[RFI] Auto quieting
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.
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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