Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uwm.edu!lll-winken!quintro!bpdsun1!rmf
From: rmf@bpdsun1.uucp (Rob Finley)
Newsgroups: sci.electronics
Subject: Re: AM Radio (slightly long but a good summary)
Message-ID: <1990Feb4.072004.21732@bpdsun1.uucp>
Date: 4 Feb 90 07:20:04 GMT
References: <90026.012605TRM900@PSUVM.BITNET> <26374@cup.portal.com> <3044@optilink.UUCP> <9602@nigel.udel.EDU>
Reply-To: rmf@bpdsun1.UUCP (Rob Finley)
Distribution: usa
Organization: Harris Broadcast Div., Quincy, IL
Lines: 60

0000000014 0000000001 n
alt.hotfut 0000000000 0000000001 y
rec.arts.startrek.info 0000000002 0000000001 m
comp.sys.laptops 0000000191 0000000000 y
alt.hackers 0000000069 0000000000 m
clari.feature.dave_barry 0000000006 0000000000 m
clari.feature.mike_royko 0000000012 0000000000 m
clari.feature.miss_manners 0000000007 0000000000 m
clari.feature.kinsey 0000000000 0000000000 m
clari.feature.lederer 0000000002 0000000000 m
alt.cobol 0000000056 0000000000 y
alt.fan.dave_barry 0000000091 0000000000 y
news.software.nn 0000000057 0000000000 y
sci.virtual-worlds 0000000025 0000000001 m
Inventor of the Year (or whatever)
by the National Association of Broadcasters.  PDM is used in our SX series 
of 1, 2.5 and 5Kw AM transmitters.  To go to 10Kw, we have come up with an 
even neater scheme.

Phase modulation for audio works like this:  The audio is fed into one
side of a comparator while a 60Khz triangle wave is fed into the other side.
There are two comparators for the two phases of PDM.  (We also have implemented
a single phase but it isn't as efficient (uses fewer parts though)).  The
other comparator has a triangle wave 90 deg out of phase with the first and
also sees the inverse of the audio signal.  When the triangle wave level 
is higher (or lower, in the second phase) than the audio signal, the comparator
output turns on and drives a MOSFET-based class-D amplifier which allows
the carrier signal to reach the output filter network.  The higher the signal
level is, the longer the amplifier is going to feed carrier into the antenna.
Radio Electronics ran two construction projects of audio amplifiers based on
our PDM technology.  I disagree with their "digital" labels but Check 'em out.

For higher power levels (up to 50Kw now, higher when we get this project done),
we have gone to another technique developed by Mr. Swanson using digital
controlled arrays of class-D amps to produce levels of modulation (DX series).
We take an audio signal, feed it into a 12bit A/D converter with a sampling
rate higher than 400khz, decode the output into individual levels, and
turn off and on an array of switches.  It's kindof like a bar graph:  the
higher the signal level, more modules are turned on.  We also do a bit
of encoding of Big step and Little steps to reduce the number of modules,
but that discussion gets involved.
The neat thing about both of these techniques is the abundance of ways
to bypass sections if something dies (just in case...).  All of the
digital modules are redundant and plug-swappable.  If one quits, you can't 
see it on an 'scope, and you would have to lose quite a few before the 
listner can even tell.  Module failures are tracked with a generous number
of indicators.

For more information, visit the latest issues of broadcasting electronics
textbooks.  I apologize if any of this information is incorrect.  I am
not an engineer here but I am working on that problem too.  

-----
"We as a company know what we are talking about and have sales leadership
to prove it.  I am an individual and since I am not an engineer, I am trying
to help you guys out by typing in stuff as I read the instruction manual.
Please don't start a flame war because I misunderstood what I read.  I knew
what I was talking about once but have since forgotten quite a bit..."

quintro!bpdsun1!rmf@lll-winken.llnl.gov    uunet!tiamat!quintro!bpdsun1!rmf