[Elecraft] K3 RFI in transmitted audio

[Joe Subich, W4TV]

 > This is not a good idea. Repeating W4TV's warning with a different
 > choice of words....

Actually, using +13.8V *from the transceiver* is not a major issue
since there is no current return via the microHAM interface (or any
other accessory device).

The big problem is with using the same supply as the transceiver
and connecting at the power supply end of the transceiver power
cable because a portion of the transceiver return current flows
through the microHAM interface (or other accessory device).  That
return current modulates the V- line of the accessory and puts
the voltage drop on the accessory V- line in series with the
microphone audio!  The variable voltage on the accessory V- is
often in the 10 to 50 mV range - equivalent to the level of many
microphones!

The problem with the K3 +12V jack is the two series diodes and the
power switch FET between the +13.8V input and the switched power
output.  The voltage drop is rather significant and can be quite
variable under load ....

 > Always use separate DC power supplies for SSB transceivers operated
 > concurrently:  one supply per transceiver.

This is good advice ... and always bond the chassis of each piece
of equipment to the common ground point and not "chassis to chassis."

73,

    ... Joe, W4TV

On 9/11/2010 3:02 PM, Guy Olinger K2AV wrote:
> Hi Felipe,
>
>>> microham is being fed from K3 12v port.
>>>
>
> This is not a good idea. Repeating W4TV's warning with a different
> choice of words....
>
> The 12V at the K3 will vary up and down according to the audio current
> draw, at an audio rate.  This variation in the 12V will now modulate
> anything else that uses it including audio circuits in the uHam gear.
> Depending on the characteristics this could cause the transmission
> from one rig to appear in the other. Even without uHam or other
> utility low level audio boxes, the same thing can happen with the two
> rigs if they share the same power supply. It would likely be a muddled
> sounding audio crosstalk, but based upon the degree of DC filtering in
> a rig or device, could be quite loud.
>
> Always run a separate small power supply to feed uHam devices and low
> level audio station utility gizmo's.  Always use separate DC power
> supplies for SSB transceivers operated concurrently:  one supply per
> transceiver.
>
> You can get away with mixed supply leads on CW, but NOT on SSB.
>
> 73, Guy.
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