K3 Receiver Noise Level -- Evaluation and Adjustment

[John Lemay]

Hi Joe

My measurements were taken from the Line Out on the K3, with the line out
setting of 15. No modifications.

Regards

John


-----Original Message-----
From: Joe Subich, W4TV [mailto:[hidden email]]
Sent: 20 December 2009 17:58
To: 'John Lemay'; [hidden email]
Subject: RE: [Elecraft] [K3] K3 Receiver Noise Level -- Evaluation
andAdjustment


John,

Where did you make your measurements?  There are significant
differences among the line, speaker and headphone outputs as
well as the IMD/harmonic performance depending on the level
of modification.

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of John Lemay
> Sent: Sunday, December 20, 2009 11:33 AM
> To: 'Sverre Holm (LA3ZA)'; [hidden email]
> Subject: Re: [Elecraft] [K3] K3 Receiver Noise Level --
> Evaluation andAdjustment
>
>
> Hello Sverre
>
> Thanks for putting your spectral analysis on the web page.
> It's very interesting and I decided to do my own test. I used
> Spectrum Laboratory, but it displays similar information to Spectran.
>
> I have deviated from white or pink noise, because most of the
> time we are listening to real signals (or qrm!). I decided to
> inject a signal: I used my K3 and XG1 to perform the test.
> The line out from the K3 was connected to line in on my PC.
>
> With reference to a fundamental audio signal at 0db, a third
> harmonic of this was observed at about -40, and a fifth
> harmonic at around -50dB. The even order harmonics are well
> down in comparison.
>
> I don't know if this is a good result or bad. But it's clear
> to me that if I am listening to a 700Hz CW note, no amount of
> audio roll-off at 4kHz or so is going to change the relative
> levels of those first few harmonics.
>
> Just for fun, I ran the same test using an FT-847. The
> results were very different. Firstly, harmonics were down at
> -55 or better. Secondly, the even order products (2nd and
> 4th) were louder than the odd ones. And lastly, the FT-847
> graph showed several close-in spurious alongside each
> harmonic, which had been completely absent with the K3.
>
> I hope this all makes sense. And apologies if my use of
> technical terms is not spot-on. I'm just an amateur, with no
> training in electronics or radio !
>
> Regards
>
> John G4ZTR
>
>
> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Sverre
> Holm (LA3ZA)
> Sent: 20 December 2009 13:13
> To: [hidden email]
> Subject: Re: [Elecraft] [K3] K3 Receiver Noise Level --
> Evaluation and Adjustment
>
>
> Concerning the last few days' discussion of noise and
> artifacts, here is a spectral analysis plot of my K3's
> output: http://www.qslnet.de/member/la3za/bilder/K3-CW-spectran.jpg
>
> First it shows the signal related artifacts in the 10-13 kHz
> range as mentioned by Wayne N6KR below. It also shows line
> noise at approximately every kHz (harmonics of 1.025-1.030
> kHz). The highest of these is at approx 2050 kHz and is at
> about -40 dB relative to the main signal, i.e. at a level of
> about 10^(-40/20) = 0.01 or 1%. Thus these lines could be
> audible and be part of the explanation for the complaints
> about the K3's sound. Apart from the lines, the background is
> down to -60 dB above 2-3 kHz.
>
> It is interesting to note that my spectran plot has discrete
> line noise as opposed to that of G4ILO in his blog from today
> (http://www.g4ilo.com/blog.html), plot here:
> http://www.g4ilo.com/uploaded_images/k3freqresponse-736797.gif
>  . His background noise level is similar to mine.
>
> Could this be due to differences in individual K3's as has
> been hinted to on this list?
>
> My setup is:
> K3 with PRE on at 14.102 MHz, RF gain at max,
> CW with 400 Hz roofing filter and bandwidth set to 400 Hz,
> AGC-F on and with AGC parameters SLP=9, THR=3, I use a lot of
> averaging in Spectran to make the lines more visible as seen
> from the setup info in my plot
>
>
> wayne burdick wrote:
> >
> > This filter rolls off everything at about 4.5
> > kHz, including some low-level AF sampling artifacts from 10-14 kHz  
> > that some operators with excellent ears can hear.
> >
>
>
> -----
> Sverre, LA3ZA
>
> http://www.qslnet.de/member/la3za/K2/mod.html LA3ZA
> Unofficial Guide to K2 modifications
> --
> View this message in context:
> http://n2.nabble.com/K3-Receiver-Noise-Level-Evaluation-and-Ad

justment-tp418
7070p4194554.html
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[n7ws]

As others have stated, using the Line Out as a proxy for the headphone and/or speaker outputs is probably not a good idea.

The only time it is valid is if you are using the Line Out into the same sound card for digital modes.

Otherwise, if you are using headphones, you should be looking at the spectrum with your 'phones plugged in. If you are using an external speaker then look across it.  If you are using the internal speaker, stop and avoid it like the plague (unless you play electric guitar and like the sound of distortion).

The other caveat: avoid over-driving the sound card.  Good lab practice when making spectrum measurements with any instrument is to insert some attenuation between the source and the analyzer and look at the effect.  If you put in 10 dB and some product drops 20 dB, you have an issue.

Of course, all of this assumes that you have clean power supplies and no ground loops adding line frequency spurs to your plot.

Wes  N7WS


--- On Sun, 12/20/09, Julian, G4ILO <[hidden email]> wrote:


However - and this is the interesting bit - the level of the harmonics
relative to the fundamental can be varied by changing the LIN OUT level. On
my K3 reducing LIN OUT to 003 and they just about disappear. (Obviously that
also depends on the strength of the input signal.)

It seems to me that there is some non-linearity in the audio stages that
sets in at a fairly low level. I'm not an expert either, but might the
isolating transformers in the line output be the cause of this?




     
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[Leigh L. Klotz Jr WA5ZNU]

Sverre,
You might also consider an even lower LINE OUT level.
I use 3, and the K3 manual recommends no more than 10.
Leigh/WA5ZNU

Sverre Holm (LA3ZA) wrote

It appears that there are several sources for lines in the spectrum of the output on the LINE OUT jack.
What I measured earlier today was at a LINE level of 015,