[Elecraft] Cable length for KPA/KAT100 remote; importance of KPA100 upgrade

[Don Wilhelm]

Jim.

Jack Brindle and I just had an off-list email exchange in which he did 
state that the conductor capacitance and resulting waveform distortion 
may be a factor.

I really don't know the relative conductor to shield capacitance of the 
4 conductor cable may be, but common sense tells me that it is variable 
depending on which conductor you select - at least your implementation 
of CAT5 twisted pair provides consistency.

As far as your "transmission line" suggestions go, I am going to go out 
on a limb and say that YES, the AUXBUS signalling is a transmission line 
when external to the K2 (even though it is uncontrolled).  The fact is 
that it "floats around" inside the K2 to all the option processors and 
the KIO2, it is not treated as a transmission line, and level sensing at 
the receivers is sufficient.  When one goes external with that signal, 
the conditions change - to get the signal from "here to there", it must 
appear on some sort of transmission line (the cables in the Elecraft 
KRC2, XVxxx transceivers, KPA100 for some examples) do constitute a 
transmission line whether that is intended or not - but bottom line, the 
receiver is an input to a CMOS firmware chip, and that input is level 
sensitive.
While the drivers and receivers may not have been designed for a 
transmission line ( I believe they were not), they are acting as drivers 
and receivers on whatever transmission line is presented between them.  
Be careful - amateurs often think of transmission lines as coax and 
parallel lines to antennas, but the world of transmission lines is much 
greater then that.

Given the characteristics of transmission lines regarding distortion of 
digital square wave signals, the receiver "interpretation" 
characteristics become important.  I do not see exotic line drivers and 
line receivers in any Elecraft devices, and so the obvious conclusion is 
that the "receivers" are level driven.  If the data pulse is distorted, 
the timing may be off, and if the levels are influenced by line 
capacitance, switching may not occur at all.

In other words, Jack Brindle has tested the K2 AUXBUS up to 6 feet with 
the standard 4 conductor cable, and all seems to be in order.  For those 
who would want to go beyond that length, a lot of careful observation 
for potential problem situations may be required - in other words, go 
ahead and try it - it may work in your setup. but I cannot provide any 
assurance that it will work at that length for all situations.  I am am 
a retired IBM Assurance engineer whose business was testing that a 
product met specifications under all possible conditions.  So I can say 
that it might work for you, but that is no proof that it will work in 
another controlled setting unless you provide detailed data (well beyond 
"it works") to me and the general Elecraft community.

I have not personally tested the command/response timing, but I do 
reflect Jack's comments which I do consider as regsarding "good 
engineering practice".

73,
Don W3FPR


On 3/23/2012 8:01 PM, Jim Brown wrote:
> On 3/23/2012 3:24 PM, Don Wilhelm wrote:
>> I  recall that 6 feet was
>> considered an OK length because it had been tested - longer lengths had
>> not been tested.
>> Of course, this applies to the K2.  The K3 may or may not be different.
> Thanks Don.  At the very least, if this is a critical issue it ought to
> be tested and the results noted.  There's also the issue of cable
> capacitance, which can be a big deal for high speed lines that are not
> impedance-matched (like RS232).  CAT5 has a big advantage there too --
> it's pretty low capacitance as cables go. Perhaps Jack can tell us if
> that circuit is impedance matched (that is, treated as a transmission
> line).
>
> 73, Jim K9YC
>