[Elecraft] K3 Noise Floor 160m - 75m

[Paul Christensen]

Problem solved.  The RS232-to-USB adapter between the K3 and PC is 
responsible for 100% of the noise.  I now recall running into this issue 
before with the Elecraft USB adapter used on my LP-100 wattmeter.  10T 
through a #31 Fair-Rite toroid cleaned it up completely.

Paul, W9AC

----- Original Message ----- 
From: "Paul Christensen" <w9ac at arrl.net>
To: <elecraft at mailman.qth.net>
Sent: Sunday, July 19, 2009 2:15 PM
Subject: [Elecraft] K3 Noise Floor 160m - 75m


>I was recently looking at the noise floor of my K3 with SDR-IQ with antenna
> disconnected.  As I went from the upper bands down to 75m and 160m, I
> noticed that the noise floor was not particularly clean on some portions 
> of
> the low bands.  I have attached links to two print-screen shots of what I 
> am
> referring to.  In both cases, the antenna is disconnected and the pre-amp 
> is
> engaged.  I can hear the high noise floor as well as see it on the SDR-IQ
> screen, even with the SDR-IQ disconnected.  A few more important points:
>
> 1) The radio is not being coupled to a nearby magnetic field.  The shots
> taken are isolated from the radio shack and the rig is battery-powered as 
> is
> the laptop PC.  Same result when the rig is in the shack and connected to 
> a
> desktop PC;
>
> 2) The result is the same if I toggle the ANT2 port with no antenna
> attached.;
>
> 3) Although I am sampling the IF with the SDR-IQ, the noise floor remains
> with the SDR-IQ completely disconnected from the K3.  The only relevant 
> mod
> is the change to the IF voltage divider as described by N8LP; and
>
> 4) With the pre-amp disengaged, the noise floor drops as expected, but the
> noise floor rise as shown in the screen capture is still evident.
>
> In the first link, I have captured the effect on the upper end of 75m.
> Notice that the broadband noise bump is generally just outside the band 
> but
> even so, the noise floor is noticeably higher than when measured down 
> toward
> 80m as shown in the second link:
>
> http://tinyurl.com/ltyc2o
>
> Down toward 80m...
>
> http://tinyurl.com/kpxhgb
>
> I can replicate this on 160m as well.  All other bands are clean.  Can
> someone confirm this effect by using their K3 (with N8LP buffer mod) and
> SDR-IQ unit and get back to me by private E-mail?  Thanks!
>
> Paul, W9AC
>
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