[RFI] Auto quieting
jim at audiosystemsgroup.com
Mon Oct 12 02:48:48 EDT 2015
On Sun,10/11/2015 11:15 PM, Roger (K8RI) wrote:
> Put one end of the heavy zip cord (the red and black stuff) in a vise
> and the other in an electric drill motor. Pull the trigger and you
> have instant twisted pair at ant twist you want. I've found it to
> generally be cheaper than seperate wires if you don't purchase it at
> the audio stores.
Be skeptical with the red/black zip cord. I've seen stuff that is
labeled at least two wire gauges larger than it actually is. Commonly
sold at hamfests by vendors with roots in CB radio. Put a micrometer on
anything you are considering buying.
> I purchase CAT 5 and 6 in the 1000' boxes. I've never found one yet
> that spooled out the way it should, at least to begin.
Remember that CAT5/6/7 is a mass market product, distributed by
The good stuff is made by vendors like Belden, with pairs that are
molded together to maintain constant impedance. There's a lot of junk
> BTW I'm not fond of the usual tiny, solid wires used in CAT-5 and 6
> cables. OTOH the tiny stranded wires have not proven any better for me
> and they take different connectors for the best results.
Remember that CAT5/6/7 cable is specifically designed for SMALL SIGNAL
circuits that transmit and receive simultaneously. Each pair in the
cable has a different LAY -- that is, the twist ratio -- with the design
purpose of minimizing crosstalk between pairs. These cables are
specifically designed for Ethernet systems. We can use them in our
systems by understanding what they are and their limitations.
> NOTE: you can not just run the wires straight through. Color for
> color they are pin for pin, but there is a specific order. The 2
> standard pin outs are listed on the net!
Clarifying -- these are PAIRED cables, that are USED as pairs. The
signals they transmit are broadband signals (into the 10 MHz region), so
they MUST be treated as transmission lines in that context. OTOH, we can
USE them as generic transmission lines if we understand their physical
design. For example, CAT5/6/7 makes EXCELLENT telco cable for up to 4
pairs. It can also be used in RS232 circuits, with one pair per
signalling circuit. In this application, the solid color of each pair
goes to pins 2, 3, 4, and 7, while the /white conductor goes to signal
return (pin 5, or the connector shell to avoid Pin One Problems).
While thinking about RS232, remember that the conductors are NOT used as
matched transmission lines, the sources are low Z and the loads are High
Z, so the limitation is the capacitance of the circuit. CAT5/6/7 cable
has pretty low capacitance between conductors, so it can go several
hundred feet with a pretty good data rate. Study the later versions of
the RS232 standard which define the bandwidth in terms of circuit
capacitance to understand this. In the pro audio world, we regularly ran
RS232 circuits 200 ft or more on low capacitance cables to tune sound
In other words, the cable in RS232 systems is NOT a transmission line,
it's TX and RX connected by low capacitance cables, and bandwidth is
limited by cable C. This doesn't mean that we can't USE these cables in
matched applications, like connection Beverages to an RX.
73, Jim K9YC
73, Jim K9YC
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