Re: Zo of a transmission line
From: John Larkin (jjlarkin_at_highlandSNIPtechTHISnologyPLEASE.com)
Date: 07/07/04
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Date: Wed, 07 Jul 2004 08:37:37 -0700
On Wed, 07 Jul 2004 11:14:47 GMT, "mook johnson" <mook@mook.net>
wrote:
>The Zo is being calculated to determine the necessity of using a matching
>transformer when using this customer supplied line (could be any kind of
>line matched only in voltage and current capability) to extend our known
>instrumentation cable which measures 110 ohms Zo through that range.
>
>These cables will be used for 2-way telemetry that will occupy the 10Khz to
>50Khz frequency range and we want to reduce the reflections by terminating
>the cable on both sides and driving the cable with source impedances that
>are matched to the Zo of the cable. I have never tested what the tolerance
>of mismatch can be before our telemetry is scrambled by standing waves
>(bi-phase Manchester) so, if the effort is reasonable, I prefer to match it
>as close as possible.
>
Such a 5-mile cable is almost surely dominated by resistive losses. 5
miles of #20 twisted pair will have about 500 ohm resistance, so much
loss there will be no significant reflections or standing waves. The
phone companies have traditionally considered their lines to be 600
ohms, even though the actual impedance of a short length of a
telephone pair is closer to 100 ohms. Don't worry about reflections,
but do worry about frequency response... it will drop off radically
with frequency.
John
>thanks for the replies.
>
>
>"mike" <spamme0@juno.com> wrote in message news:40EB79D2.1030102@juno.com...
>> mook johnson wrote:
>> > If I were to measure the Zo of a transmission line (long sucker at 5
>miles
>> > long) for frequencies in the 10kHz to 50kHz range and all I had access
>to
>> > was a scope and function generator (50 ohm output) and few various
>passive
>> > components (resistors, pots, etc.) and transformers. What method would
>be
>> > best to use to measure such a beast?
>> >
>> > take a guess a the range based in the construction of the cable (~120
>> > ohmsfor twisted pair, 50-75 ohms for coax) then adjust the terminating
>> > resistance until the square wave input in one end looks good on the
>other?
>> >
>> > Or do I use input voltage and current (phase and amplitude) to calculate
>the
>> > Zo for several frequencies across this range. It should flatten out as
>the
>> > line becomes a transmission line.
>> >
>> > Any other methods of corrections to the above methods?
>> >
>> > The reason for the sparse equipment list would be that this will be done
>in
>> > a remote location.
>> >
>> >
>> >
>>
>> I'd use a pencil.
>> If it's twisted pair, write down 120 ohms.
>> For a 5 mile cable, I'd guess that the error in this "measurement"
>> is well swamped by a bunch of other issues...losses, radiation,
>crosstalk...
>>
>> So, satisfy my curiosity...why do you need to know the exact value?
>> mike
>>
>> --
>> Return address is VALID.
>> Bunch of stuff For Sale and Wanted at the link below.
>> Toshiba & Compaq LiIon Batteries, Test Equipment
>> Yaesu FTV901R Transverter, 30pS pulser
>> Tektronix Concept Books, spot welding head...
>> http://www.geocities.com/SiliconValley/Monitor/4710/
>>
>
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