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@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Roger
> Sent: Sunday, April 11, 2010 2:45 PM
> To: amps@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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