[UK-CONTEST] OT- Attenuation and IMD3

[Clive Whelan]

Not directly contest related of course, but something we all have to 
deal with, so hopefully of interest.


Let me declare that I am not  a techie, at least not in the engineering 
or radio field, just a hopeless appliance operator. We  all know I 
suspect, that in a less than perfectly  linear receiver that two 
interfering signals on F1 and F2 will generate spurious ( phantom) 
signals on 2xF1-F2 and/or 2xF2-F1, and if this product falls within our 
passband will generate QRM. The CW operator recognises this instantly as 
the headless chicken effect, i.e. a CW signal present only when both F1 
and F2 are simultaneously key down. Quite how it is manifest on SSB I 
confess I do not know, but suspect just as general " mush" which could 
equally be an overdrive artefact from an adjacent signal, so perhaps a 
little more insidious?.

Now the question: Let's say we have a fairly average rx with a 3rd order 
dynamic range of 70dBm at 2kHz spacing. But if money were not an object 
we could go and buy a rx with a 90dBm range. We don't, so we have to 
deal with the issue in crowded band conditions. Typically modern 
receivers have step attenuators to deal with the matter, whereas in days 
of yore we would have had to turn the RF gain down, possibly losing some 
AGC performance in the process. Something in the dark recesses of my 
brain suggests that all things being equal,  attenuation is more 
effective than reduction of RF gain, and that x dB of attenuation will 
produce better than x dB improvement in the dynamic range, perhaps as 
much as 2 times x dB? If true I am not certain why this should be: is it 
perhaps because both interfering signals are being attenuated?

 Put another way, how much attenuation do I need in my average rx to 
improve the IMD3 performance to the level of the super rig. Would this 
perhaps be (90-70)/2=10dB? Please feel free to disabuse me of this view 
if incorrect, but otherwise, having regard for the fact that virtually 
all modern rigs have this level of attenuation available, and also that 
IMD3 is likely to occur with generally high signal levels anyway, if I 
am prepared to flick a few attenuator buttons when the going gets tough, 
do I really need to spend mega bucks to obviate the issue in the first 
instance?

Thanks for the bandwidth

73


Clive
GW3NJW