Tacos Digest, Vol 84, Issue 21
rabruner at aol.com
rabruner at aol.com
Tue Feb 16 22:06:06 CST 2010
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Sent: Tue, Feb 16, 2010 8:36 pm
Subject: Tacos Digest, Vol 84, Issue 21
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oday's Topics:
1. first unmanned transatlantic crossing (Mike O'Dell)
2. Re: "And now for something completely different..."
(bbruhns at erols.com)
3. Re: AM modulation (rabruner at aol.com)
4. nice to have in the summer (wb5mmb)
5. Re: AM modulation (bbruhns at erols.com)
6. Re: AM modulation (Mike O'Dell)
7. Re: AM modulation (Mike O'Dell)
---------------------------------------------------------------------
Message: 1
ate: Tue, 16 Feb 2010 15:04:40 -0500
rom: "Mike O'Dell" <mo at ccr.org>
ubject: first unmanned transatlantic crossing
o: Tacos AMRAD <tacos at amrad.org>
essage-ID: <4B7AFA58.3010300 at ccr.org>
ontent-Type: text/plain; charset=ISO-8859-1; format=flowed
http://rucool.marine.rutgers.edu/atlantic/
--
Of course it's hard!
f it was easy, we'd be buying it from somebody else!"
-----------------------------
Message: 2
ate: Tue, 16 Feb 2010 20:35:49 +0000
rom: bbruhns at erols.com
ubject: Re: "And now for something completely different..."
o: tacos at amrad.org
essage-ID:
<1064535042-1266352491-cardhu_decombobulator_blackberry.rim.net-733619471- at bda600.bisx.prod.on.blackberry>
ontent-Type: text/plain; charset="Windows-1252"
Complexity is relative. Ampliphase may have been more efficient than plate
odulation, but I don't think that it caught on that much in the USA. I assume
hat station owners didn't see enough of an advantage to offset their worries.
hey didn't really understand even the simple plate modulation technology, in my
pinion, or I think that they would have gone with triode audio power tubes and
etrode or pentode RF power tubes in the conventional class-AB / class C
late-modulated transmitters pretty much from the 30s. So convincing them to go
ith some remarkable vector combination system like Ampli-Phase, as cool as it
eally was, was a tough sell. To me, the reduction in overmod spatter, the good
fficiency, and the improvement in audio quality, would have made the sale. No
ig heavy modulation transformer to blow out, either. But they were nervous
nvestors.
Bob, WA3WDR
-----Original Message------
rom: andre kesteloot
o: Bob Bruhns
c: Tacos AMRAD
eplyTo: andre.kesteloot at ieee.org
ubject: Re: "And now for something completely different..."
ent: Feb 16, 2010 11:28 AM
Bob Bruhns wrote:
In the old days, there were a few other alternatives to plate modulation that
id get reasonably good efficiency. One was called
AmpliPhase, it belonged to RCA and it combined two oppositely phase-modulated
utput stages. [...] Ampli-Phase
was commercially viable, and some stations used it. The inherent linearity of
he Terman-Woodyard and Taylor systems were not so
good, but I understand that with enough negative feedback, they were OK.
The problem with those systems was complexity, and adjustment was difficult,
nd multi-frequency operation was tricky. That was an
issue with the old Conelrad requirements for stations to be able to transmit
n 640 KHz or 1240 KHz, and also on shortwave where a
transmitter might be expected to change bands a few times a day.
I am somewhat familiar with Ampliphase transmitters. The Conelrad
egulations did not apply overseas and medium-wave transmitters were
asy to adjust, and the lower power consumption made it easier on
entilation (or A/C) requirements (not to mention the electric bill).
or short-wave operation, tuning, unfortunately, was not quite as
imple; as the transmitting station usually has to switch from one
requency to an other (and one short-wave band to another) in a very
hort time.
73
ndr?
ent from my Verizon Wireless BlackBerry
------------------------------
Message: 3
ate: Tue, 16 Feb 2010 19:08:20 -0500
rom: rabruner at aol.com
ubject: Re: AM modulation
o: tacos at amrad.org
essage-ID: <8CC7D7C72B6B2FE-5158-1A69 at webmail-d006.sysops.aol.com>
ontent-Type: text/plain; charset="us-ascii"
s several have said, high level modulated Class C AM transmitters have a
reater overall efficiency than linear amplifier transmitters. Straight linear
mplifiers transmitters are actually usually run in class AB2 and have an
verage plate efficiency in the final of about 30% or so. The efficiency off the
ower line is even worse because they usually operate at lower gain than Class C
ransmitters and have more stages, more parts to stock, etc. RCA's Ampliphase
mproved on this. Western electric also had a high efficiency linear amplifier
hey marketed successfully based on the Doherty amplifier. This, like the
erman Woodyard, is a carrier tube/ peak tube arrangement with the main
ifference being the Doherty is an amplifier and the Terman Woodyard is a
odulated stage. W-E transmitters were modulated at very low level, about two
atts, and used a series of Doherty amplifiers to achieve powers up to 50kW.
