentially sensitive. The typical table-top radio, the
5-tube AC-DC model, would only have one RF and one IF stage. The
local oscillator, the RF amplifier, and the converter amplifier
would occupy one tube (conveniently called a pentagrid
converter), the IF amplifier would use the second tube, the audio
detector and pre-amplifier would be the third, the audio power
amplifier the fourth, and the rectifier to convert AC to DC would
be the fifth.
I actually have worked on, and even have a couple of really
high-class AM receivers that have three tuned RF stages, two of
them amplified, and three amplified IF stages, and use both
common frequencies for IFs. The radio tunes in four different
bands: 100-200, 200-410, 410-850, and 850-1750 kHz. This covers
everything from extreme LW to above the MW band. There are 26
tubes total in the receiver, some of them very specialized, and
others found in any radio repair shop of the day. The two
different IFs are used for different bands, for image-rejection
purposes. Images are ghost signals caused by the interaction of
the local oscillator with harmonics of the received frequencies.
They couldn't entirely be eliminated, but they could be mitigated
by having the IF sufficiently far out of the tunable range.
These receivers were part of an aircraft Automatic Direction
Finder. The sensitivity specification was that they would home
on a signal of 100 microvolts/meter of antenna. I could get them
to do it at 10 microvolts, these receivers were that good. I
used to listen to the BBC Dagenham at 206 kHz from North Carolina
using the run of the shop sets installed on the aircraft, and I
have great hopes for the two receivers I have. The were taken
out of service in the 1950s, when they were relatively new. I
shouldn't have to work too terribly hard to put them in use.
--
Doug Wickström