[Amps] A tale of two IMs What happens?

[Carl]

You can easily here the results during a SSB contest. Even a clear frequency 
outside the SSB window has an elevated noise floor caused by all the IMD 
products from DC to daylight mixing randomly. It becomes white noise.

This was covered in the early 80's in a paper dealing with the IMD 
requirements of CATV trunking amps in order to meet near studio level video 
quality.

Carl
KM1H

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----- Original Message ----- 
From: "Gary Schafer" <garyschafer at comcast.net>
To: "'Roger'" <sub1 at rogerhalstead.com>; <amps at contesting.com>
Sent: Sunday, April 11, 2010 4:35 PM
Subject: Re: [Amps] A tale of two IMs What happens?


> Roger, you are indeed a trouble maker. :>)
>
> Generally IM products from the exciter and the amp will add. But they 
> could
> subtract too if the phase happens to be right between the driver IM 
> products
> and the amp generated IM products.
>
> Another thing that happens is the peak powers in the driver add together. 
> In
> other words the IM products peak power and the wanted signal peak power 
> add
> together in the driver to slightly increase the overall PEP. This also
> happens in the amp. The amp has to have more headroom to handle that
> increase in PEP even though it is not useful or wanted additional power.
>
> The IM products from the driver will create additional IM products in the
> amp in addition to what the main signal products cause. Those additional
> products will be rather small as the driver IM products are rather small 
> to
> begin with compared to the main signal. But it all adds to the overall IM.
>
> 73
> Gary  K4FMX
>
>> -----Original Message-----
>> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
>> On Behalf Of Roger
>> Sent: Sunday, April 11, 2010 2:45 PM
>> To: amps at contesting.com >> Amps Amps
>> Subject: [Amps] A tale of two IMs What happens?
>>
>> No one has touched the question on how reducing power without returning
>> affects IM I now have another one.
>>
>> Given that most of todays transceivers have an IM or roughly -35db "so
>> I've been told", and we put a amp behind it that also has an IM of
>> -35db, what is the resultant IM?  What if the amp has an IM of -55db?
>> Do they add, subtract, or go with the lowest number?  IOW it is the amps
>> job to "faithfully" reproduce the input signal, but that really only
>> happens when running class A, if the user is lucky.
>>
>> Intuitively, "I would think" that the two figures would add, but if that
>> were the case the amp with -35db and the exciter with -35db would have a
>> pretty ratty signal. OTOH in the case of the -55db amp behind the -35 db
>> exciter does the amp "clean up" the exciter signal? Doesn't seem likely.
>>
>> It'd be interesting to see the IM figures for today's exciters, (and
>> amplifiers) rather than than adds that just say "Provides amazingly low
>> IM".
>>
>> I also see I need a refresher on working with logs.<:-))
>>
>> 73
>>
>> Roger (K8RI)
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>
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