[Elecraft] K3 RFI in transmitted audio

[Don Wilhelm]

  Joe,

Let me rephrase your point and add a bit more.
Any accessories that are used during transmit would best be powered by a 
supply different than the transceiver power supply.  Strange modulation 
of the V- line to the accessory can result, especially with SSB 
transmission, but possible with other modes as well.
If the accessory is only used during receive, it can use the same supply 
as the transceiver because the current draw of the transceiver is 
constant during receive.

73,
Don W3FPR

On 9/11/2010 8:06 PM, Joe Subich, W4TV wrote:
>   >  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,
>