KX3 VFO tuning noise reduced or eliminated

[wayne burdick]

Some KX3 owners hear a pulsing or clicking noise when tuning the VFO on higher bands, especially when atmospheric noise is low. We discovered that the KXFL3 filter module is responsible for nearly all this noise, and that it can be reduced or eliminated with a simple modification to the module. This modification, which does not require any desoldering, is described in a document on our website:

    http://www.elecraft.com/manual/KX3%20Tuning%20Noise%20Reduction%20Mod.pdf

Factory assembled KX3s from serial number 4668 up, and kits from serial number 4753 up that include the KXFL3 option have been shipped with the updated board. (Note: The modification performed at the factory is slightly different from the one described in the document, but the resulting decrease in noise is the same.) Any KXFL3s purchased after July 4th for field installation will also include the modification.

You can tell if your KXFL3 board has been factory modified by opening the KX3 case, as if to change batteries. Inspect the KXFL3 module. If pins 11 and 12 are removed from the board, the KXFL3 has been modified. If not, you can easily perform the modification yourself using the instruction sheet.

In rare cases, such as when an extremely low-noise antenna is in use, you may still hear some very weak tuning noise even with a modified KXFL3 or with no KXFL3 module installed. In such cases we recommend trying the following firmware-based approaches to reducing the noise:  (1) set MENU:VFO NR to ON; (2) turn on the noise blanker and set it to a low level; (3) set MENU:RX SHFT to 8.0 instead of NOR. These settings are all saved per-band, so you only need to use them on affected bands.

73,
Wayne
N6KR




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[wayne burdick]

TJ Campie wrote:

> How come this works? What was the function of those pins that caused the problem?  

The three pins removed in the KXFL3 modification provide control signals for accessing a small EEPROM on the module. This memory device was intended to store DSP gain/phase parameters for opposite-sideband nulling. However, it turns out that gain/phase parameters are also dependent on hardware on the RF board. So we have never used the EEPROM, and it will be eliminated from future production runs.

The noise heard when tuning the VFO is due to RF radiation from the digital control lines into adjacent circuitry including the synthesizer. When the three pins are cut or removed, the bus is no longer loaded by the EEPROM on the KXFL3, and the signal path is shortened.

73,
Wayne
N6KR

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[Mike WA8BXN]

The instructions suggest that realignment of the filter may be necessary if
some of the capacitors are pressed on the board. Is there any simple way to
tell if such realignment is necessary? Is it very likely that if those parts
are not touched realignment is not needed?
 
73 - Mike WA8BXN
 
 
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[wayne burdick]

Mike WA8BXN <[hidden email]> wrote:

> The instructions suggest that realignment of the filter may be necessary if some of the capacitors are pressed on the board. Is there any simple way to tell if such realignment is necessary?

As long as you handle the module carefully and don't press down hard on the large, white, rectangular film capacitors on the back side of the module, there should be no need to do any realignment.

If you're concerned, you could spot-check opposite sideband suppression on 20 meters. To do this, first go to CW normal mode and normalize the filter passband (400 Hz, centered). Next, tune in a strong signal in the clear (S9 or so), check the S-meter reading, then tune the VFO downward by an amount equal to your sidetone pitch x2. Recheck the S-meter reading. Typically it will be 50 dB or more down (where an S-unit is about 5 dB). Sideband suppression can vary a bit with pitch, filter, and AGC settings, but this will give you a rough idea if the module is still properly aligned.

There an app note on our KX3 web page describing all-band RXSBNUL alignment. This can be done manually (with any RF signal generator), or in an automated fashion when used with an Elecraft XG3.

73,
Wayne
N6KR





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[Rich Heineck]

Mike,

What you would be checking for is opposite sideband suppression and
you'll need a very strong single tone signal source.  S9 + 20 or
thereabouts is a good level, and this can be on or around 20m. There's a
procedure to do the calibration in the KXFL3 installation instructions,
but the following is a fairly quick approach if you have a handy signal
to use.

- Turn AGC MD (menu) to OFF.  Set AF gain to 1 before tuning to a strong
signal.  Set the PBT BW for 2 kHz, which should be FL1 setting.

- Set mode to CW and  check your PITCH setting.  Multiply the pitch by 2
and set up RIT for minus this amount.  For example, if PITCH is 550 Hz,
set RIT to -1.10 kHz.  Turn RIT OFF once it's set up.

- Tune to the signal and adjust the preamp setting if needed to try and
get the level to S9+10 or more.

- Turn RIT on to hear the opposite sideband.  The signal should be very
weak and likely close to the noise floor.  You'll also normally need to
turn the AF gain way up to hear it.

- Turn the AF gain back down then reduce the PBT BW so you get a change
to FL2 and then again to FL3.  The opposite sideband should be very weak
for those settings as well.  If it jumps up considerably, or if it's
just not very weak for any of the BW settings, a calibration would be
helpful.  In that case, follow the procedure in the KXFL3 installation
instructions.

- Don't forget to turn AGC back on when done :-)

73,
Rich  AC7MA




On 07/08/2013 10:12 AM, Mike WA8BXN wrote:

> The instructions suggest that realignment of the filter may be necessary if
> some of the capacitors are pressed on the board. Is there any simple way to
> tell if such realignment is necessary? Is it very likely that if those parts
> are not touched realignment is not needed?
>  
> 73 - Mike WA8BXN
>  
>  
> ______________________________________________________________
> 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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