[Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax cable?

[Hank P]

There are certainly other possibilities.

Making an amplifier unconditionally stable with any combination of source 
impedance ,load impedance , drive level , and supply voltage and achieving 
any reasonable gain ,  efficiency , bandwidth , and able to withstand about 
anything a customer can dream up at the same time is a real challenge . And 
what these source and load impedances are in band or well out of band have a 
strong bearing on stability.

Then to add to the fun, if you are not a high quantity buyer of the 
semiconductors , and therefore cannot demand there be no changes  in 
processing, you can do a design and a couple years later all of a sudden you 
have a stability problem that never existed because the vendor decided to 
make a process change such that the device still met the typical specs ,but 
had a much stronger propensity for  half freq oscillations.  Some process 
changes that raise low frequency gain without much effect on high frequency 
gain can make an amp have a tendency for low frequency oscillations because 
the decoupling /low frequency  circuitry loading  is now not sufficient to 
suppress that .

Enough of that , what CAN be happening here is with certain cable lengths 
between the amp and the K3 ,  in band or out of band impedances presented to 
the K3  could well make the K3 PA want to have a spurious output like half 
frequency oscillation or low frequency oscillations . I do not recall if the 
KPA500 has any resistive broadband padding at the input in normal operation 
, I believe they say they switch in a 3 dB pad when extreme overdrive is 
applied. Resistive pads are a great way to limit in band or out of band 
impedance excursions - BUT you give up amp gain dB for dB.

Same is true at the  output of the amplifier -(or combinations of in and 
output)  certain loads may make the amp more prone to a spurious output at 
half the driving frequency - even though the load may look like a perfect 50 
ohms in band at the fundamental frequency . This is another joyous feature 
of semiconductor power amp designers get to experience whether bipolar or 
FET .

The designer has to be careful what the out of band impedances of the output 
filters are - and just because you have a 1:1 SWR in band with your antenna 
, the out of band impedances can be darn near anything .

As I recall , in this case  , he had the same problem even into a dummy load 
on the output. So just as an  experiment it might be interesting to see of a 
3 dB resistive pad between the amp and the K3 made the whole thing totally 
independent of cable length. I am m not talking about cable loss , I am 
talking about limiting the possible impedance excursions especially out of 
band .

Enough of this - I spent  30 years of my life designing and putting into 
production these beasts , I enjoy the fruits of the Elecraft designers 
efforts .have no interest whatever in doing that again.  I like antennas 
much better they don't half f or oscillate at low frequencies- at least so 
far .

Hank K7HP

-----Original Message----- 
From: Richard - HB9ANM
Sent: Wednesday, November 19, 2014 12:58 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] Strange K3 & KPA500 Issues fixed with a longer coax 
cable?

Well, this is surprising, to say the least. Inasmuch as it seems to have
happened in several places. I normally don't post remarks of the
"no-problem-here" sort but I don't think this has anything to do with
RFI or inductance. Neither are ferrite chokes likely to solve the
problem. My K3->KPA500 cable is only 65 cm (26") long (the only piece of
RG58 in my setup) but nobody mentioned any distortion.

BTW: Both the K3 AND the KPA500 are earthed (grounded, if you prefer)
separately to a common earth... I mean: grounding point, as well as the
(manual) tuner, the SteppIR controller, etc.

If a longer cable can solve the problem, it's OK but the main question
(the "WHY?") remains unanswered... Strange, indeed.

73
Richard - HB9ANM

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