Time Domain Reflectometer

[cerickson]

I am in my RV a long ways from home (Las Vegas area) and I have a coax cable that goes from the front of the RV all the way to the back and there is a problem with it someplace in the middle. Because RV's are packed so tight and installed cables are so hard to access, I was wondering if there was a simple TDR setup that could be fabbed together using a 1280n development board, which I happen to have with me as part of my traveling hobbies. There is a local Fry's Electronics that has a nominal selection of electronic components too.

Is the 1280n fast enough for this project?

How complex of a circuit am I looking at?

Thanks in advance for any and all replies,

-Christopher Erickson

[twesthoff]

Try going to your local cable TV company and see if one of the techs will put their TDR on it.  If you have an oscilloscope and some way to produce a fast short pulse, you could get an indication of sorts, but short cables are a bit tricky.
If you suspect a shorted cable, you could use an ohmeter to know for sure.  You also could measure the capacitance from center conductor to the shield and roughly calculate the distance if the cable was open... just some thoughts.

General wrote:

[cerickson]

Here is what I know so far:

RG-59U coax.

About 40 foot long.

Ends tested.

Center conductor is connected end-to-end.

Outer conductor is not connected end-to-end.

Suspect there is a coupling someplace in the middle of the cable that I have not yet found and it is either loose or bad.

I do have a small Leader 30MS/s model 300 LCD O-scope with me and I just found this circuit:

www.epanorama.net/circuits/tdr.html

Not sure I can find a 74AC14 here in Las Vegas and I don't have a mailing address at the moment so I can't order one.

-Chris

[twesthoff]

You could go to Fry's and get a capacitance meter (some digital multimeters have one) then measure the capacitance of the cable at both ends.  The cable has ??pf/foot that you can look up.  Make sure you find the exact cable you use because the dielectric used makes a difference.  Usually it is foam or polyethelene.  Then calculate the length from each end measurement and see if they will help you find the break.

General wrote:

[cerickson]

I have a Fluke 85 multimeter and it can measure capacitance.

I will try your suggestion!

[twesthoff]

Remember to allow for the capacitance of the test leads you are using, for best accuracy.
Measure them when not connected and subtract that value from your reading...

General wrote:

[cerickson]

Dealing with tiny capacitances is a PITA. I figured I would remove the multimeter leads altogether and use a rigid F-BNC-Banana jack adapter configuration to try to eliminate variability as much as possible.

[stevech]

any hope you can just pull the coax through (not fastened anywhere along the way) and drag in a new one? Probably not.

and

Fry's Electronics
684 Las Vegas Blvd S
Las Vegas, NV 89101
Phone: (702) 932-1400

[cerickson]

AFAICT, the cable takes an incredibly convoluted route from the front to the back. I have found the cable at various points between the front and the back inside various accessible panels and covers. From what little I can see from those accessible points, the builders tie wrapped all of the wiring (coax, 12VDC, 120VAC) into bundles with tie wraps about every foot.

I sure wish they had used plastic conduit and junction boxes instead.

I am still hoping that I can find a concealed cover or other accessible point with a loose coupling behind it but I have really taken this thing apart already without finding any such location.

And I don't think my little 1Mhz LCD O-scope is gonna work well as a TDR. My 100Mhz Hitachi O-Scope is back home and not much use to me right now. In fact I have a $1,500 network cable analyzer (TDR) there too!

Today I will be trying to get capacitance readings from each end of the cable with my Fluke 85.

Thanks to everyone for all of the assistance so far!

[Don_Kirby]

FWIW...

In my years of being an electrician, I have never seen a piece of RG-6 (or RG-59U in this case) 'go bad'. I've also never seen a connector 'go bad'. I have seen water creep in and muck things up, but that usually only happens outside.

That said, I propose that there is something else at play here.

There is certainly the possibility that there is a cut in the cable due to vibration induced chafing, but most major vehicle manufacturers have already learned how to properly put together a wiring harness.

To be thorough however, I've been in the business long enough to have learned that there will always be something to surprise me, or something that I haven't seen before. I've also learned that I should always check if there's gas in the car before I start wrenching on spark plugs and the like.

If this is a simple video connection, perhaps the source or the destination is at fault (sounds like you're talking about a rear view camera setup). If it's an antenna, perhaps the receiver is faulty, or more likely, the antenna itself took some damage.

After you've double and triple checked everything, you can build a simple TDR setup like this one, but keep in mind that it will only give you the distance to the fault, and you'll need to account for al of those twists and turns in the convoluted route from front to back.

-Don

[cerickson]

My speculation is that losing the entire shield (no end-end continutity and no short between center-shield) but not the center conductor implies a hidden coupler as opposed to a chafed wire. I can't imagine any other scenario that would explain what I am seeing.

It is the coax video connection for the rear TV. I has worked in the past but went bad about as month ago. The RV ('93 Fleetwood Bounder 34C) is about 15 years old.

I strung a wire through the RV so I could verify that the center conductor has continuity from end-to-end but the shield does not. Neither end has continuity between the center and shield so there are no shorts.

I have verified the equipment on each end. An LCD TV (and DVD player) in the back and a 1Ghz CATV splitter (and LCD TV) in the front.

The troubleshooting continues!

-Chris

[DocJC]

Chris,

Seems to me you are 'in luck' that it is the shield that has lost connectivity.

As the cable is routed inside the vehicle you do not require absolute integrity for the outter plastic protective cable cover.

So you have the option of testing continuity at your various access covers by sticking a pin through the outter plastic protective cover to hit the shield. Aim the needle to just hit the shield, not to pierce the inner insulator or conductor if you press to hard.

You can now "pin" the location down to between two access covers and verify continuity to both of the other ends of the cable.

JC

[cerickson]

In fact that is EXACTLY what I was doing to try to get the fault narrowed down! I used the pin from my Victorinox Tech-40 Swiss Army Knife.

I now know that it is between the back of the electrical panel and the back of the RV.

Unfortunately the cable isn't overly-easily accessible between those two spots.

-Christopher Erickson