[Elecraft] Added LIN OUT measurements

[Paul Fletcher]

Jim Brown-10 wrote:
> 
> The DSP is not operating at baseband audio either -- it is another 
> IF just above the audio passband. The only way it's going to 
> generate an audio product as distortion is the IM of two or more RF 
> signals. 
> 

Jim,

I completely understand the K3 architecture and that the DSP operates at a
low IF. DSP is still software though and can still have bugs - no aspertions
being cast just a simple statement. This can produce weird effects and
analogies to analogue techiniques are only really applicable when describing
DSP topologies - implementation is a whole different ball game. As I'm sure
you are aware from your audio work (very interesting articles by the way)
digital processing of any kind of signal introduces a raft of new pitfalls
such as Nyquist aliasing, clock jitter, differing data formats etc etc.

I see strange things happening that I cannot attribute to non-linear effects
of the AF stage. Let me explain (maybe my fourier transform theory is a bit
rusty - not used it for 20 years or so).

As far as I can remember when a sine wave starts to clip odd order harmonics
are introduced at varying phase and amplitude in relation to the
fundamental, depending on the level of clipping. So far so good. Now, I have
decoded a harmonically related PSK31 signal (i.e. a "ghost") with no problem
whatsoever. This implies that the phase relationships are as per the "real"
signal. Also the bandwidth of the ghost signal is exactly the same as the
"real" signal with Lin out set at 4 (yes I realise this is not the phones
output but trust me the ghost signal appears on the phones output as well).
The thing that puzzles me is this. When a PSK signal is idling it is two
tones a set frequency apart. For the purposes of this excercise let's say
the tones are 150Hz and 200Hz. The third harmonic of each tone is 450Hz and
600Hz respectively so the ghost signal should have a bandwidth of 150Hz but
it doesn't - it's still 50Hz (50Hz may not be the real spacing of the but
you get my point).

If my reasoning is off beam I would welcome an explaination (still learning
something new each day).

Regards,
Paul
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