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Author Topic: AM and Linears  (Read 2289 times)
K8EU
Member

Posts: 3




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« on: October 31, 2004, 08:13:31 AM »

Question; If I drive a linear amplifier with a 100% plate modulated AM signal, is the amplified output still 100% modulated?
 
        Tnx   Glenn

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HEDGEHOG
Member

Posts: 46




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« Reply #1 on: October 31, 2004, 09:01:59 AM »

Yes, a linear is by definition "linear" in action.  The only thing that gets changed is the amplitude of all signals.  But, be sure you don't exceed the plate dissipation of the linear.  The rated input power on most amps is (or at least was in "the day") based on SSB or CW duty factor.  AM is 100% duty factor, and you should check for max AM input for the amp.

Bob - AE6RV
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K7KBN
Member

Posts: 3693




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« Reply #2 on: October 31, 2004, 01:11:53 PM »

Also, don't lose sight of the fact that if you're intent on running 1500 watts PEP on AM, you can't exceed 375 watts RMS.
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73
Pat K7KBN
CWO4 USNR Ret.
WB2WIK
Member

Posts: 21837




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« Reply #3 on: October 31, 2004, 02:10:32 PM »

The answer is "yes," if you stay below the PEP output where the amplifier maintains linearity.  If you don't, then, "no."

With most linear power amplifiers, if you adjust your carrier level to be no more than 25% of the available PEP, you'll stay within the linear region.  Adjusting carrier level to be well below that almost assures linear operation.  But don't forget to tune the amplifier at its full PEP output, and *not* at the carrier level!

WB2WIK/6
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BEEKER7104
Member

Posts: 1




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« Reply #4 on: October 31, 2004, 08:05:12 PM »

Not really a question, More of a statement actually......I've been in contact with the F.C.C for Ohio, and they have sent a letter to our local police stating that unless we are running OVER 600 Watts in either our Mobiles, or Homes there is nothing illegal about having or using them. They did however say that it is a "Good Neighbor" policy to put a filter on your antenna/Linears to keep neighbors from screaming at you for "bleeding" anything within their houses.
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N3ZKP
Member

Posts: 2008




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« Reply #5 on: November 01, 2004, 06:36:13 AM »

Jerry:

<< and they have sent a letter to our local police stating that unless we are running OVER 600 Watts in either our Mobiles, or Homes there is nothing illegal about having or using them. >>

What a crock!!!

First, the police department has no authority of FCC licensed stations and second that 600w figure is bogus. Someone is pulling your leg on this one.
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W8JI
Member

Posts: 9748


WWW

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« Reply #6 on: November 04, 2004, 04:19:35 AM »

The biggest problem with non-linearity is how you tune your PA. Simply running a 1500W PEP PA at less than 1500W (or more than that) does not guarantee linearity, but proper tuning does.

Let's assume you have a BIG linear amp that is well-designed.

In order to be linear, you MUST load the amplifier up to deliver the full PEP power or slightly beyond. Then you have to reduce drive power until the peaks just reach or stay slightly below maximum power.

With a 50 watt exciter and normal symmetrical modulation of 100%, you would have to adjust your amplifier so it delivered the maximum obtainable output with 200 watts of drive or more!!! Then when you applied 50W carrier and had 200W PEP, everything would be linear.

With a 100 watt exciter and a 100W input amp, you could tune to full power and then run 25W of carrier (assuming you had normal 100% modulation).

The rub is in AM mode, PA's at carrier condition have very low efficiency. About three times the carrier power is dissipated in the PA tubes under carrier conditions. If you have higher efficiency and are using a linear amplifier, you can bet it is tuned wrong.

Another way you can tune is to watch the relationship of input to output power on a scope. You can feed vertical plates the RF output, horizontal plates the RF input. The angled line that is drawn should be straight, or if it is curved it should be a very smooth very gradual curve. Any sharp bends or quick changes in direction indicate non-linearity that will cause splatter.

Or you can watch peaks, and make sure you find the maximum peak, and then back off til you never reach the peak.

If you do that, THEN the amp will faithfully reproduce the drive. If you don't tune one of the above ways, then you run a high risk of having splatter and change the symmetry of the waveform regardless of how big the amp is. It is also very tough on the amp to not load it properly.


http://www.w8ji.com/loading_amplifier.htm


73 Tom
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W8JI
Member

Posts: 9748


WWW

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« Reply #7 on: November 04, 2004, 04:26:21 AM »

By the way the correct power definitions are PEP and AVERAGE , not peak and RMS.

When we measure PEP, we are measuring the highest peak of RMS power over the entire length of the measurement period, which for our case is a whole transmission. When we measure Average power, we are measuring the short term average of the RMS power.

This is why using a scope to determine peak power is almost useless, unless you have a storage scope.

In both cases we are measuring RMS, one is short term average and the other is long term peak of RMS.

73 Tom
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WA4MJF
Member

Posts: 1003




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« Reply #8 on: November 13, 2004, 02:10:14 PM »

N3ZKP, If his state has the enabling
legislation, as NC does, the local
authorities CAN enforce Part 95
ONLY.  Congress allowed this in
the a couple of sessions ago, because
of complaints (valid, I would say)
that the FCC was not up
to the task of enforcing Part 95.

73 de Ronnie
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