----- Original Message -----
From: "Dr. David Kirkby" <david.kirkby@onetel.net>
To: "Roger" <sub1@rogerhalstead.com>
Cc: "'AMPS'" <amps@contesting.com>; <garyschafer@comcast.net>
Sent: Thursday, April 22, 2010 6:52 AM
Subject: Re: [Amps] A tale of two IMs What happens?
> Roger wrote:
>>
>> Gary Schafer wrote:
>>> IM and cross modulation are the same thing. With the catv amps there are
>>> many signals operating at nearly the same level that mix with each other
>>> to
>>> produce new products, some of which fall on the frequency of existing
>>> signals. The only way for IM products or cross modulation products to
>>> appear
>>> is by multiplication.
>>>
>>> The levels of the IM or cross modulation products increase in strength,
>>> with
>>> cascaded amplifiers because those product levels add together from one
>>> amplifier to the next in a linear fashion. Each amplifier produces its
>>> own
>>> IM products that fall on the same frequency as those produced by the
>>> previous amplifier.
>
> To be more precise it is the *voltages* which add. The voltages must be
> added as
> vectors, not scalers, so it is not true to say the powers will add. They
> could
> cancel under some highly unlikely set of circumstances.
>
>> I believe it's rare for any amp to have the output either in phase of 180
>> degrees out of phase with the input. BUT IIRC it's not all that
>> complicated to set the phase to what every you desire in the design.
>
> For a spot frequency, all one needs to do is have the right length of
> cable on
> the output - you can get any phase shift you want.
>
> However, that will change the phase of the IM products by the same amount,
> so it
> does not achieve you anything useful.
>
>> If it's true that each amp generates identical IM products in amplitude
>> and frequency then it'd seem to be a relatively simple matter to set
>> the output phase 180 degrees from the input and end up with a net zero
>> IM for each even number of amps while each odd number would have only
>> those IM products from that amplifier, but I don't think it's quite that
>> straight forward or simple. Then again I was wrong one other time.
>
> You are not wrong about it not being as simple as that.
>
> I think to do this analytically one would have to consider the non-ideal
> amplifier as being a mixer and an amplifier. Perhaps I'll look at this at
> some
> point. Using Sage
>
> http://www.sagemath.org/
>
> should take out some of the hard work of the maths! (Though to be honest I
> know
> Mathematica better, but Sage is free, whereas Mathematica is several
> thousand
> pounds or dollars a copy).
>
> Dave
>
Note that CATV specs are written in dBuV at 75 Ohms and not dBm as is common
in the 50 Ohm world. Switching between the two methods sometimes several
times a day used to require sticky notes on the test equipment!
Then there was Return Loss vs VSWR to confuse a bit more.
It all took some getting used to.....similar to being bilingual.
Carl
KM1H
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