On Thu, 2007-04-26 at 17:45 +0930, Tim wrote:
> Tim:
> >> Sounds like they need to, also, employ someone who used to be a teacher.
> >> Someone who's used to the idea of having to train, as the main thing
> >> that they do.
>
> Gene Heskett:
> > On the face of it, that is a good idea. Till that old saw about
> > "those who can't do, teach" comes crawling up out of the back of my
> > mind, having had it quite amply demonstrated in my nearly 57 years of
> > chasing electrons for a living. The other corollary to that is that
> > those who can do, and then try to teach, have a hell of a time trying
> > to reduce the language to something that actually works for TV-101
> > classes.
>
> ;-) Generally, we had the opposite problem at college. Teachers who
> learnt electronics at college, then became teachers, were worse than
> those who worked in the industry, then became teachers. For one thing,
> they knew the difference between theory and practice.
>
> I could never get any lecturer to give a sane explanation of AM. They'd
> tell us that the carrier was a fixed amplitude. I'd argue that AM was
> modulating the carrier, therefor it has a varying one. I'd even
> demonstrate by cranking the pot up and down to give a 1 Hertz AM. None
> of them could give a reasonable explanation. Yes, they could give
> strange ones, but none that fitted the situation demonstrated.
>
> Yes, I know that you can put a 1 kHz signal on top of a 1 MHz one, and
> then filter one away from the other. Theoretically, that's fine. But
> it doesn't get around the fact that I had grabbed the pot and changed
> the carrier level. It sure didn't have a constant carrier level in my
> hands.
>
> --
> (This box runs FC6, my others run FC4 & FC5, in case that's
> important to the thread.)
>
> Don't send private replies to my address, the mailbox is ignored.
> I read messages from the public lists.
>
>
Hi, Tim,
That is changing total power, not adding modulation. That can be
detected as AM, but only if the signal averages out to some carrier
level. True AM is not changing the powerlevel of the carrier. It is
adding modulation sidebands to the carrier. When you put the modulation
on the plate, signal strength of the carrier remains constant, its power
comes from the power supply fo the RF stage, and the modulation power
comes from the audio stage. The full power is then greater than the
original system power on peaks (by 100% if you are modulating to that
level, and to zero on the modulation negatives if you are modulating to
100% on that side as well. one half of the power transmitted is from
the RF stage, and one half from the audio modulation stage.
If you took your power cranking up and down, and looked at it on a
spectrum analyzer, you would observe a different effect. But the effect
of demodulating that signal would produce a carrier and sidebands, but
the total power available would never exceed the total of the RF power
supply input. In this case the mixing would occur in the receiver (or
spectrum analyzer balanced mixer).
Hope this helps.
Regards,
Les H
