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Plate Modulation vs Linear Amplifiers




 
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Author Topic: Plate Modulation vs Linear Amplifiers  (Read 18992 times)
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k4kyv
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Don
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« on: July 13, 2005, 12:58:08 PM »

Q:  Can't the best quality audio of all be gotten from low level modulation and a linear amplifier?

A:    A linear amplifier has the same kind of distortion as a class-B modulator.

With tubes, the best quality audio can be had from low distortion plate modulators such as class-A series or Heising modulation, or pushpull plate modulators running class A or AB1.

Pulse-width series modulators produce perhaps the best audio.

I suspect the best quality of all comes from the new solid state class-E rigs.

According to the tube manuals, class-B audio service has inherent distortion levels on the order of 3-5%.  It can be reduced with negative feedback.  My Gates BC1-T manual claims less than 2% distortion at 100% modulation.

The signal driving a linear amplifier has its own distortion, since the original signal has to be produced somehow.  Control and screengrid modulation is inherently less linear than high level class-C plate modulation.  Pushpull transformer coupled class-A or class-A series modulators, with negative feedback, might be good candidates for the driver stage of a linear.  If the linear is run properly in class AB1, that would be near to the best possible audio out of a tube transmitter, even though the efficiency is not all that good.

Speaking of efficiency, an AM linear or grid modulated amplifier has close to the same overall efficiency as plate modulation, when calculated from the ratio of power drawn from the a.c. mains, to rf carrier output.  A linear amplifier running AM has exactly the same efficiency as when it runs SSB.  It's just that the duty cycle is different.


Actually, since with the human voice, the average power is 7-8 dB lower than peak power (equivalent to around 30% modulation), the average efficiency of a SSB linear is similar to that of an AM linear because the efficiency of a linear is a function of the amplitude of the signal (0% at idling current, and a practical real-world maximum of about 67% at maximum peak output just below the point of saturation or flat-topping).  AM linears got their reputation as "low efficiency" on AM because the 100% duty cycle carrier runs about 30% efficiency to allow enough headroom for the positive peaks.  With an AM linear, you can see the glow on the plates DECREASE when you whistle into the mic to produce 100% tone modulation.  The DC input is the same regardless of modulation, but the rf output is higher with modulation, since sideband energy is now included.  That power has to come from somewhere, so the efficiency of the amplifier goes up to generate the sidebands.

A major advantage of plate modulation with amateur AM is the ease of tuning up and QSY'ing.  You simply dip the final and load to the desired carrier output, while maintaining enough grid drive to assure class-C service.  With low-level modulation (linear or grid modulated), the rf drive level and degree of antenna coupling are critical to the modulation linearity of the final.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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K1JJ
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« Reply #1 on: July 13, 2005, 02:16:18 PM »

Good article for the archives, Don!
We need more of this kind of stuff written up.

As a side note:

For low level driven linears and QSYing.... all it takes to tune a true
class AB2, etc linear is to peak the output of the linear for max
power. Just peak C1 and C2 and you're OK. The heavier loading
of C2 for better linearity is not really mandatory, but should be done
when time permits.  

However, the tuning hassle starts with a class C grid or screen
modulated amplifier. They ARE critical and need a scope to insure
they are loaded right.  Axe Frank/KB1AHE about the time required for him
to finally get his class C, screen modulated 4-1000A tuned and loaded
so it sounded decent.

As for cleanliness of a low level linear.... lets assume you had a
perfect driving low level signal, like something that comes out
of an NE-602 or similar balanced modulator. A normal linear
power amplifier using common zero biased triodes [designed
for linear service] like the 3-500Z, 8877, etc are down at
LEAST -30 db of 3rd order intermodulation distortion. I cannot
remember exactly, but -30db equates to something like <1%
distortion. Very clean. So, in reality, I believe that a low level
driven linear power grid triode can probably produce some of
the best audio out there -  BUT at a price. The efficiency sux.
If you run a BIG mawl, the room heats up big time.

So, as you say, Don, probably a class E rig being as clean as
it is, in additon to the 85%++ efficiency is the best all
around compromise.

Personally, I like all three: plate, class E and linear. They
all have a purpose. For fast QSYing around the various bands,
I love my FT-1000D/FT-102's and various preset/ pretuned linears.
For just sitting on one AM area and generating little heat and
sounding good overall, my class E rig can't be beat.  

In the Winter and when you feel old buzzardly, a big plate
modulated rig is FB. Mix it up baby! That's what keeps it fun.

