Help. K2 noise level variation with band selected

[raycollins]

I have noticed that as I change frequency bands on my K2 in SSB mode, the audio noise level changes in frequency content.  On the five lower bands (3.5 to 18.2 MHz), the noise sounds typical for a 2.2 KHz audio bandwidth but on the three higher bands (21.0 to 28.8 MHz) it is predominantly high frequency with greatly reduced low frequency content.

The appropriate sideband is correctly selected for each band, receiver sensitivity appears to be well within specification on all bands and the set works perfectly in all other respects.  Could the band pass filters for the top three bands require adjustment?  If anyone can explain why this is happening and suggest a fix (if one is actually necessary), it would be greatly appreciated


Thanks for your help,

Ray  
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[John Oppenheimer]

Hi Ray,

I believe that your observation is normal. I performed a fresh CAL FIL
alignment on the two SSB modes, and tried your test, I get similar results.

When on 21 MHz, USB has lower frequency noise, LSB has higher frequency
noise.

John KN5L
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[Don Wilhelm-4]

Ray,

You would have to use a noise generator for a proper test - the
atmospheric noise varies from band to band and from antenna to antenna.

On the bands above 17 meters, the K2 local oscillator uses low side
injection as opposed to high side injection on 17 meters and below.
That means that the audio passband for 15m USB (and 12 and 10 meters
should be the same as LSB on bands below 15 meters.  There will be no
difference.

If you have doubts, use a wideband noise generator (Elecraft N-Gen) and
look at the audio passband on an audio spectrum analyzer (like
Spectrogram or SpectrumLab) and you should see the same audio passband
shape on 40 meter LSB as you do on 15 meter USB.

The bandpass filters are not likely to make a lot of difference unless
they are severely mis-tuned.  For the best response from the bandpass
filters, tune them in transmit mode for peak output at 100 kHz above the
low frequency end of the band for 40 through 12 meters, on 10 meters
tune them at 26.400 MHz and for 80 use 3750 kHz and if you have 160
meters, start at 1850 kHz and you may have to find a compromise setting
between 160 and 80 meters since the inductors are shared and the
capacitor are fixed (but that bandpass is quite broadbanded abd that
compromise works just fine).

73,
Don W3FPR

On 7/24/2013 6:05 PM, Ray Collins wrote:
> I have noticed that as I change frequency bands on my K2 in SSB mode, the audio noise level changes in frequency content.  On the five lower bands (3.5 to 18.2 MHz), the noise sounds typical for a 2.2 KHz audio bandwidth but on the three higher bands (21.0 to 28.8 MHz) it is predominantly high frequency with greatly reduced low frequency content.
>
> The appropriate sideband is correctly selected for each band, receiver sensitivity appears to be well within specification on all bands and the set works perfectly in all other respects.  Could the band pass filters for the top three bands require adjustment?  If anyone can explain why this is happening and suggest a fix (if one is actually necessary), it would be greatly appreciated
>
>
>

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[pa0pje]

Hi Ray and John,

KSB2 manual page 22:
Note: The BFO settings for LSB and USB are swapped on 15, 12 and 10
meters due to the K2's frequency
mixing scheme. If you use 17 m or below for making BFO changes, the
following rules apply:
LSB: moving the LSB BFO lower in frequency will raise the voice pitch
and improve carrier rejection.
USB: moving the USB BFO higher in frequency will raise the voice pitch
and improve carrier rejection.

That's the reason for what you observe.

73,
Peter - PA0PJE

John Oppenheimer schreef:

> Hi Ray,
>
> I believe that your observation is normal. I performed a fresh CAL FIL
> alignment on the two SSB modes, and tried your test, I get similar results.
>
> When on 21 MHz, USB has lower frequency noise, LSB has higher frequency
> noise.
>
> John KN5L
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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[Don Wilhelm-4]

John,

You will likely notice the opposite difference between LSB and USB on
bands lower than 21 MHz.
You may not have the SSB filters aligned correctly.  It is almost
impossible to get the SSB filters aligned for best results using the "by
the numbers" method used in the manual (although those numbers will
produce workable filters.  The crystal centers marked on the crystal
envelope for the K2 and KSB2 are marked to the nearest 100 Hz while the
crystals themselves are matched to the nearest 10 Hz to create a crystal
set.  That means that the numbers in the manual may be off by as much as
90 Hz.  Refining that would take too much detail in the manual and might
just confuse builders.

There is a solution beyond the manual --

To refine the filters Both SSB and CW as well as RTTY if you want to add
data modes), use a wideband noise generator (Elecraft N-Gen) and observe
the passband setting(s) with an audio spectrum analyzer such as
Spectrogram or Spectrum Lab.

Part 3 of the K2 Dial Calibration article on my website www.w3fpr.com
has information that may be helpful to you.  If you need Spectrogram,
look for the links near the bottom of the home page.

My experience in repairing and aligning K2s is that most all K2s that
come onto my workbench need refinement in the filter settings. I have
had many unsolicited comments about the improvement in the SSB audio
Both for RX and TX) after I align those filters properly. Place the -3dB
point of the low frequency slope for the SSB filters at 300 Hz and they
will work great.

73,
Don W3FPR

On 7/24/2013 6:27 PM, John Oppenheimer wrote:
> Hi Ray,
>
> I believe that your observation is normal. I performed a fresh CAL FIL
> alignment on the two SSB modes, and tried your test, I get similar results.
>
> When on 21 MHz, USB has lower frequency noise, LSB has higher frequency
> noise.
>

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