[AMPS] radios/imd/intercept points

[Peter Chadwick]

Igor says:

>secondly the  receivers  3rd order intercept points

Unfortunately, this still fails to address the fact that a high intercept
point in a receiver is totally useless without adequate phase noise
performance to go with it. A +20dBm third order input intercept point needs
at least  -131dBc/Hz phase noise performance at the offset at which IMD is
measured.

For a 10dB noise figure and SSB bandwidth, the noise floor is -140dBm. To
produce IMD signals at noise floor with a +20dBm TOIP, the inputs are at 
-33dBm - that's about 5mV, which is a big signal - S9 + 40 on the 'usual' S
meter standard. In practice, noise at the antenna will be at least 10dB
above this. 

Admittedly, the number of IMD products is n(n-1)+ 1/2(n-1)(n-2) for a system
in which the front end bandwidth is less than a half octave, where n is the
number of input signals intermodulating. This does suggest a somewhat higher
intercept point is required than would be established just from
consideration of the highest level signals, while the phase noise
contribution is n times the number of signals. However, signal power
distributions show that there are relatively few big signals (which make the
most phase noise contribution), and more lower level signals which porduce
the IMD noise contribution. Thus there is justification for having the IMD
dynamic range equal to the phase nosie dynamic range. See my paper at RF
Expo, 1986.

73

Peter G3RZP


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