[Elecraft] K3/100 input voltage and TX IMD

[Rick Shindley]

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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