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Noise Reduction in Signal Cable

R

Rileyesi

Jan 1, 1970
0
We make instrument systems where the sensors generate analog DC voltages and
are connected to the display panel with a signal cable. We have always used a
multi-condictor cable that has a braided wire overall shield (i.e. no
individual shields between the condunductors). We have always used analog
indicators. I've never had complaints about signal noise showing up on the
indicators.

However, I have a new installation where digital indicators will be used. I
THINK that if there is any noise may show up more on the digital indicators
than on the analog. So, I am looking for ideas on noise reduction.

To be more specific, the current signal cable has 7 condictors that are gage 18
in size. 2 are to power the sensors and the other 5 are signal. The range of
singals is about 0-5 VDC. I recommend to the customer to ground only one side
of the braided shield. One last thing, the singal cable run is anywhere from
50 to 2,500 meters. On the 2,500 meter run, we might use an RF link.

The problem is that our products are installed in a wide variety of
environments. Anything from insustrial plants to remote fields with nothing
for miles!

Any tips or information sources on noise reduction would be appreciated.

Thanks.
 
C

Charles Edmondson

Jan 1, 1970
0
Rileyesi said:
We make instrument systems where the sensors generate analog DC voltages and
are connected to the display panel with a signal cable. We have always used a
multi-condictor cable that has a braided wire overall shield (i.e. no
individual shields between the condunductors). We have always used analog
indicators. I've never had complaints about signal noise showing up on the
indicators.

However, I have a new installation where digital indicators will be used. I
THINK that if there is any noise may show up more on the digital indicators
than on the analog. So, I am looking for ideas on noise reduction.

To be more specific, the current signal cable has 7 condictors that are gage 18
in size. 2 are to power the sensors and the other 5 are signal. The range of
singals is about 0-5 VDC. I recommend to the customer to ground only one side
of the braided shield. One last thing, the singal cable run is anywhere from
50 to 2,500 meters. On the 2,500 meter run, we might use an RF link.

The problem is that our products are installed in a wide variety of
environments. Anything from insustrial plants to remote fields with nothing
for miles!

Any tips or information sources on noise reduction would be appreciated.

Thanks.

Okay, first, change to a twisted pair cable. That will reduce your
noise tremendously, even though it doubles your number of signal wires.
Use differential signalling of your data.

Next suggestion... :cool:

Charlie
Edmondson Engineering
Unique Solutions to Unusual Problems
 
R

Rileyesi

Jan 1, 1970
0
Rileyesi said:
Okay, first, change to a twisted pair cable. That will reduce your
noise tremendously, even though it doubles your number of signal wires.
Use differential signalling of your data.

Thanks for the reply.

Just to be sure I understand...

I would use a twisted pair for each signal. One of the pair would carry the
signal voltage and the other would be tied to ground? In other words, for all
signal pairs, one of all pairs would be tied together to ground.

Question, if I use twisted pair signal cable, do I need the outer or overall
shield too??

Thanks.
 
C

Charles Edmondson

Jan 1, 1970
0
Rileyesi said:
Thanks for the reply.

Just to be sure I understand...

I would use a twisted pair for each signal. One of the pair would carry the
signal voltage and the other would be tied to ground? In other words, for all
signal pairs, one of all pairs would be tied together to ground.

Question, if I use twisted pair signal cable, do I need the outer or overall
shield too??

Thanks.
Wellll....
Actually, I would ground the second wire only at one end, probably the
transmit side, and would differentially measure the voltage at the
other, probably with a balanced termination. You ground the sheild only
at one end to help limit ground loops. I real life, I would use digital
signalling and optoisolation on a long run like this to protect the
receiver from transients...

Charlie
Edmondson Engineering
Unique Solutions to Unusual Problems
 
R

Roberto Ramirez

Jan 1, 1970
0
Like Charles said, i would use differential signalling.

