Re: RF interference issue again (esp. for E Drucker and Jim Weir and other RF wizards)

On 6 Nov 2003 05:34:08 -0800, [EMAIL PROTECTED] (Snowbird)
wrote:

>Roger Halstead <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>> On Wed, 05 Nov 2003 11:10:29 -0800, Jim Weir <[EMAIL PROTECTED]> wrote:
>> >I think we are chasing our tails here, folks.  
>
>Well, I'm not sure the decision tree is quite this binary from
>what folks are saying.
 
<snip>

It's still a binary decision tree and a process of elimination.

>But what I'd like to understand is this:
>
>How could my nav radios (or my handheld) be contributing
>to this problem when they are *powered off*?

I'm trying to think of a plain language explanation and Jim could
probably do it more eloquently...and most likely with better accuracy.

The power off problem usually uses a different mechanism to produce
the problem than one that is powered up.

Today's radios have a transistor amplifier on the receiver input.
If you put them in a strong enough RF field (close to a powerful
transmitter) there will be enough voltage on the input transistor to
cause it to conduct.  The only thing is, it can conduct in only one
direction so it acts like a switch that turns on and off with each
cycle of the signal.  Unfortunately the transistor doesn't start to
conduct until the voltage is already on the upward cycle and this
causes the transistor to switch on abruptly.  It's sorta like the
rusty fence/poor connection/rubbing a screwdriver on the guy wire sort
of thing, but it's not just noise.  It can generate signals although
they usually sound muffled, or garbled.

Actually the same thing can happen in the audio stages of receivers
with power on, but it normally happens with those using long speaker
leads.  here the speaker leads act like an antenna and pick up the
signal.  The audio transistors act like a switch and rectify the
signal producing a garbled sound on top of the audio to which the user
was trying to listen.

Without getting in too deep, intermod (with power applied) can be
caused by a signal so strong it exceeds the design limits of the
amplifier and it goes into what is called a non linear operation (It
becomes a mixer instead of just an amplifier). 
>
I hope this makes sense. I've found the longer I've been in a
particular field the more difficult it becomes to explain things in
plain language.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)
>Could someone explain this to me please?  I'm not an electronics
>wizard (obviously) but I do know a little bit and this just seems
>very "twilight zone".
>
>Thanks,
>Sydney