[Boatanchors] Receiver Technology
Fuqua, Bill L
wlfuqu00 at uky.edu
Fri Jan 14 16:47:34 EST 2011
I am going to snip away a lot of the following long messages and comment.
>>I've been reading all the way back to Armstrong and the more I read the
>>more I wonder! I think there are tremendous articles wauting to be
>>written on this subject, especially lab tests on original equipment.
I think it would be interesting to test performance with modern test equipment.
A communication receiver in the mid 20s was more stable than just about any
signal generator then. On the air checks were probably the best test for
sensitivity, since these were MW or SW receivers and as long as the receiver noise
was less than atmospheric, it was fine.
>>Looking at pictures of coherers, which I've done all my life, it's hard to
>>imagine one working on any reasonable signal level. Have you ever seen
Coherers were quite real. I am an engineer in a Physics Department and one grad student came
to tell me of an interesting device that they discovered. These guys were working with powdered
materials and nano-particles. He was asking about some application for something they discovered while
measuring the conductivity of some powdered materials. The resistance dropped when they applied a current and
went back to normal after tapping on the tube containing the material. I told the gradstudent this was discovered in
the late 1800s and referred him to an interesting book on coherers.
As far as sensitivity of coherers, I doubt they were any better than a few hundred mV. Marconi did 3 things just before
receiving the signal he expected. One was he went to a broadband tuner, the second he went to an experimental device
called a coherer but I suspect it was actually an early semiconductor device and third was switching to headphones.
I still doubt he actually received a signal but I believe he believed it.
In very early 1900s they discovered heterodyne reception and that if you left an oscillator on very near the
crystal receiver's frequency you increased sensitivity a hundred fold. This oscillator had to be farly pure by the standards those days.
It became known as the local oscillator, one near the receiver or coupled to it. And heterodyne referred to the mixing of two different
signals (hetero) to produce a third that you could hear.
In the early days of Superheterodyne radios the mixer stage was called the first detector, but we retained the term local oscillator.
>>I'm going to start with a fairly modern regen, like with a 19 or 30, and
>>then work back and see what I can do with 01s, etc. Armstrongs regen
>>would probably work fine with decent tubes, but it was years getting into
>>common use. Then there were several more years of limbo before superhets
>>I built regens as a kid and still have a 1L4 that I can return to
>>operation. It was great on BC and up to 40M...I heard BC all over the
>>country at night, usually from bed...on bats, of course.
The milestones I see are: diode, regen, superhet, rationalized bandspread in
the HRO, and "modern" constant rate bandspread through double conversion (Collins?).
Looking back like this, I'd say we were not innovative at all and that we
suffered with poor receivers that were hard to tune above 40m for too many years.
There's nothing in the 75A-1 that couldn't have been built in 1920!
There was a great deal of change in the 20's. However, the 75A1 did have something
that was not available then and that was crystal oscillators and filters. Readily available
quartz crystals was quite an improvement.
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