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[JGO]

"Ernelli is using two 1:1 100ohm baluns to connect the noise generator.  I must admit, it's an area about which I know next to nothing     How does it work?!  Any clues?! "

Balun is short for balance to unbalance transformer -  (or should be it tends to also get used for  devices like the transformer in the I plate, which works in a balanced line and acts as a high series inductance to an unbalanced current with negligible effect on balanced)

The simplest form is a 1:1 transformer, centre tapped on the balanced side. The centre tap and one side of the unbalanced winding are earthed. With clever transformer construction  this will convert a balanced line to unbalanced, or vice versa over a considerable frequency ratio. ( For ADSL2+ the ratio is 84.5:1)
This is the simple and comprehensible form; at HF/ VHF/Microwaves MANY other variants are possible.

Regarding another point, a modem is a bit like a radio receiver, which can be designed for maximum sensitivity, e.g. for a radio telescope, or with less sensitivity to avoid overload and distortion by a strong signal - you can't have both at once and building in adaption costs money.  Yes, it would be nice if someone told you what you were buying.
 

[asbokid]

Thanks JGO... 

What specifically remains beyond my understanding is how two 100:100 BALUNs placed back to back can impedance match the output of the 50ohm noise generator.

This looked like a suitable BALUN for connecting the unbalanced 50ohm output of the signal generator to the 100ohm balanced twisted pair of the cable under test. Except, the price of that BALUN ($150) renders it completely incompatible for the budget!   It might be easier, and certainly more profitable to wind one by hand!



From: http://www.northhills-sp.com/store/cgi-bin/cp-app.cgi?rnd=9535125&pg=prod&ref=0301BB

cheers, a

[les-70]

http://www.ebay.co.uk/itm/North-Hills-NH14023-Balun-Transformer-50-to-100-Ohms-300MHz-/190808888099?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item2c6d189323

 See above -- they are cheaper on ebay but I think they really just ensure an impedance match and good transmission with no nasty reflections.  Video ones are very cheap (99p) but I suspect the impedance is wrong.   As a tiny lad I once made one to connect an aerial to a valve radio (before transistors were much used!) -- that did not end up working very well!! 

 I think the basic setup would allow "relative" testing of the modems with no balun and just the transmission line with noise injection in the middle. 

[JGO]

Les  - are you saying the device covers  300 MHz  to 10 KHz ?!!!!!! I wouldn't have thought that is possible.

 " What specifically remains beyond my understanding is how two 100:100 BALUNs placed back to back can impedance match the output of the 50ohm noise generator."   

Me too !
I would be inclined to look at for unsymetrical resistive attenuators so you can separate the impedance a device looks like  and the load impedance it sees. As Les says you can do a relative test that way, just allow for the attenuation. 
 I have used this (once) for matching a 75 ohm  source and a 50 ohm receiver, but it seems more complicated with 3 devices; or I've forgotten how to do it in the intervening years !

[asbokid]

Thanks guys for the advice. By coincidence, a couple of those 75ohm video baluns (50p each) were ordered from China, just to see what they did. That was over a month ago!  When they eventually arrive, I'll cut one open and photograph it.   The North Hills BALUNs are still a bit expensive,for what is just an amateur experiment, but maybe they're the more professional answer.   There is/was a course given at Univ. New Brunswick on performing these sorts of tests:

http://www.ee.unb.ca/Courses/EE3822/BC/Lab_3.pdf

cheers, a

[benji09]

 First point I want to make is that ` POTS '  has not been 600 ohms since year dot. Underground cable put paid to that. Old type ASTIC telephones had to have a balance network to simulate a UG cable to cancel out sender's speech in the earphone. That balance was NOT a nice 600 ohm resister.

 A  1:1 transformer will reflect the impedance that is on one side to the other side. In this case, 50 ohms in parellel with 100 from one side, to the other. As I see it the transformers, although required to deal with the generator's unbalance problem, are in this case a bit of a red herring to the understanding of the way the test set up works due to the 1:1 turns ratio. My thought is why the need for two of them. Not only do transformers EACH normally considered to introduce a 0.5 dB loss, they don't operate over an unlimited frequency range.
  I personally would think at one transformer on the output of the generater, and TWO high value resistors in each leg of secondary winding to provide both generator balanced output, and a form constant current drive to the line with it's varying impedance over frequency.     Would be interested in members comments to my comments....   

