[Elecraft] K3 on-channel strong signal overload?

[Geoffrey Mackenzie-Kennedy]

Hi Mike,

There is a penalty to be paid if diodes are used to clamp the level of 
strong signals in a receiver's signal path, and that is the generation of 
intermodulation products. In a "no-split" pile-up situation where two or 
more closely spaced strong signals from other callers are in the passband, 
but not zero beat with the DX, chances are that an intermodulation product 
will land on top of the DX's signal if he returns to somebody before the 
others stop calling. You may ask how a weak DX station could be heard at all 
in this situation even if intermodulation products did not exist, to which 
my reply must be that it is possible.

>From reading the posts, my impression is that this case of  in-passband 
strong signal overload could be viewed in the same fashion as a classic 
example of 3rd Order Dynamic Range droop or "bottoming". There is a 
similarity in the causes even though only a single signal is involved in the 
overload case. Among the usual causes for the dynamic range droop problem 
are (1) too much gain in the front end before the roofer to overcome the 
insertion loss of the roofer, and to maintain a useful overall noise figure, 
or (2) The Input IP3 of the roofer itself is too low, or (3) the 3rd Order 
Dynamic Range of the IF cascade and what follows is inadequate, or (4) a 
combination of these three. All of these are relevant when considering 
overload.

According to my back of envelope calculations, the 3rd Order Dynamic Range 
of the K3's  8.215 MHz IF amplifier and the second mixer is in the region of 
80db before the onset of hardware AGC, the second mixer being the dominant 
element. This calculation does not include the stages after the second mixer 
nor the roofer nor LO phase noise, so I believe that the actual in-passband 
dynamic range looking into the roofer would be less than 80db. I can only 
speculate on the effect that the application of hardware AGC might have on 
the dynamic range of this part of the receiver.

What this means of course is that while the overall 3rd Order Dynamic Range 
of the K3 is very good in an enviroment where strong unwanted signals are 
outside of the roofer's passband, one should expect some decrease within the 
roofer's passband, likewise possible overload. The cure could be difficult 
to implement.

73,
Geoff
GM4ESD


Mike Harris" <mike.harris at horizon.co.fk> wrote on Monday, May 04, 2009 at 
7:20 PM

> Just been looking through my archives.  I remembered that there was a
> simple mod to the K2 to overcome audio distortion in an extreme overload
> environment, I installed it on my K2 and it was totally immune to
> co-channel audio distortion when my very near neighbour VP8LP transmitted:
>
> "K2 - Improved Handling of Extremely strong (and nearby) Signals".
>
> Maybe the K3 architecture offers similar opportunities.