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Re: [Amps] How intermod limits your PEP

To: "amps@contesting.com" <amps@contesting.com>
Subject: Re: [Amps] How intermod limits your PEP
From: "Dr. David Kirkby" <david.kirkby@onetel.net>
Date: Wed, 17 May 2006 07:06:45 +0100
List-post: <mailto:amps@contesting.com>
Gary Schafer wrote:

> Hi David,
> 
> Yes I am in error, thank you, but only on the examples where the PEP is
> greater than 1000 watts (1123 watt example) I believe.

But as you say, as soon as you add any IMD, to a signal, whilst keeping 
the signal level constant at 1000 W PEP, the PEP power will exceed 1000 
W due to the distortion.

Are you talking about an AM signal, with a carrier and two sidebands, or 
an SSB one, with no carrier? I assume it is AM.

> The signal should be 223 (not 233 as you noted).

No, see below.

> My attempt is to show how distortion products, however small they may seem,
> add to the PEP.

Agreed.

> Keeping PEP to 1000 watts would require reducing the signal level as you
> noted and keeping the IM level the same. 

No!! It distorts (pun intended) the figures.

If you run up your amp to something over 1000 W, then decide to turn it 
down to 1000 W (by for example lowing the input drive), whilst keeping 
the IM products the same amount down (-30 dB to take one of your 
examples), then *both* the signal *and* IMD signal must be reduced - not 
just the signal. So the power of the IM products must fall below 1 W 
each or 7 V RMS.

Symbols (P1, P2, f1, f2 ... etc) would make it a lot easier to see. One 
could then do it for any signal levels or IMD you want.

> My first example of PEP being greater than 1000 watts would indeed give a
> ratio where the distortion products would be better than 30 db below PEP but
> not by much. It would be 30.5 db rather than 30 db if I am now doing that
> right.

I am a bit confused exactly what you are doing, so are not going to work 
it out. But I think your method is flawed.

> It would probably be easier to just relate distortion products to the level
> of one of the signals of the two tone signal rather than to PEP and show how
> PEP changes.

Well this is one of the things you can do any number of ways - it just 
depends on what you want.

As long as you state it, then it does not matter. But as it is, I don't 
think the explanation is clear. Others clearly disagree, as several have 
commented on it being clear. Perhaps it is just me.

> Thanks for the comments.

You are welcome.

> 73
> Gary  K4FMX
> 
> 
> 


-- 
Dr. David Kirkby BSc MSc PhD CEng MIEE
Chartered Engineer
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