[RFI] Auto quieting

CR ka5s at earthlink.net
Mon Oct 12 08:34:59 EDT 2015

On 10/12/2015 2:15 AM, Roger (K8RI) wrote:
> On 10/10/2015 3:15 AM, Jim Brown wrote:
>> On Fri,10/9/2015 10:31 PM, Roger (K8RI) wrote:
>>> I fail to see where two runs of RG8 size cables in a car, large SUV, 
>>> or long cab pickup truck would be cost prohibitive.
>> But it's wasted dollars. Most RF guys fail to realize the noise and 
>> RF rejection inherent in good twisted pair. For more than a century, 
>> telco went cross country on unshielded twisted pair running next to 
>> power lines. CAT5/6/7 is four high quality, 100 ohm twisted pairs, 
>> each with a different twist ratio to further minimize crosstalk 
>> between pairs. 

War story.

Some years ago, I had to verify the performance of a  GPS timing pulse 
buffer design over a range of temperatures and supply voltage. This 
signal was a relatively high amplitude, short, fast, unipotential pulse 
carried on a 50 Ohm twisted pair.  The complete equipment was being 
automatically tested in another lab, who, not having  50 Ohm STP, used 
Cat 5 instead.  This didn't work until I showed them they could use two 
Cat 5 pairs of the same length in parallel.

I would later have to deal with an angry customer engineer, who'd 
thought one measured the signal on 50 Ohm twisted pair by using a 50 Ohm 
setting  or terminator at the scope, and isolated the scope from the 
twisted pair  by using a  10 MOhm 10:1 scope probe connected "upside 
down" WRT to the pulse.

I talked to his tech, and didn't hear from him again.


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