At the TX side, convert from one wire signal, to 2 wires differential
signal.
At the RX side, convert from 2 wires differential, to 1 wire signal.

Connect ground only at one side.

There are plenty of chips out there on the market to perform that job with
ease, and high speed.


regards,
Antonio Sergio Sena




---------------------------------------------
Antonio Sergio Sena
(Field Applications Engineer)

Primetec - Engenharia de Sistemas, Lda.
Rua Porto Alegre, 9 - 1º Esq.
2780-031 Oeiras
PORTUGAL

e-mail: [email protected]
WEB: www.primetec.pt







..
 
F

Fred Bloggs

Jan 1, 1970
0
We make instrument systems where the sensors generate analog DC voltages and
are connected to the display panel with a signal cable. We have always used a
multi-condictor cable that has a braided wire overall shield (i.e. no
individual shields between the condunductors). We have always used analog
indicators. I've never had complaints about signal noise showing up on the
indicators.

However, I have a new installation where digital indicators will be used. I
THINK that if there is any noise may show up more on the digital indicators
than on the analog. So, I am looking for ideas on noise reduction.

To be more specific, the current signal cable has 7 condictors that are gage 18
in size. 2 are to power the sensors and the other 5 are signal. The range of
singals is about 0-5 VDC. I recommend to the customer to ground only one side
of the braided shield. One last thing, the singal cable run is anywhere from
50 to 2,500 meters. On the 2,500 meter run, we might use an RF link.

The problem is that our products are installed in a wide variety of
environments. Anything from insustrial plants to remote fields with nothing
for miles!

Any tips or information sources on noise reduction would be appreciated.

Thanks.

If the sensor reading was accurate before then there will be no
difference with the digital meter if you LP filter the hell out of it-
using the Fc of the previous analog meter would be a good start-
absolutely nothing else changes, same cable, same sensors, same people.
 
M

Mac

Jan 1, 1970
0
We make instrument systems where the sensors generate analog DC voltages
and are connected to the display panel with a signal cable. We have
always used a multi-condictor cable that has a braided wire overall
shield (i.e. no individual shields between the condunductors). We have
always used analog indicators. I've never had complaints about signal
noise showing up on the indicators.

However, I have a new installation where digital indicators will be
used. I THINK that if there is any noise may show up more on the
digital indicators than on the analog. So, I am looking for ideas on
noise reduction.

To be more specific, the current signal cable has 7 condictors that are
gage 18 in size. 2 are to power the sensors and the other 5 are signal.
The range of singals is about 0-5 VDC. I recommend to the customer to
ground only one side of the braided shield. One last thing, the singal
cable run is anywhere from 50 to 2,500 meters. On the 2,500 meter run,
we might use an RF link.

The problem is that our products are installed in a wide variety of
environments. Anything from insustrial plants to remote fields with
nothing for miles!

Any tips or information sources on noise reduction would be appreciated.

Thanks.


I have read over a few of the posts, and I have some thoughts. I think
Fred Bloggs is right that if it worked before, it can be made to work now
with a suitable capacitor, with no cable changes. The only caveat is that
the real part of the impedance on the sensing circuit must be comparable
or higher. Another way of saying this is that the DC current flowing in
the SIGNAL cables should be the same or lower. If it is, then adding a
capacitor to the sensing end of the cable should do the trick. You may
have to experiment a bit to find the right value.

I have to admit that sensing DC values across 2.5 km sounds a bit sketchy
to me, but if it worked before, it should still. Also, if you are sending
power over a 2.5 km cable run, you better calculate the voltage drop in
the wire and make sure you can live with it.

Now, if you would consider making more changes, then you might consider
designing a unit with one or several analog to digital converters. Then
you can use some kind of digital communications scheme to send the
information over the wire. This could save you a lot on wiring, because
you could probably use a single shielded twisted pair for data. Unshielded
twisted pair would probably work, too. Afterall, DSL works with fairly
long wire runs, and has only a single pair of wires with virtually unknown
impedance characteristics. And it can handle pretty high data rates.

