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Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!

Subject: Re: [Amps] Fwd: Linear Amplifier Tuning---PROPERLY!
From: "Roger (K8RI)" <>
Date: Wed, 28 Dec 2011 18:48:54 -0500
List-post: <">>
On 12/27/2011 1:23 PM, Commander John wrote:
>   That is about what I said.  But it doesn't make the signal any wider.
> please read the previous posts as to what I was commenting on.
> john W9ZY

Let me take a shot at this:
I'm sure some one else can do this more clearly than I but...

First, lets separate the lower stages from the driver and final.
Lets say your transceiver has an audio pass band of 0.3 to 2.8 KHz or 
2.5 KHz.   Now lets hit it with two tones. One at 300 Hz and one at 2.8 
KHz and make the foolish assumption that this is still a clean 2.8 KHz 
wide signal when it gets to the final.

  We now have two tones that on upper side band are your carrier 
frequency plus 300 Hz and plus 2.8 KHz or still 2.5 KHz wide if all went 
well.     Now that final can not only pass and amplify these two signals 
at 750 watts each, but if it has only - 20 db 3rd order IM we also now 
have signals at 2.8 KHz +/- 300 Hz or 2.5 Khz and 3.1 KHz. 20 db down 
(If I understand this correctly)  we also have 300 Hz +/- 2.8 KHz or 3.1 
KHz and a - 2.5 KHz  3.1  and a - 2.5 KHz below the carrier  for 5.6 KHz 
band width instead of 2.5.  The signal is over twice as wide.   What if 
we have 2KHz and 2.8 KHz.  Now we have 800 Hz (still in the pass band) 
and 4.8 KHz but we also have 2 KHz - 2.8 KHz or a - 2 Khz (below the 
carrier)  and we are now up to 6.8 KHz wide.  IOW in a two tone test we 
have the sum and difference between *both* frequencies. In the latter 
example both sums fall on the same frequency while but the differences 
fall on different frequencies.  Those that fall on the same frequency 
depend on the phase as to whether they add, subtract, or fall some where 
in between.

In either case the IM does cause the signal transmitted to be wider than 
we'd hear from an amp with -50 db IMD products and I think they'd be 30 
db stronger.  You can keep taking the results and running the sum and 
difference on all of those and then repeat with all of those results 
with diminishing strengths each time.

I'll leave it to others to correct my work as I've never gone through a 
math problem without making a mistake yet even though I have a minor in 
it. <sigh>
It may not be correct but it should convey the general idea.


Roger (K8RI)

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