[Elecraft] K3/100 input voltage and TX IMD

[Don Wilhelm]

Rick,

It sounds like you have a good low resistance connection with each 
connection as tight as it can be.

That voltage drop is about the best you can do. If I calculate the 
resistance of a 12 gauge cable 5 feet long, it is 8.1 mv for each 
conductor. The APP connector contact resistance is a maximum of 578 
microohms. So the total resistance in the cable and APP connector is 
17.4 milliohms. That would be a voltage drop of 313 millivolts maximum 
at 18 amperes. That is as measured inside the K3. The voltage drop 
measured outside the K3 at 18 amps would be 292 millivolts due to the 
cable alone.

Bottom line, a voltage drop of 0.3 to 0.35 volts can be blamed on the 
cable and the APP connector resistance. Anything more than that can be 
blamed on something else - insufficiently tight connection to the power 
supply terminals, power supply 'droop', bad connection to the cable and 
the terminations (both the APP connector barrels and the power supply 
lugs if any are used), or any fuses used between the power supply and 
the K3. Fuses are good and should be used, but they have contacts too, 
and each contact adds its own contact resistance.

73,
Don W3FPR

Rick Shindley wrote:
>
> A solid state linear RF PA will generate more IMD if the supply 
> voltage sags on current peaks. How much distortion vs voltage change 
> depends on the amplifier design and components used.
>
> Someone reported here that he increased the supply voltage a bit and 
> noted a 1 to 2 dB reduction in TX IMD. He also reported a 400 mV drop 
> in the supply at the back of his K3/100 during Tx. Increasing the 
> supply voltage but still having a 400mV “droop” and yet seeing a 
> slight reduction in TX IMD hints at the sensitivity of the 
> relationship between supply voltage and TX IMD.
>
> I measured the voltage drop at the Power Pole connector at the back of 
> my K3/100 while transmitting 100W CW out on 14.350 MHz into a dummy 
> load. I found a 270mV drop due to the standard length power cable from 
> Elecraft (IR loss) and 20mV due to the supply (Astron RM-35) load 
> regulation capability for a total “droop” of 290mV.
>
> So on SSB voice peaks or CW at 100W there is at least a 290 mV (and 
> some say 400mV) drop in the supply voltage to the radio. It may even 
> be a bit more inside the rig at the PA due to connector contact 
> resistance and trace routing on the PCB. The slight drop in voltage 
> will contribute to the amount of distortion in the output. Could this 
> be the cause of the reported variations in measured TX IMD among rigs?
>
> Rick
>
> KC0OV
>
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