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

[Jim Brown]

On 3/23/2012 6:17 PM, Don Wilhelm wrote:
> I really don't know the relative conductor to shield apacitance of the 4 conductor cable may be,

That's something I had to learn about from a practical, hands-on point 
of view for my work in pro audio, and also as a member of the AES 
Standards Committee.

> 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.

In the world of pro audio, for a 10 year period beginning about 15 years 
ago we used RS232 a lot to communicate with DSP used in sound systems, 
and a common connection was for a computer in the audience connected to 
a DSP unit in an equipment rack buried in the bowels of the building. We 
learned by thoughtful experimentation that we could reliably do that 
over 200 ft or more of cable if it was low capacitance cable like CAT5, 
but were limited to far less distance with ordinary cables. The 
capacitance of CAT5 is on the order of 12 pF/ft, while the capacitance 
of ordinary twisted pair is on the order of 35 pF/ft, and it's even 
greater for commonly used cables.

Cable capacitance can be quite important, even with baseband audio, 
where harmonic components coming out of a mic picking up musical 
instruments can extend to 50 kHz and above, and an 38,400 Hz RS232 
circuit will have components well above that..Some output stages can 
ring and even go unstable when driving highly capacitive loads.

BTW -- if you dig up a recent copy of the standard that defines RS232, 
it talks about cable length limitations in terms of cable capacitance, 
not length. In simple terms, it boils down to waveform distortion based 
on capacitive loading of the output stage and high frequency attenuation.

> As far as your "transmission line" suggestions go

What I'm getting at here is that to treat it as a transmission line it 
simply needs to be approximately matched at each end, and the output 
stage needs to be capable of driving the impedance it sees. CAT5 (and 
digital audio cables) are about 100 ohms; most practical jacketed 
cables, twisted or not, are on the order of 70-80 ohms.

With un-matched circuits like RS232, there are three key issues beyond 
timing -- capacitance, loss, and noise. Another important benefit of 
CAT5/6 cables is that their HF attenuation is a lot less than ordinary 
cables.  The method I described for wiring CAT5 at each end helps 
prevent the Pin One Problem from coupling noise, and also minimizes IR 
drop in the return conductor that can be present if the units being 
connected are powered or grounded at different points.

My comment that Elecraft ought to be testing longer cable lengths is 
based on the expectation that, especially with the KX3, some users will 
want to separate their KX3 from their power amp and antenna tuner by 
sticking the KX3 on the dash and the amp/tuner in the trunk. I could see 
cable lengths on the order of 15 ft in a typical car, longer in a few.

I'm with you on specifications too, and simply saying that the bus will 
work for 6 ft is hardly a specification, or even an adequate 
description. What is needed is a more thorough technical description of 
the bus from a SYSTEM point of view, so that a technically educated user 
can look at it, understand its limitations, and understand how to 
implement an optimum interconnection when he needs to approach or 
stretch those limits.

73, Jim K9YC