Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. I am concerned with noise,
and I don't think shielded cable will work well enough. So I am
considering using coax cable. But I don't understand coax. There are
numerous "RG" ratings and ohm ratings. Suppose I used RG62 with 50
ohm terminations. What do I need to know about the termination and
the cables in order to properly use this? What's the difference
between RG62 and say RG59? What are the ohm ratings referring to, and
what terminations would I need for an ohm rating of 50? Is there a 50
ohm load which gets put somewhere in the system? Clearly I have many
questions. Any help would be appreciated.

thanks

On Mar 12, 11:18*am, Andy wrote:
Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. *I am concerned with noise,
and I don't think shielded cable will work well enough. *So I am
considering using coax cable. *But I don't understand coax. *There are
numerous "RG" ratings and ohm ratings. *Suppose I used RG62 with 50
ohm terminations. *What do I need to know about the termination and
the cables in order to properly use this? *What's the difference
between RG62 and say RG59? *What are the ohm ratings referring to, and
what terminations would I need for an ohm rating of 50? *Is there a 50
ohm load which gets put somewhere in the system? *Clearly I have many
questions. *Any help would be appreciated.

thanks

Are you looking for the 5 mV signal to be a DC or an AC signal, if AC
up to what frequency?

On Mar 12, 9:18*am, Andy wrote:

I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. *I am concerned with noise,
and I don't think shielded cable will work well enough.

What do you mean, 'noise'? Pickup from heavy machinery?
Thermal resistor noise? Limited ADC resolution?

And what IS your signal? Source impedance and bandwidth are
important.

There are lots of signals (like a thermocouple output) that would
work better without coaxial cable, and lots of solutions (like
a 4-20 mA current transmitter) that a 'sensing device' could
conceivably support. Researching coaxial cable is not
yet called for.

"Andy"

I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. I am concerned with noise,
and I don't think shielded cable will work well enough. So I am
considering using coax cable.

** Co-axial cable is a shielded cable.

But I don't understand coax. There are
numerous "RG" ratings and ohm ratings. Suppose I used RG62 with 50
ohm terminations. What do I need to know about the termination and
the cables in order to properly use this?

** Probably nothing - unless your mysterious "instrument" outputs signals
in the MHz range.

A run of co-axial cable with no special termination will present a
capacitive load to the source of about 22pF per foot - so in your example
about 770pF for RG59. Long as that capacitance is OK, along with whatever
load the input of the sensing device presents - you will be fine.

BTW

Co-axial cable has similar interference cancelling properties to twisted
pair cable.


...... Phil

5mV is down in the range of microphones and phono pickups. I would not run
such a signal through 35' of coax.

You need a balanced system -- like a mice cable -- where the shield does not
carry the signal.

I meant mic cable. Though mice are known to nibble cables.

"William Sommer****** Fool "

5mV is down in the range of microphones and phono pickups. I would not run
such a signal through 35' of coax.


** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.



..... Phil

On Mar 12, 7:56*pm, "Phil Allison" wrote:
"William Sommer****** Fool "

5mV is down in the range of microphones and phono pickups. I would not run
such a signal through 35' of coax.

** Co-ax works perfectly with microphones.

* *Every bit as good as shielded twisted pair cable.

.... *Phil

The OP has yet to tell us what kind of a dignal source he has, AC vs
DC, etc.

On Mar 12, 5:56*pm, "Phil Allison" wrote:
"William Sommer****** Fool "

5mV is down in the range of microphones and phono pickups. I would not run
such a signal through 35' of coax.

** Co-ax works perfectly with microphones.

* *Every bit as good as shielded twisted pair cable.

.... *Phil

Using physically realizable materials, the only signal carrying system
one can construct in a manner that will be the most immune to external
noise sources is a shielded balanced twisted
pair system. Coax is prone to adding noise to the signal, since the
shield has impedance, carries the signal, and therefore can add noise
to the signal as the shield carries noise.

However, even poorly made twisted pair cable can actually add noise to
the signal as the shield attempts to do its function. For example,
Belden's high quality HDMI cable, if used to carry analog, only
shieldls in the 35 to 60dB range. Of course its worse at the higher
frequency range.

Robert Macy, PE

"Robert Macy"
"Phil Allison"

** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.


Using physically realizable materials, the only signal carrying system
one can construct in a manner that will be the most immune to external
noise sources is a shielded balanced twisted pair system.

** Fraid that is dead wrong.


Coax is prone to adding noise to the signal, since the
shield has impedance, carries the signal, and therefore can add noise
to the signal as the shield carries noise.

** Only true where the source has a common ground path with the receiving
device and an earth loop is created by the shield.