he transmitter arm of W-E became Continental Electronics and th!
ey continued to make and sell Doherty amplifier transmitters. They developed a
ariant where the Doherty amplifier was high level screen modulated which
chieved total efficiencies greater than Class C amplifiers. The Continental
17C used only 9 tubes, and drew only 86 kW off the power main to make a 50,000
att carrier. The two final tubes, 4CX35000s were screen modulated by a pair of
CX3000s. Both final tubes were biased to operate in Class C, giving them good
F efficiency and modulation linearity. The two audio tubes, were driven in
arallel, the plate of one being connected to one final tube, and the cathode of
he other to the second tube. The transmitter could modulate flat to 30 kHz on a
ummy load and into the subsonic region. The distortion on a dummy load was in
he region of a couple of tenths of a percent.
Many of these are still in service as are the RCA Ampliphase transmitters.
he tendency, though, is toward PW modulated transmitters with even greater
fficiencies.
One other note. When analyzing AM modulation it is incorrect to say that the
carrier goes away,' or that 'the carrier goes to zero.' In a correctly
odulated AM transmitter of any design, with the exception of clamp tube
odulated transmitters, the carrier level is invariant. The FCC Rules in fact,
equire that the carrier amplitude not vary by more than 4% under any condition
f modulation. I realize this conflicts with high school physics books, and
opular technical magazines, but checking a few references of the physics at a
igh level will bear this out. Collins' book on SSB is a good place to start.
ven though we have all seen the AM carrier 'go to zero' on oscilloscopes, the
act is, it doesn't happen at all. The normal presentation on oscilloscope is
he resultant of the carrier and sidebands --which are out of phase -- adding
and canceling) in the scope. Observing AM modulation on a spectrum analyzer
ill reveal that the carrier just sits there doing noth!
ing while the sidebands come and go with the modulation.
ob Bruner
9TAJ
ansmitters were almost always plate-modulated
why didn't they AM the signal when it was "small"
and easy to do and then just use a linear final
so all the muscle went into the signal instead
of heating the modulation transformer?
here was a reason. A linear amplifier is most efficient at maximum power, and
e efficiency decreases for lower power, reaching
ro at zero output.
n the old days, there were a few other alternatives to plate modulation that
d get reasonably good efficiency. One was called
pliPhase, it belonged to RCA and it combined two oppositely phase-modulated
tput stages. Another was the Terman-Woodyard
plifier,
Bob, WA3WDR
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Message: 4
ate: Tue, 16 Feb 2010 19:21:02 -0500
rom: wb5mmb <wb5mmb at pobox.com>
ubject: nice to have in the summer
o: tacos at amrad.org
essage-ID: <7.0.1.0.2.20100215214437.03db46b0 at erols.com>
ontent-Type: text/plain; charset="us-ascii"; format=flowed
http://www.physorg.com/news185463943.html
ttps://intellectualventureslab.com/?page_id=563
------------------------------
Message: 5
ate: Wed, 17 Feb 2010 00:37:16 +0000
rom: bbruhns at erols.com
ubject: Re: AM modulation
o: tacos at amrad.org
essage-ID:
<1384277114-1266366986-cardhu_decombobulator_blackberry.rim.net-1332011594- at bda600.bisx.prod.on.blackberry>
ontent-Type: text/plain
That's right, that high-efficiency linear with the combiner that got increased
ower from the carrier tube on modulation peaks was the Doherty amp, and it was
ot designed by Terman as I misrecalled, but only described by him.
Communications engineers are also thinking about these amplification and
odulation techniques today because radio systems are using methods that involve
ariable power, and in some cases also rapidly varying amplitude, and the old
fficiency issue is still with us. The simple method of reducing drive to
educe the output of an FM power amp results in poor efficiency, and that means
igher power consumption, shorter battery life, more heat, etc.
Bob, WA3WDR
ent from my Verizon Wireless BlackBerry
-----Original Message-----
rom: rabruner at aol.com
ate: Tue, 16 Feb 2010 19:08:20
o: <tacos at amrad.org>
ubject: Re: AM modulation
_______________________________________________
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-----------------------------
Message: 6
ate: Tue, 16 Feb 2010 21:34:54 -0500
rom: "Mike O'Dell" <mo at ccr.org>
ubject: Re: AM modulation
o: bbruhns at erols.com
c: tacos at amrad.org
essage-ID: <4B7B55CE.2070002 at ccr.org>
ontent-Type: text/plain; charset=ISO-8859-1; format=flowed
almost all the cellular base-station PAs are Doherty amplifiers these
ays. they have been the savior of both DMOS RF transistors and the
ellular industry in general.