Good luck!

--Mac
 
B

Bryan Swadener

Jan 1, 1970
0
Rileyesi said:
We make instrument systems where the sensors generate analog DC voltages and
are connected to the display panel with a signal cable. We have always used a
multi-condictor cable that has a braided wire overall shield (i.e. no
individual shields between the condunductors). We have always used analog
indicators. I've never had complaints about signal noise showing up on the
indicators.

However, I have a new installation where digital indicators will be used. I
THINK that if there is any noise may show up more on the digital indicators
than on the analog. So, I am looking for ideas on noise reduction.

To be more specific, the current signal cable has 7 condictors that are gage 18
in size. 2 are to power the sensors and the other 5 are signal. The range of
singals is about 0-5 VDC. I recommend to the customer to ground only one side
of the braided shield. One last thing, the singal cable run is anywhere from
50 to 2,500 meters. On the 2,500 meter run, we might use an RF link.

The problem is that our products are installed in a wide variety of
environments. Anything from insustrial plants to remote fields with nothing
for miles!

Any tips or information sources on noise reduction would be appreciated.

Thanks.

Since we're talking DC, I don't see a serious issue. The digital indicators
use an ADC, and likely will have internal signal-conditioning (low-pass
filtering and overvoltage protection). To ensure you're not getting any
noise at the receiving end, you might want to look at it with an
oscilloscope.

A few other posters suggested using a differential transmission scheme (with
twisted-pair cable), where neither conductor is ground. For that to work,
the negative side of the source needs to NOT be at ground potential and, the
input of the digital indicators needs to not have the low side tied to
ground. The beauty of differential (or balanced) transmission is that
longitudinal or common-mode currents are all-but ignored. I used to do
carrier system maintenance for a well-known Telco, and we used that method
in T1, T148, and Duobinary transmission. Their repeaters are typically
placed about 5000ft/1500m apart and though the pulses (effectively, 770KHz
rate) were pretty rounded-off, there was hardly *any* noise.

You're right on the mark in recommending grounding the shield at only one
end (to avoid nasty ground loops that might induce error-producing AC hum
and/or random noise). I'm thinking simplest is bestest! :^)

Bryan
 
T

Tim Shoppa

Jan 1, 1970
0
We make instrument systems where the sensors generate analog DC voltages and
are connected to the display panel with a signal cable. We have always used a
multi-condictor cable that has a braided wire overall shield (i.e. no
individual shields between the condunductors). We have always used analog
indicators. I've never had complaints about signal noise showing up on the
indicators.

However, I have a new installation where digital indicators will be used. I
THINK that if there is any noise may show up more on the digital indicators
than on the analog. So, I am looking for ideas on noise reduction.

To be more specific, the current signal cable has 7 condictors that are gage 18
in size. 2 are to power the sensors and the other 5 are signal. The range of
singals is about 0-5 VDC. I recommend to the customer to ground only one side
of the braided shield. One last thing, the singal cable run is anywhere from
50 to 2,500 meters. On the 2,500 meter run, we might use an RF link.

The problem is that our products are installed in a wide variety of
environments. Anything from insustrial plants to remote fields with nothing
for miles!

Others have suggested differential signaling with digital or analog
voltages... but I'm going to take a different tack:

Send currents, not voltages. High-impedance voltage circuits by
their very nature pick up noise. Current loops are potentially vulnerable
to induced currents from nearby high AC currents, but twisted pairs
really do help.

Use one copper pair for each signal you need to send. This means that
you probably have to go from 7 conductors to 12 conductors, which will
have some cost.

But at the other end of the cable you just put milliammeters. Be
they digital or analog, it doesn't matter.

Even better: go to the industrial standard of 4-20mA current loops!
There's a wide variety of prepackages 0-5V and 1-5V to 4-20mA converters
already out there.

Tim.
 
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