[Elecraft] DX Doubler/K3 issue

[Joe Subich, W4TV]

> There actually is a much more common and much more severe 
> issue that gets almost totally ignored, dc current paths 
> from external power supplies.

Tom is absolutely correct.  However, a much easier "fix" is 
to use a separate power supply for the low voltage accessories 
and ground (bond) any power supply chassis ONLY to the station 
common RF/safety ground.  

By connecting the DC return from the high current device 
directly to the power supply and only to the power supply 
as well as keeping the heavy "bonding" paths to the common 
safety/RF ground, the impedance of the alternate paths 
through low voltage accessories is generally high enough to 
prevent the return current diversion.  

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Tom W8JI
> Sent: Wednesday, March 31, 2010 6:09 AM
> To: Jim Brown; elecraft at mailman.qth.net
> Subject: Re: [Elecraft] DX Doubler/K3 issue
> 
> 
> > There are two common causes of this problem. One is the potential 
> > difference between the two radios. Bonding and using
> > the same outlet will fix that.
> >
> > The second common cause is a magnetic field that couples
> > into
> > the loop between the two radios and the DXDoubler. That 
> > field is
> > most often is established by a big power transformer very 
> > close
> > to the rig (either the linear supply for the rig(s) or the 
> > power
> > transformer for the power amp. The field is coupled to the 
> > audio
> > two ways. First as the induced voltage, and second as 
> > current
> > flowing into a pin 1 problem at either (or both) end(s).
> 
> There actually is a much more common and much more severe 
> issue that gets almost totally ignored, dc current paths 
> from external power supplies.
> 
> The high supply current radios draw causes  a ground loop 
> when we use low voltage supplies that have the negative 
> supply voltage grounded at the power supply or any other 
> point in the system other than at the radio. When the radio 
> has any sort of amplitude varying RF output, the rapidly 
> varying dc input current creates a pretty good voltage drop 
> across the negative supply lead. Since that lead is grounded 
> at more than one point, it drives the supply negative to a 
> different potential at different points in the system, and 
> the rig's chassis and other chassis "wiggle around" at 
> different potentials at the RF envelope's modulation rate as 
> PA current varies from quiescent to 20 amps or more peak 
> current on envelope peaks.
> 
> The effect is almost indistinguishable from rectification of 
> the RF envelope. As a matter of fact many blame this problem 
> on RFI because it sounds like RFI even though it is "AFI".
> 
> While bonding all the chassis helps a great deal, the real 
> issue is a design shortfall in the negative rail wiring of 
> the power supply and our equipment. The best fix for this 
> problem is to not have common chassis grounds on low voltage 
> sensitive signal leads, a good design isolates the sensitive 
> signal leads with isolation transformers. This problem is a 
> whole lot like the problem we create in vehicles when we run 
> the negative radio lead to the battery negative post. People 
> designing gear have to learn to not create a harmful path. I 
> can't imagine having an audio interface device that connects 
> audio and signal lead grounds all together at low 
> frequencies or dc!
> 
> The voltage induced this way is typically hundreds of times 
> higher in magnitude than voltage induced by flux leakage 
> from power transformers. Worse yet, it is an extremely low 
> impedance source making it very difficult to "bypass".
> 
> 73 Tom 
> 
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html