[Elecraft] KX3 12kHz audio spike

[Jim Brown]

On 8/6/2013 5:51 AM, Lyle Johnson wrote:
> It is always best to use headphones or external speakers that have 
> limited response above 5 kHz with the KX3.  The radio will not pass 
> audio information above this frequency, so the only thing wide 
> response headphones or speakers will provide is additional noise. 

Horsepucky.  This is nothing more than an excuse for poor design. There 
are several possible solutions, perhaps in combination, but using lousy 
headphones or speakers is not one of them -- headphones and speakers 
with poor HF response are also likely to have very bumpy amplitude and 
phase response in midrange.

First, the audio bandwidth ought to be limited in the radio -- a few 
poles of low pass around 6 kHz would make a nice dent at 12 kHz. Second, 
if there's that much ripple, it sure sounds to me like the power supply 
is either inadequately filtered or poorly regulated, or there's 
insufficient decoupling somewhere in the audio chain.

Or perhaps even a circuit layout issue. On several occasions, I've urged 
the Elecraft engineering team to attend one of Henry Ott's excellent EMC 
workshops. Henry talks about keeping track of where the current is 
flowing -- ALL of the current, not just the "intended" current, and he 
talks about the invisible schematic hidden behind the "ground" symbol, 
which is one of the most common ways we lose track of the return 
current. If, for example, the current associated with that 12 kHz clock 
happens to share a return current path with an audio gain stage, it gets 
added to the audio. It's the same sort of mechanism that if it's at the 
junction of the box and the outside world we call a "Pin One Problem."

73, Jim K9YC