[Elecraft] KX3 12kHz audio spike

[Rich Heineck]

Nick,

I think Lyle meant to type RX I/Q, not RX ISO.  Turning on the RX I/Q 
outputs (which is accompanied by the DSP load leveling) will reduce the 
12 kHz spur by 15 to 20 dB typically.  Doing this also increases overall 
supply current by about 10 mA.

When looking at these spurs on an audio spectrum analyzer that uses a 
very narrow resolution bandwidth, they're quite prominent.  But they are 
at or below the noise in a 12 kHz (or greater) bandwidth. For quite a 
few of us we would never know they were there without the 
instrumentation.  I guess good hearing can be a blessing or a curse, 
depending on what you're listening to... :-)

73,
Rich  AC7MA


On 08/06/2013 07:41 AM, Nicklas Johnson wrote:
> Lyle: I gave setting RX ISO to ON a try, and though I can see some small
> changes in the spectrum analyzer, it doesn't seem to change the 12 kHz
> spike or its harmonics.
>
> Granted, my hearing is abnormally good for my age.  I can still hear up to
> about 18 kHz, which is pretty unusual for an almost-38-year-old I'm told.
>   Flyback transformers in old TVs and monitors are the worst thing in the
> world.
>
> I wonder if connecting a 100uF capacitor across the headphone output would
> be about right to attenuate noise above 5kHz.
>
> Johnny: I did this test by routing the headphone output of the KX3 into the
> input of a Roland Quad Capture, which is effectively just a really
> high-quality sound card, turned the gain on the Quad Capture up about
> halfway, and sampled the audio at 192kHz, 32-bit mono into Sound Forge.
>   Then I ran Sound Forge's spectrum analysis tool on the captured audio.
>   You could probably perform a similar test using an ordinary sound card to
> see whether you have the spike at 12kHz, but you might not see the ones at
> 24, 36, and 48kHz if your sound card won't sample above 48000 Hz or won't
> provide enough gain.  Free tools like Audacity also provide spectrum
> analysis if you don't already have something like Sound Forge.
>
> Thanks for the input; it's nice to know at least that I'm not crazy ;-)
>
> 73,
>
>     Nick
>
>
>
> On 6 August 2013 05:51, Lyle Johnson <kk7p4dsp at gmail.com> wrote:
>
>> This is most likely power supply ripple from the variable loading of the
>> DSP, which occurs at a 12 kHz rate.
>>
>> You may be able to reduce it by setting MENU:RX ISO to ON.  This increases
>> average current in part by preventing the DSP from "sleeping" between
>> tasks, thus smoothing out some of the power supply load.
>>
>> 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.  This is
>> particularly true if your ears haven't "aged" to provide some low-pass
>> function for you :-)
>>
>> 73,
>>
>> Lyle KK7P
>>
>>   While doing some tests tonight to compare the white noise of my Icom
>>> IC-7000 to the white noise of the KX3, both having the AF gain turned all
>>> the way down, I noticed...
>>>
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