Hi Frank,
'
> Second harmonic of 27 and 28MHz are 54MHz and 56MHz.
> So a 54MHz signal is added two a 28 MHz signal, with some non
> linearity function after that. So I get the sum and products: 26 and
> 70MHz. Doing the same with 56 and 27, I get 29 and 83MHZ. With
> filtering, I will only keep the contents near the IF bannd: 26, 27, 28
> and 28Mhz.
> Ok, I it matches so far.
> >Now, it gets more tricky - if the level of BOTH of the input signals
> >is increased by 1dB, then the levels of the 3rd order IMD products
> >will increase by 3DB.
> I am still struggling here. I would assume that a model of
>
> y(t)= a x(t)+ b x(t)^2 + c z(t) + d x(t) z(t) +e z(t)^2
> x(t)=input signals = x1(t) + x2(t)
> z(t)=local oscillator
>
> I am trying to map this idea to the explanation you provided. I just
> don't see how come in the below graph
> http://en.wikipedia.org/wiki/Image:Interceptpoint.png
You do not need any local oscillator to get IM3. You will have
it in amplifiers and almost anything that you pass signals through.
IM3 is a problem in professional antennas for example.
This is all you need:
y(t)= a x(t)+ b x(t)^2+ c x(t)^3
Here x= A*{cos(w1*t)+cos(w2*t)}
w1 and w2 are the angular frequencies, 2*pi*freq
Products like cos(w1*1)*cos(w1*t)*cos(w2*t) are generated
by the third order term c. When going from product to sum
with standard trigonometric formulae one will see that
such products contain the IM3 frequencies. If you do the math
you will find that they grow in third order. 3 dB for a 1dB
increase of A.
There is a discussion here:
http://www.sm5bsz.com/dynrange/intermod.htm
There is also a link to the math details, but expressed
in terms of sin(w*t) which is clumsy (I did not know when I
wrote that sub-page.)
73
Leif / SM5BSZ