[burakkucat]

My understanding is that the transceivers at each end of the link operate in differential mode. The noise generator needs to be coupled into a balanced pair such that its signal is not rejected by the differential circuit at the receiving end.

Consider the balanced transmission line stretching across the screen, just like this sentence. At the midpoint of each wire of the pair, insert a 1:1 transformer in series with the wire. For one of those transformers, the second winding is connected to the signal generator. For the second transformer, the second winding is left unused. How does that appeal to the electronics wizards?


                                            T1
                                    -----ooooo-----
A --------------------------------------ooooo----------------------------------- A
Twisted Pair                                                    Twisted Pair
B --------------------------------------ooooo----------------------------------- B
                                    -----ooooo-----
                                    |        T2      |
                                    |                  |
                                    |                  |
                                    - Noise Gen -

[benji09]

  I personally can't see the need to inject a noise signal in series with the line, as the differential bit means  that one leg of the pair is pushing, whilst the other leg is pulling a signal around it !  If the signal was injected into both legs of the pair in the same phase, then as you say, the interfering signal would cancel itself out at the modems. The fact that the differentially injected signal goes to both ends of the circuit does not stop the signal from being injected......... If my assumption was not correct, the there would never be a problem with crosstalk on cables.
  I must confess that I have not had the time to study all of the postings, or the original university paper on this topic, but in what I have seen, I am not convinced that a number of the assumed facts were correct in the paper. The other point I wish to make is that if the line were a Virgin Media using coax cable to the cabinet, it would be a proper 75 ? ohm transmission line that would be terminated with the correct impedance. Therefore test equipment would be available for the correct impedance to get meaningful results.
 But as I suggested in an earlier post underground cables behave more like a capacitor than a proper transmission line. Therefore the impedance will tend to drop with frequency. Due to this fact, how do you measure what power of noise signal you are actually injecting into modem, in relation to the signal being received.
   In my opinion, because of BT lines being of varying characteristics, perhaps the best way of evaluating the different modems is to just to compare them in the field on various lines. In fact I am sure that BT has probably carried this out at some point already, and already know the answer !

[burakkucat]

Methinks that the schematic diagram (Figure 3.4, on the previous page and attached below) does not tell the truth of what was actually used. 

[asbokid]

I took the plunge and bought a secondhand 100:50 BALUN from ebay!  Ironically, the seller works for a company that performs these very tests! 

There are a number of other "curiosities" in that master's thesis.   The calculations for selecting resistor values to create specific loop attenuations do not match my own sums.  At best, the calculations in the thesis seem over simplified:



As for measuring noise, the $100 Hantek noise generator planned for these tests comes with Windows-only software    But if that handicap can be overcome, precise levels of white noise (dBm/Hz) will be injected into a zero-loop.  Allowing the response / compensation from each modem to be measured.





cheers, a

[burakkucat]

I took the plunge and bought a secondhand 100:50 BALUN from ebay!

I am wondering how you propose to connect it to/into the zero-loop. Have you yet decided? 

[asbokid]

The "zero" loop will be a couple of short lengths of cat5e twisted pair, with an RJ11 crimped on one end of the first length,  and the other end of the second length will be punched into the IDCs of the DSLAM patch panel.   The other ends of the two lengths will be stripped of insulation, and probably soldered together.   Oops, correction! A BNC to BNC will connect the noise generator to the BALUN, and the BALUN will probably be soldered to the twisted pair of the zero loop.  I'll take some photos when it arrives!

cheers, a

[burakkucat]

I see it! Thanks for the description. 

[edward]

Hope you don't mind me asking a question. My connection is via a long line 800m-900m, supplied with an HG612 and have a matching DSLAM in the cabinet, would I be right in thinking reading through this thread that I might be better off using an ECI V-2FUb/r Rev B with this length of line?

[asbokid]

The tests, at least the tests I performed aren't to be relied upon. They weren't conducted scientifically.  For example, the linecard/port should have been reset between each modem - to flush any residual DLM behaviour from previous tests.   At some point the tests can be performed again, but this time under "laboratory conditions" !   As such it's probably best not to read anything much in to them!
As an aside, they are surprisingly difficult tests to frame.  A modem can be synced-up half a dozen times, with static line conditions, and yet it will perform differently on each occasion.  Discovering why that happens, if not because of DLM, is crucial!

cheers, a