[Elecraft] KX3 12kHz audio spike

[Nicklas Johnson]

I decided to build a thing to work around the problem when using
headphones. I did find that turning I/Q on reduced the spike significantly,
but didn't eliminate it entirely.  I found that a pair of .47uF capacitors
and a 3mH choke did a nice job of reducing the noise over 6kHz though:

http://i.imgur.com/bAiquCc.png

I'd never built a pi-network before, so this was a decent learning exercise
too.  The reality matched up with the math quite well, I'm pleased to say.
 The spur at 12kHz went from about -59dB to -78dB, almost exactly 18dB (as
before, I'm also adding a lot of gain in the capture device to emphasize
the problem; in practice, the spur isn't anywhere near that loud).

It takes the spur at 12kHz down about 18dB, which is enough to get it well
under the noise threshold for normal listening conditions for my ears.  Of
course it would be much better if the 12kHz spur weren't there in the first
place so it wasn't necessary to try to filter it out after the fact.

One thing that's kind of interesting in the spectrum analysis is that
filtering above 6kHz looks like it emphasized the spectrum below 6kHz.  I
suspect that this is not actually what happened (given that the filter is
all passive components), but rather the filtering either slightly changed
the results of the FFT calculations, or possibly the responsiveness of the
audio capture device.  I'd be interested to hear others' theories on this
unexpected result, too.

   Nick



On 7 August 2013 10:25, David Gilbert <xdavid at cis-broadband.com> wrote:

>
> I didn't say that RF current in a ground plane wasn't "well-behaved",
> whatever that means.  I said that it wasn't uniform, as in uniform density.
>  It's not.
>
> 73,
> Dave   AB7E
>
>
>
> On 8/7/2013 2:10 AM, Jim Brown wrote:
>
>> On 8/6/2013 8:49 PM, David Gilbert wrote:
>>
>>> Currents most certainly do not flow uniformly in ground planes, and
>>> coupling effects can be real whenever significant gain is involved.
>>>
>>
>> Actually, what happens is that a trace above an ideal "ground plane" (the
>> ground layer)  forms a transmission line, with the return current flowing
>> in the ground plane directly below he trace -- UNLESS the ground plane
>> below the trace is broken, for example, by the circuit board layout guy
>> realizing he left something off the main layer, and putting on the ground
>> layer instead. In which case there is no longer a transmission line, the
>> return current flows wherever it can. inductance is added to the path,
>> which causes magnetic coupling to other circuits, and it forms an antenna,
>> so it can radiate, into other circuitry, or outside the box, or both.
>>
>> So the RF current in a ground plane is VERY well behaved and predictable.
>> The problems arise when the PC layout guy doesn't understand the function
>> of the ground plane, and that it CANNOT be interrupted without
>> consequences. Like zipper noise on tuning in the K3.
>>
>> 73, Jim K9YC
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