> Jim Thomson wrote:
>>
>> ### 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.
>
> There are lots of claims, but I have not seen anybody post anything to
> substantiate these claims.
>
> I think it is a quite complex issue. I suspect an exact analysis is
> possible,
> which would put absolute limits on the parameters, but it would not be
> trivial.
>
> The fact that the 3rd order products of one device makes higher order
> products
> in another, means you can't simply add powers.
>
>> 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.
>
> Well, irrespective of whether there is a pad or cable loss, or
> pre-emphasis, the
> fact remains the situation is completely different from amateur radio,
> where the
> aim of the amplifier is to produce a bigger signal, rather than to just
> overcome
> losses. As such, the analysis from the CATV book seems totally
> inapplicable to me.
>
>
> Dave
I consider the CATV amp analogy a good way to understand what is happening
in a receiver. There is no need to cascade amps, simply add xx number of
sources into a single amp which becomes the radios front end. The 2 signal
IMD method of testing an amp or a radio is equally useless except for
marketing.
Carl
KM1H
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