For a floating source, like a microphone, co-axial cable works just as well
as STP despite not being balanced.

This is not a very well known fact and derives from the inherent symmetry of
co-axial cable which rejects external magnetic field induction in a similar
way twisting a pair of wires does.


..... Phil

In article ,
Phil Allison wrote:

"William Sommer****** Fool "

5mV is down in the range of microphones and phono pickups. I would not
run such a signal through 35' of coax.


** Co-ax works perfectly with microphones.

Every bit as good as shielded twisted pair cable.

That must explain why pretty well all pro microphone cable is twisted
pair, then. And always has been even before phantom etc power.

--
*Who are these kids and why are they calling me Mom?

Dave Plowman London SW
To e-mail, change noise into sound.

"William Sommer****** Fool "

5mV is down in the range of microphones and phono pickups.
I would not run such a signal through 35' of coax.

Looks like I spoke too soon.

Andy wrote:

Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. I am concerned with noise,
and I don't think shielded cable will work well enough. So I am
considering using coax cable. But I don't understand coax. There are
numerous "RG" ratings and ohm ratings. Suppose I used RG62 with 50
ohm terminations.


RG-62 is a 93 ohm cable. It has a lower capacitance per foot than
most other coax in that size because of its construction.


--
Lead free solder is Belgium's version of 'Hold my beer and watch this!'

In article
,
Andy wrote:

Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. I am concerned with noise,
and I don't think shielded cable will work well enough. So I am
considering using coax cable. But I don't understand coax. There are
numerous "RG" ratings and ohm ratings. Suppose I used RG62 with 50
ohm terminations. What do I need to know about the termination and
the cables in order to properly use this? What's the difference
between RG62 and say RG59? What are the ohm ratings referring to, and
what terminations would I need for an ohm rating of 50? Is there a 50
ohm load which gets put somewhere in the system? Clearly I have many
questions. Any help would be appreciated.

From your description of your application, not one single thing you
mentioned is of any significance at all.

With a 5 mV signal, it's almost a certainty that you'll need a shielded
cable, but shielded may not be enough. It might be necessary for you to
choose a cable that has low microphonics (some cables can generate a
voltage when you whack them). Generally, better-grade microphone cables
have less of problem with it.

Another thing you might look into is the percentage of coverage some
particular shield configuration offers. RF coaxial cables in particular
usually do not have great shield coverage. Foil shields can offer much
better shielding.

All cables have capacitance, and longer cables have more. As a very
general rule, larger-diameter cables will likely have lower capacitance
than skinny ones. Make sure the capacitance of the cable you choose will
not affect your signal by attenuating the higher frequency components.

Perhaps best of all would be to amplify the signal *before* you send it
down the cable.

Isaac

On Mar 12, 12:18*pm, Andy wrote:
Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. *I am concerned with noise,
and I don't think shielded cable will work well enough. *So I am
considering using coax cable. *But I don't understand coax. *There are
numerous "RG" ratings and ohm ratings. *Suppose I used RG62 with 50
ohm terminations. *What do I need to know about the termination and
the cables in order to properly use this? *What's the difference
between RG62 and say RG59? *What are the ohm ratings referring to, and
what terminations would I need for an ohm rating of 50? *Is there a 50
ohm load which gets put somewhere in the system? *Clearly I have many
questions. *Any help would be appreciated.

thanks

Does your "sensing device" support differential inputs.. I would think
that would determine if twisted pair is the solution or not.

On Mar 14, 3:23*pm, Gus wrote:

Does your "sensing device" support differential inputs.. I would think
that would determine if twisted pair is the solution or not.

That's not the whole story; if the source is floating
(not locally ground-referenced) the twisted pair is a good solution
for non-differential input, as well.

"Andy" wrote in message
...
Hello,
I am working with an instrument which outputs 0-5mV, and that signal
has to reach a sensing device 35' away. I am concerned with noise,
and I don't think shielded cable will work well enough. So I am
considering using coax cable. But I don't understand coax. There are
numerous "RG" ratings and ohm ratings. Suppose I used RG62 with 50
ohm terminations. What do I need to know about the termination and
the cables in order to properly use this? What's the difference
between RG62 and say RG59? What are the ohm ratings referring to, and
what terminations would I need for an ohm rating of 50? Is there a 50
ohm load which gets put somewhere in the system? Clearly I have many
questions. Any help would be appreciated.

thanks
The RG terminations refer to the RADIO FREQUENCY and pulse properties of
the line. If you are only measuring voltages which change slowly (not more
than 50Hz components) ANY coax will do. Think of it as a screened test lead
( of course you should earth the outer screen)
Hope this helps
P.W.