Date: Sun, 18 Apr 2010 09:43:15 +0300
From: "Alex Eban" <alexeban@gmail.com>
Subject: Re: [Amps] A tale of two IMs What happens?
Not quite right:
.
Intermodulation products are multiplicative, not additive: if the first
stage amplifier generates IM products of, say -6o dBc , a second identical
amplifier will generate its own IM products PLUS the Im of the first stage
multiplied by the first stage gain SQUARED. That stems from the higher IM
products of the second stage, driven by the higher sinal: the output of the
first stage.
Alex 4Z5KS
### per ssb systems and circuits, et all... the consensus was/is
that if 2 x IDENTICAL.. IMD circuits are in series... say a TX.. with
-40db imd3... driving a linear amp also with -40db imd3... the
author's claim the worse case you would get is -34db imd3.
The best case you could get achieve is -37db to -38db IMD3.
## Now IF the TX was say -60db IMD3.. and the amp was only
-40db IMD3..... the end result is -40db imd3.
## If the TX is even just a few db better than the amp.. [-45db IMD3]
... the end result is -40db IMD3. Now if the amp is better than the
TX.. end result is the IMD3 is the same as what the TX is.. when
measured by itself.
## part of the issue is.. the amp's own IMD could easily cancel
out some of the TX's imd.
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
## IF I remember correctly, the gain of the CATV amps was much
greater than the cable loss. I THINK the input of each successive
CATV amp was padded down. Sorta like 25 db gain - 20 db CL -
then 5db padding - then CATV amp, then another 20db cable loss, etc.
The CATV amps all used equalizers too, since the cable attenuation
was not uniform... but rises on the higher freqs. IE: pre-emphasis
employed, whereby higher freqs get amplified far mote than lower
freqs. Also known as slope equalizer's.
## analog telco circuits did much the same thing, including
pre-emphasis..esp for Broadcast feeds to local AM/FM
transmitter's... [STL] 'studio TX links' . The equalized
levels we pumped done the cable pair were extremely high in level,
then padding and 'mop up equalizers' used at the broadcast TX end..
usually in the bush some where. The idea of course, was to end up
with a dead flat freq response... and a high S/N ratio. The mop up
eq's were used to flatten the residual pre-emphasis left over.
## back in the late 40's-50's- and into the 60's... the total loss/gain
on an analog LD telco circuit, from west to east coast of north America
was phenomenal. Same idea as the cascaded CATV amps.. but on a
bigger scale. You wonder how they ever worked.
later.. Jim VE7RF
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.
Dave, G8WRB
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