Hi,
I am learning some RF knowledge. The following is a post years ago on
this group. I don't understand how the last poster got -89.5 dBM.
Could you tell me that? Thanks in advance.






http://groups.google.ca/group/rec.ra...3f 1163952dd9
-----------------
am designing an RF stage that needs to have quite low
intermodulation
distorsion.


When reading data sheets for various amplifiers they usually give a
figure
fo IP3.


What I haven't found anywhere is if IP3 is related to the input or
the
output. Can ayone help me on this point?


I need ntermodulation distorsion to be at least 100 dB down at the
output of
the amplifier at a level of 1.5 dBm, and I need a gain of
approximately
30-40 dB.

-------------
Tomas wrote:
Is IP3 related to the input of the amplifer.


In general, IP3 is related to the output for amplifiers and to the
input for
mixers.

In the example using the Mini-Circuits amplifiers, the output 3rd
order
intercept point will remain at essentially +47 dBM. With an output
signal at
the +1.5 dBM level, the distortion products will be at a level of
-89.5 dBM.
This is only 91 dB down from the output level and not the 100 dB
desired.


In order for the distortion products to be at a level of -98.5 dBM,
which is
100 dB down, the output intercept point of the last amplifier must be
at
least 51.5 dBM.


A Mini-Circuits MAR-8 will make things worse because of the lower
OIP.


Gain has nothing to do with these calculations. However, something to
keep
in mind is that increasing gain reduces dynamic range at the high
signal
level end. While an increase in noise figure reduces dynamic range at
the
low signal level end.


73, AL, w6wqc,