On Sat, 15 Jan 2011 19:47:23 -0800, David Nebenzahl
wrote:
Talk about off topic... sigh.
1. TRF:
In the section on modulation, demodulation and other radio-related stuff
one book brings up "the tuned radio-frequency receiver" before
discussing superhet, as one would expect. But they say;
During the evolution of radio, the tuned-radio-frequency (TRF)
receiver was used to receive AM signals. Today, a few special
applications still use TRF receivers.
Now, they go on to explain why TRF is inferior to superheterodyne. But
I'm curious: are there still any radios that use TRF? and why? (Keep in
mind this book was written in 1979).
Yes, but it's not obvious or really TRF. The reason superheterodyne
receivers were invented was that decent narrow band LC or crystal IF
bandpass filters were not tuneable and didn't work well at higher RF
frequencies. About 45MHz was as high as they went before going exotic
with SAW devices.
These daze, dramatically improved semiconductor technology has
produced chips that work at almost any useful RF frequency. No more
need to downconvert when the IF filtering is done by a DSP (digital
signal processor). Instead of TRF, it's now called "direct
conversion". There's no local oscillator, no mixer, for fixed IF
filter, and probably no LC devices anywhere. Just a ceramic bandpass
filter (or duplexer) some gain, an A/D converter, and a DSP for
demodulation. Most GPS, Wi-Fi, and cellular chipsets work this way.
http://en.wikipedia.org/wiki/Direct-conversion_receiver
There are some rather simple TRF devices still around. A freqency
selective voltmeter is sometimes a TRF device. So are some field
strength measurement instruments, and remotes (car alarm, TV remote,
etc).
2. Thermionic converters & magneto-hydrodynamic generators:
Another book (which I frankly don't like as much since it's so
math-heavy: wouldn't electronics be so easy to learn if all that goddamn
math didn't get in the way?) covers these somewhat fantastic devices in
its chapter on "Energy Conversion Phenomena". The thermionic converter
is especially intriguing, as it seems a fairly efficient (20%) direct
conversion from heat to electricity. I seem to remember seeing a program
on PBS about something like magnetohydrodynamics being developed for
deep-space exploration propulsion.
Are either of these devices being seriously researched or used nowadays?
Keep in mind that *this* book was written in 1966.
Yep. Just about anything that has to do with plasma research involves
MHD. However, the generator part is effectively dead because of lousy
efficiency. I think the best it can do is about 20% thermal
efficiency. Light reading:
http://en.wikipedia.org/wiki/MHD_generator
The only hope for commercial use of MHD technology is generating
electricity (and some cooling) from waste heat, such as nuclear
reactor coolant.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558