73,
Tom, K1JJ
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
ve6pg
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« Reply #2 on: September 02, 2005, 10:38:23 PM »

...don..great stuff..we need more of this kind of thing to remind us and send us back to our text books to re-read alot that has been forgotten..tim...sk..
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...Yes, my name is Tim Smith...sk..
Steve - WB3HUZ
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« Reply #3 on: September 03, 2005, 02:53:37 AM »

Keep in mind that most of the concern about distortion levels at less than 2-3 percent is moot. Your receiver creates more distortion than that (some a lot more), not to mention the effects of the ionosphere. So, no matter the modulation method, if the distortion level is less than that of the receiver, they will all sound equally good.
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Vortex Joe - N3IBX
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« Reply #4 on: September 08, 2005, 09:23:37 AM »

Don - That was a good meaty dissertation on modulation. I thoroughly enjoyed it!
Regards,
           Joe Cro N3IBX
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Joe Cro N3IBX

Anything that is Breadboarded,Black Crackle, or that squeals when you tune it gives me MAJOR WOOD!
kc2ifr
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« Reply #5 on: September 08, 2005, 05:24:02 PM »

Steve,
How true....how true.
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Steve - WB3HUZ
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« Reply #6 on: September 08, 2005, 09:32:08 PM »

Glad someone agrees. That said, it's not entirely without merit to pursue lower distortion (at least to some reasonable level), because the lower your distortion the less bandwidth you will occupy. I'm not sure what a reasonable cut-off level of distortion would be. Anyone have a good number?
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WA1GFZ
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« Reply #7 on: September 09, 2005, 12:11:23 PM »

Steve,
I bet 5% sounds as good as 1% on most receivers.
30 dB is .1 % power Tom Vu so 1500 watts third order side bands are at 1.5 watts crank up the kompressor and now 15 watts of crap.

I like the run class A approach then talk louder when required.
Tubes with handles required.

When they double the electric rates this winter Class E will become very popular.
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K1JJ
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« Reply #8 on: September 09, 2005, 01:43:52 PM »

Steve,
I bet 5% sounds as good as 1% on most receivers.
30 dB is .1 % power Tom Vu so 1500 watts third order side bands are at 1.5 watts crank up the kompressor and now 15 watts of crap.


Thanks for the clarification, Frank. Wow, so a normal linear with -30db 3rd order has 0.1% distorsion. I remember Chuck telling me that once, but couldn't remember if he said 1% or 0.1%.

My question is: Lets say a receiver had 2% distorsion. If a transmitter had 1.5%, would it then degrade the 2% receiver figure further to something higher than 2% or be totally masked by higher RX figure? ie, is there any kind of cummulative factor occurring?

For example, a driver with -30 db IMD driving a linear that is -55db IMD will still have a final output system value of like, -29 db, etc. Never better than the worse component - slightly worse as a total system. Thus, the RX question above has me curious.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
k4kyv
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Don
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« Reply #9 on: September 09, 2005, 07:37:40 PM »

[
My question is:  Lets say a receiver had 2% distorsion. If a transmitter had 1.5%, would it then degrade the 2% receiver figure further to something higher than 2% or be totally masked by higher RX figure? ie, is there any kind of cummulative factor occurring?

Dont' expect the distortion figure to be directly cumulative.   You wouldn't necessarily have 3.5% as a result.  Most likely the total distortion would fall somewhere between 1.5% and 3.5%.  It is possible that the total distortion could even be less than that of either the receiver or the transmitter.  Sometimes the nonlinearity of one device complements the nonlinearity of the other so that the final result is improved linearity.

Of course, in a real situation, distortion produced by propagation effects would have to be factored in.

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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
WA1GFZ
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« Reply #10 on: September 14, 2005, 12:14:46 PM »

10 dB is 10 %
20 dB is 1 %
30 dB is .1%

-30 driving -55 dB will never improve. If you shove in a signal with -30 crud it will be reproduced with an error of -55 dB crud. The linear will not know the difference between signal and close in distortion. It will just amplify everything.
A receiver that has crud it it's bandpass will process it like a normal signal through the analog section. DSP can help with some crud but the basic receiver circuits don't know the difference between crud and real signal.
The problem with most receivers is filtering. The ultimate rejection isn't good enough to tell the difference between bleed through and high order distortion. Most IF filters have ultimate rejection about 60 dB so a strong signal off to the side will go right around the filter and sound like QRM.
Then you have the issue of dynamic range when signals mix and generate more crud. After all that is fixed there is phase noise from the synthesizer
dumping in crud. Battle mode dx window operation requires 100 dB of dynamic range, 100 db of ultimate filter rejection and close in phase noise down 100 dB before you can complain about the guy 3 KHz away.
This is why -30 is presently an exceptable number



 
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Ian VK3KRI
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« Reply #11 on: September 22, 2005, 07:50:59 AM »

10 dB is 10 %
20 dB is 1 %
30 dB is .1%


Isn't:
 10% distortion -20dB
  1%                  -40dB
 .1%                  -60dB
?
I think Distotion % levels were based on voltage measurements , not power . My Noise & Distortion meter certainly indicates that a  10:1 distorion drop is -20dB
                                                    Ian VK3KRI
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