[Elecraft] K2 cellphone interference

[Don Wilhelm]

Brian,

I have heard another report of this noise, but no analysis of the 
effect.  Yes, the fact that the level does not depend on your audio gain 
setting indicates that it is confined to the AF Amplifier.

Note that the K2 uses the LM380N BYPASS pin (8) for sidetone input. 
Normally, the BYPASS pin is used to increase the power supply rejection 
capabilities of the LM380 and many designs simply ignore it (nothing 
connected) because the "power source is well filtered for ripple".

As a quick test, try grounding the negative side of Control Board C32 
with a very short (low inductance) wire and see if that improves the 
situation.  If so, it could be that the power supply wiring in the K2 is 
picking up the radiation and feeding it to the audio amp.  Understand 
that this is just a guess, but it would be nice to know if this is the 
source of the interference.  Of course, the real problem is the phone - 
if it radiates that much, I wonder just how well it meets other 
radiation requirements.

Note that your sidetone will not work with the test wire I suggested in 
place - I did say it was a test!  If one would want to implement adding 
the bypass capacitor as a permanent fix, then a different method of 
sidetone injection would have to be created.  A quick look at the K1 
audio may provide some ideas (add an LM386 ahead of the LM380N for more 
AF gain and isolation of the AF stream from the sidetone - no, I do not 
have time to design and test and document it, it is just a thought).

73,
Don W3FPR


Brian Lloyd wrote:
> Our K2 appears inordinately sensitive to radiation from my GSM phone. 
> It occurs with the audio gain turned all the way down so it appears 
> that this problem might be with the audio amp stage. Have others 
> experienced this and, if so, has anyone come up with a fix?
>
> -- 
>
> Brian Lloyd                         Granite Bay Montessori
>