> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Dr. David Kirkby
> Sent: Saturday, April 17, 2010 4:53 AM
> That's interesting. At first I thought this seemed silly, but then I
> thought
> about it, and I believe it may be semi-true in the case of CATV, but not
> true at
> all in the case of amateur radio.
>
> In the case of amateur radio the purpose of the amplifier is to produce
> a bigger
> signal, rather than simply overcome the losses in the cables. For CATV
> purposes,
> the amplifiers many not added to produce a bigger signal, but just to
> overcome
> losses. That is a VERY different case.
>
> If you have to get a signal over a very long distance using cables, then
> the
> losses of the cables obviously become significant, and you might need to
> have
> many amplifiers in series. So the source is connected to the load like
> this:
>
> source -> CL -> A -> CL -> A -> CL -> A -> CL -> A -> load
>
> where:
> CL = 20 dB of cable loss
> A = 20 dB gain amplifier
>
> In the above case, the 80 dB dB gain just overcomes the 80 dB cable
> loss. So the
> main signal at the load is just at the power same level as the main
> signal at
> the source. In this case, I can see how the IM level would deteriorate
> rapidly.
> At each amplifier, the level of distortion increases, but the level of
> the main
> signal is kept the same.
>
> In the case of an amateur radio power amp, the distortion gets bigger,
> but so
> too does the signal. Since the IMD is given as a ratio, it stay much
> more constant.
>
> So I don't believe this 3 dB applies in the context of the amateur radio
> amplifier stuck on the end of a rig to get a bigger signal. But I do
> believe it
> could (approximately at least) apply in the case of multiple amplifiers
> in
> series which just have enough gain to overcome the losses.
>
> Dave, G8WRB
Hi Dave,
In further reading of the catv handbook it seems that what they mean by
"well behaved amplifiers" is they provide a straight line relationship
between output and IM contributions.
A good one provides a 2 db decrease in IM products for a 1 db decrease in
output. This carries thru most of the output level range of the amplifier.
The "not so well behaved amplifier" has a dip in the output verses IM line
over the output range. In other words it does not always provide the 2 db
decrease in IM with 1 db decrease in output.
This is similar to what our ham amps do.
With the catv amps they are assuming all amps have the same amount of gain
and the same level of input signal. Then the IM product contribution of each
can be directly added together as shown before.
With our ham amplifiers they are amplifying the IM products from the driver
along with the main signal. So if the new IM products generated in the amp
from the main signal are in phase with the IM products amplified from the
driver they should add together. But that would only be true if our
amplifier was totally linear.
I would think that the addition would have to be calculated similar to how
noise figure is calculated with cascaded amplifiers of different gains and
noise contributions.
73
Gary K4FMX
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