Coax cable simulator

[martinkr]

Hi Eevblog forum

I have been looking around for a test-unit simulating a coax cable, not a pc simulator program but a unit for real life test, but without any luck.
Normally I have 700m roll of 50 ohm coax cable outside my office for bench test. But now I need to test a system with coax cable 50 ohm 4000m. I dont have a cable at thai length an before I run out and buy a cable at this length I hopede someone in here knew about a test unit simulating coax cable or a circuit so I could build one myself and so I could get rid of that cable drum outside my office, did I mention it is a armored coax with a weight of ~700kg :-O

Thanks, I really hope someone in here has some good ideas

/Martin

[bktemp]

Here is one example how a cable simulator looks like:
https://youtu.be/R40i-ZsUbHc?t=2384

[martinkr]

Actually I found that unit in my search, and it is one like that i'm looking for, but just for coax in-sted of twistet pair.

I was thinking about making a RLC circuit matching that 700m cable I have and then make a bunch of them and cascade them, does anybody has any experience with that?

[tggzzz]

You need to specify what is important in the test, in other words why you are doing it and what you expect to learn/verify.

For example, if you are using a cable on a mountain top (it happens), one very important attribute of a cable is its weight. Weight on a reel is important for transportation, and can be simulated by a block of concrete. Weight when strung between poles cannot be simulated that way.

So, what characteristics do you need to simulate? Delay? Frequency response? Impedance as a function of temperature? Etc.

[SeanB]

Have you considered using a smaller diameter cable, so your 4km roll is not a 2 ton monster? No need for the armour, but just the 50R impedance. All the simulators are is a lumped LCR circuit, and this diverges from the real cable quite badly as you sweep it, so the best test is a real cable.

[martinkr]

Right, I should have clearified what system test I was expecting to do.

The system is running 300 V DC / 300 W with a DSL signal on the positive DC. In each end the is a filter separating DC and DSL signal. I need to test the dampning, speed and reliability of the DSL signal as first priority. If the test unit could handle full load at the same time it would be a plus.

The only reason I'm using a armored cable is it was what I had at hand. I already was looking at some corresponding non-armored cables at 5000m and split it up in different lengths and connect with switches so it was easy to test different kinds of lengths.

But a simulator box would still be smaller to have on the test-bench, but it looks like it's not and alternative.

I'm still interested in what others do so please keep good suggestion coming for inspiration, thanks.

[T3sl4co1l]

Why do you have to test the full system?  Are you not simply adding bias tees, which can be qualified on their own?

Tim

[Berni]

I don't think that one of these cable simulator boxes will work for you.

Firstly the large DC levels and currents will likely burn the components inside. Then also the simulator likely will not have realistic DC resistance to correctly model the DC losses.

Then comes the DSL part. Firstly they tend to use very high frequencies (Close to 1GHz) so the simulator has to model it accurately up to very high frequencies. Also there are many kinds of coax cable out there. In DSL systems a important property of the cable is RF loss. Better (offten thicker) kinds of coax cable tend to have lower loss, while the cheap thin stuff that people tend to run cable TV trough the walls of the house is lossy as heck. The loss can be as bad as loosing 10dB over 50m. So you put 100mW of RF power in to one end and only 10mW of it makes it to the other end. With a 4km cable that lossy you would barely get anything out the other end, while some fancy waveguide could still keep most of its original power.

So if you want to really accurately test all scenarios you actually need multiple reels of cable in a few different types. Or make tests that simulate one aspect of the cable at a time. For example a power resistor to simulate DC resistance, an RF inline attenuator to simulate signal losses, a filter to simulate the frequency response etc. Most difficult to simulate is the delay of it as for that you need a coax cable that long or a ton of LC filters in a chain to create enough phase shift. For DSL you probably don't need to simulate the delay as the modems measure it during initialization and compensate it out. Usualy the reason for a poor DSL connection is too much loss or damaged cable that causes signal reflections.

[T3sl4co1l]

Surely you're confusing DSL with cable..?  DSL tops out at a few MHz.

The point about DC may be a big concern, though!  That depends how low in frequency the cable simulator goes; if it models correctly at DC (and doesn't dissipate excessive power in the process), it should be fine.  If it's only intended to model down to modest frequencies, you'll have some error.

One would hope they're also built with linear components, so you don't have inductors saturating under DC current bias, or capacitors distorting (or breaking down) under DC voltage bias.

Tim

[martinkr]

Hi

Thanks for all the good replys 

I'm sorry I has not returned earlier, but one day take the other and suddenly we are talking months 

But to round it off, I ended up by bying a reel of 3000 m to test the system on, with ok results 5 Mb/s, the system had 15 dB damping.

A T3sl4co1l says, DSL is only a couple of MHz, depending on modulation type.

Ones again thanks for inputs, I will keep on looking into some sort of simulation box, just for the fun of it.

/Martin