-mo
n 2/16/10 7:37 PM, bbruhns at erols.com wrote:
That's right, that high-efficiency linear with the combiner that got increased
ower from the carrier tube on modulation peaks was the Doherty amp, and it was
ot designed by Terman as I misrecalled, but only described by him.
Communications engineers are also thinking about these amplification and
odulation techniques today because radio systems are using methods that involve
ariable power, and in some cases also rapidly varying amplitude, and the old
fficiency issue is still with us. The simple method of reducing drive to
educe the output of an FM power amp results in poor efficiency, and that means
igher power consumption, shorter battery life, more heat, etc.
Bob, WA3WDR
Sent from my Verizon Wireless BlackBerry
-----Original Message-----
From: rabruner at aol.com
Date: Tue, 16 Feb 2010 19:08:20
To:<tacos at amrad.org>
Subject: Re: AM modulation
_______________________________________________
Tacos mailing list
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_______________________________________________
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--
Of course it's hard!
f it was easy, we'd be buying it from somebody else!"
-----------------------------
Message: 7
ate: Tue, 16 Feb 2010 21:36:13 -0500
rom: "Mike O'Dell" <mo at ccr.org>
ubject: Re: AM modulation
o: rabruner at aol.com
c: tacos at amrad.org
essage-ID: <4B7B561D.5010405 at ccr.org>
ontent-Type: text/plain; charset=ISO-8859-1; format=flowed
thanks for the great brain dump.
recalled having seen mention of "screen-grid modulated"
M transmitters but didn't know any of the details.
-mo
n 2/16/10 7:08 PM, rabruner at aol.com wrote:
As several have said, high level modulated Class C AM transmitters have
a greater overall efficiency than linear amplifier transmitters.
Straight linear amplifiers transmitters are actually usually run in
class AB2 and have an average plate efficiency in the final of about 30%
or so. The efficiency off the power line is even worse because they
usually operate at lower gain than Class C transmitters and have more
stages, more parts to stock, etc. RCA's Ampliphase improved on this.
Western electric also had a high efficiency linear amplifier they
marketed successfully based on the Doherty amplifier. This, like the
Terman Woodyard, is a carrier tube/ peak tube arrangement with the main
difference being the Doherty is an amplifier and the Terman Woodyard is
a modulated stage. W-E transmitters were modulated at very low level,
about two watts, and used a series of Doherty amplifiers to achieve
powers up to 50kW. The transmitter arm of W-E became Continental
Electronics and they continued to make and sell Doherty amplifier
transmitters. They developed a variant where the Doherty amplifier was
high level screen modulated which achieved total efficiencies greater
than Class C amplifiers. The Continental 317C used only 9 tubes, and
drew only 86 kW off the power main to make a 50,000 watt carrier. The
two final tubes, 4CX35000s were screen modulated by a pair of 3CX3000s.
Both final tubes were biased to operate in Class C, giving them good RF
efficiency and modulation linearity. The two audio tubes, were driven in
parallel, the plate of one being connected to one final tube, and the
cathode of the other to the second tube. The transmitter could modulate
flat to 30 kHz on a dummy load and into the subsonic region. The
distortion on a dummy load was in the region of a couple of tenths of a
percent. Many of these are still in service as are the RCA Ampliphase
transmitters. The tendency, though, is toward PW modulated transmitters
with even greater efficiencies. One other note. When analyzing AM
modulation it is incorrect to say that the 'carrier goes away,' or that
'the carrier goes to zero.' In a correctly modulated AM transmitter of
any design, with the exception of clamp tube modulated transmitters, the
carrier level is invariant. The FCC Rules in fact, require that the
carrier amplitude not vary by more than 4% under any condition of
modulation. I realize this conflicts with high school physics books, and
popular technical magazines, but checking a few references of the
physics at a high level will bear this out. Collins' book on SSB is a
good place to start. Even though we have all seen the AM carrier 'go to
zero' on oscilloscopes, the fact is, it doesn't happen at all. The
normal presentation on oscilloscope is the resultant of the carrier and
sidebands --which are out of phase -- adding (and canceling) in the
scope. Observing AM modulation on a spectrum analyzer will reveal that
the carrier just sits there doing nothing while the sidebands come and
go with the modulation.
Bob Bruner W9TAJ
transmitters were almost always plate-modulated
> why didn't they AM the signal when it was "small"
> and easy to do and then just use a linear final
> so all the muscle went into the signal instead
> of heating the modulation transformer?
There was a reason. A linear amplifier is most efficient at maximum power,
nd
the efficiency decreases for lower power, reaching
zero at zero output.
In the old days, there were a few other alternatives to plate modulation that
did get reasonably good efficiency. One was called
AmpliPhase, it belonged to RCA and it combined two oppositely phase-modulated
output stages. Another was the Terman-Woodyard
amplifier,
Bob, WA3WDR
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Of course it's hard!
f it was easy, we'd be buying it from somebody else!"
-----------------------------
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nd of Tacos Digest, Vol 84, Issue 21
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