Impedanc Matching Problem

Agreed, the galvanic isolation is also very helpfull.

Mark

Hello,

I have a impedance matching problem that I need help with.

I have a 1Mb data transmission system that is designed to work with 100-150 ohm cable, We would like to use it on 55 ohm cable.

If anyone thas has a good understanding of transmission line theory and would like to help out (I can probably pay for time of the problem is solvable) please e-mail me.

To e-mail me please change the z characters to o characters and the hat symbol to a at symbol. itmaster2001^yahzz.cz.uk (hopfully mailbots ar not that bright yet)

Thanks

I read in sci.electronics.design that Openhiemer wrote (in ) about 'Impedanc Matching Problem', on Wed, 23 Mar 2005:

Do you have enough signal to use L-pads at each end of the 55 ohm cable? Just two resistors in series, designed to look like 100 ohms one way and

55 ohms the other.

Use Courier font:

+----[R1]----+-----o o-----[R1]----+----+ | | | | [100] [R2] [R2] [55] | | | | +------------+-----o o-------------+----+

The calculation of R1 and R2 will keep you out of mischief for a few seconds/ minutes/hours and will be a valuable addition to your little black book of useful cribs.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Yep. One I set up between two GenRad (Phoenix) buildings had 18VAC differential between "grounds" :-(

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Try the 55-ohm cable. You will probably notice no difference.

I make R1 68 Ohms and R2 82 Ohms (nearest E12 preferred values), which will give you just over 7 dB of loss at each end. Can you afford that much attenuation?

Another option maybe to put an L pad (as above) at only the receive end. That way you shouldn't get any reflections back down the cable. If the system is bidirectional you may still get away with a pad at only one end as the reflected signal going the other way will have been through the pad and cable twice, and thus be attenuated significantly and therefore less likely to cause an error in the wanted signal.

It may well work without any matching. I would try Reg's suggestion first.

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On Wed, 23 Mar 2005 18:53:08 +0000 (UTC), Openhiemer wroth:

Twisted pair cable is usually in the 100 to 150 Ohm realm. Coax cable is usually in the 50 to 55 Ohm realm.

Is the cable suggested originally for the system twisted pair or coaxial? There are some fundamental differences other than impedance to consider.

Jim

I tend to have good luck with Coilcraft's stuff. Check out

. Those should have plenty enough bandwidth for your signal, just take a 2:1 impedance ratio and call 50 ohms close enough to 55.

Also, Coilcraft is really good about sampling, which makes it fairly easy to test whether the solution works.

James Meyer wrote in news: snipped-for-privacy@4ax.com:

Apologies for not being more specific.

The devices are multidrop and are transformer coupled to (normally) 150 ohm twisted pair cable. The devices have a master/slave relationship so there is no collission/multi transmission problems. The design limit of the transmission system in this configuration is 1KM.

The cable that I have been asked to use is screened twisted pair with a characteristic impedence of 55 ohm. I can get this working using a 100 ohm resistor at each end on a 40M cable. When I go above this length it stops working. I assum that this is due to refections caused by the impedence mismatch.

Andy

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Gareth wrote in news: snipped-for-privacy@brightview.com:

Many thanks for that,unfortunately there are 20 devices to connect so the power loss may be prohibitive.

I have been looking at transformer matching but I dont have a clue what I need to buy/make or even where to get them.

Best wishes Andy

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Verify that the characteristic impedance is from conductor to conductor and not conductor to shield. Most twisted shielded pair I've seen is between

150 and 90 ohms. 120 is nominal.

Conductor to shield measuremnt will look more like a coax with and appropriately low Zo.

Give the twisted pair a try as is. I've been able to transmit Manchester data over 55kft on Brand X twisted pair line with surprisingly low BER.

Have you taken a look with a 'scope? Maybe you need to pre-comp?

It's been a LONG time (1980) since I did a long run, but IIRC I had to pre-comp the signal, then do DC-restoration at the far end, to get it to work error-free.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

On Thu, 24 Mar 2005 12:14:01 GMT, Openhiemer wroth:

Have you tried the screened cable with no resistors? If so, did it work?

Multi-drop receivers are usually designed with a much higher input impedance than the cable's impedance simply so that they won't load the cable when many of them are connected in parallel to the cable.

Multi-drop transmitters are usually designed with an output impedance much lower than the cable to enable them to drive multiple receivers.

In other words, multi-drop TX/RX devices don't inherently match the cable at all.

Reflections, if they occur at all to a degree that might stop a multi-drop system from working, are best handled by loading only the extreme ends of the cable. A 50 Ohm resistor in parallel with the signal leads of the cable at each extreme end of the cable should eliminate the majority of the reflections. Make no connections with "matching" networks between the signal leads and the screen of the cable. Only insure that the screen is continuous.

Jim

I read in sci.electronics.design that Openhiemer wrote (in ) about 'Impedanc Matching Problem', on Thu, 24 Mar 2005:

That is an unusual cable, I think. Do you have a manufacturer's part number or a specification including the conductor insulating material and the dimensions of the cross-section? Most screened twisted pair is around 100 ohms.

--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

"Mook Johnson" wrote in news:Vny0e.12879$ snipped-for-privacy@tornado.texas.rr.com:

I have tested several lengths of the cable with a TDR between 1Mhz and 2 Mhz and the impedence is definitely 55 ohm between the pairs :(

I would agree that most transmission cables are between 100 and 150 ohm, this one has taken me by supprise.

Best wishes Andy

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My link was RG-8 and impednace-matched. The 18VAC was what forced me into transformers. I think I still have the drawings around here somewhere.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Was the tdr coorectly configured to drive the pair with a true differential signal (i.e zero common mode component) ? 55ohms is close to what I'd expect if you had grounded one of the pairs...

Charles

ohms

which

that

You must have a very good reason to spend all this time trying to work with a non compatible cable.

NT

Rob Gaddi wrote in news:d1ur92$ snipped-for-privacy@gazette.corp.bcm.tmc.edu:

Thanks so much for that. I will definitely contact them.

I tried impedance matching using John Woodgate's suggestion and values suggested by Jim. This worked great up to about 350M (sooo much better than 40M). This is just about workable, 500M would be safe.

So will get some transformers next week and see if I can get it to the safe zone.

You guys have been most helpful in a subject that I studied 20 years ago and thought "I will never need to know that", I guess you can never tell with these things.

Best wishes Andy

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