A couple of recent threads about the effects of DSL (micro)filters gave me the idea of analysing their effect under various conditions in a simulation package.
I've got several examples of filters of varying designs here and I'd really like to know how much difference and indeed what kind of differenc(es) they can make.
As some know from previous threads I'm a bit sceptical about how much they affect the DSL signal itself.
Before I can do this I need some suitable 'models' for the various bits of telecom gear involved. Can anyone point me in the right direction ?
Unlike cable (DOCSIS) modems, DSL comes in many flavors with widely varying performance specifications. Take a look at the Wikipedia article for a start
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I don't think there is an industry standard like there is for cable broadband. Each manufacturer has their own specs.
As far as the DSL filters go, since they are intended to be placed on all of the other (non DSL) equipment on the shared line, their performance might also depend on the nature of the telephone equipment connected downstream of them.
You can do a simple test by unplugging the filter and putting a lead from the ADSL modem straight into the phone socket. Although the different quality of the cable might be as important as the filter.
They are bound to attenuate slightly. Even a straight through extensions lead three inches long would have some attenuation. Whether it makes enough of a difference is the issue.
Yes I know this kind of thing. It misses the point though. I want to *quantify* in decibels of loss, frequency response etc the effect of such things using a modern electronics simulation package.
Absolutely not. If a cable wasn't twisted pair that might make a difference though. I wouldn't call that 'quality' myself. I like to see things called by their correct names.
If you wish to model the filters you will need to measure the inductance and capacitance of the elements of the filter.
If you can do this you probably have the equipment to measure the real performance so why not simply measure the performance under the conditions you are interested in?
Yes. The Capacitance values are printed on the caps btw.
Not from the exchange end. I don't have access to that for one thing ! The whole entity of the line from exhange to subscriber is what I intend to simulalte. It's quite simple as long as I can get the numbers to punch in for various pieces of kit such as phones.
Let me put it another way. Right now I'm looking for phone schematics so I can make a model of them. Can you point me in the direction of any ?
Because I want to simulate any number of generalised (and specific) conditions and measure the effect of individual changes.
Measuring performance the wayyou suggest is so passe these days and hoplessly time consuming.
You can measure the physical parameters R, L and C and put these into an analysis package to get the passband, attenuation, phase shift and other parameters, expressed in compatible units such as db.
This won't be much help because the filter assumes input and output impedance parameters that are laboratory design concepts, that don't match real life, so your analysis package graphs won't match real life. In fact you may find that what appears to be a cheapo sub-standard filter gives better results because it creates a better match between the incoming lines impedance, and the modems input impedance.
Like a lot of HF electronic design you can impedance match (often with difficulty) at a specific frequency, but the match rarely holds over a range of frequencies where the source and load are complex, and frequency dependent, numbers. Complex numbers in this sense are numbers/vectors with real and imaginary components, where you get into hyperbolic functions.
Like your TV antenna should be 75 ohms across its frequency range, and your TV's antenna socket should be 75 ohms across the full range of TV channel frequencies, it's a pie in the sky concept. In the heyday of CB radio it was all about matching, many and various, were the weird and wonderful combinations of coils and capacitors, pie, half pie, etc.
But *does* the filter (did the filter designer) assume that ? It certainly doesn't have to be that way. That's why I fancy simulating some.
At the lowish frequencies used for ADSL I'm not clear how much that 'matching' is relevant. It certainly won't be a big issue at ~ 30kHz for example, rather more of one at 300kHz and 1MHz..
All of that is still around... it's quite visible with the HF amateur radio guys, and of course in something like a cell phone it's all there but just much, much tinier.
When you're talking about transmitters, not only is proper matching desirable from the point of view of getting as much signal into the air as possible, it's also important in that serious mismatches can create voltages that simply blow up the transmitter's finals. :-) (At least with solid state devices -- tubes apparently are much more immune to this.)
Find someone nearby with DSL, and ask them if they have a spare filter. My DSL provider gave me a handful of filters - just see if you can get somebody to give you one, and hack it and find out what's actually inside the thing.
By Occam's razor, it's probably just an ordinary LC low-pass filter, probably balanced and impedance-matched to the line(s).
What you are up against is that an ADSL filter designer has to start from a set of R, L, C parameters that create the impedence of an incoming exchange line. The problem is that someone a quarter mile from the exchange sees a different incoming line impedence to someone 2 miles from the exchange. In the early days of telephony loading coils were used to level the frequency response of lines over the speech band, don't know if they are still used. In essence twisted pair street cables are very capacitive, and loading coils were inductive to compensate. You have to assume what comprises a typical ADSL line. Then you have to hope that the typical ADSL modem matches the line, and would be a good match when plugged in without a filter. Having made these sweeping assumptions you now decide how sharp the filter needs to be, and what configuration to use. You end up with a filter that hopefully does as good a job as possible in most situations.
Matching is always important because the launch energy at the source has to go somewhere, it can't just be destroyed and disappear, "conservation of energy" and if it's not absorbed it is likely in 200 pair street cables, to appear as noise in adjacent (more than likely) ADSL pairs.
Unfortunately a lot of street cables in the UK are immediate post war, though there is still a bit of paper insulated, lead overall, still around. Never designed for ADSL.
These cables were never speced for Near End Cross Talk (NEXT), if we were starting again they would be, but then if we were starting again fibre to the kerb would be a better model.
But if you wan't to dig deeper
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a good starting point for understanding what is involved.
I talk to myself and find I have 3 spare filters ! :~)
2 of them are basically simple 2nd order LC filters feeding the POTS socket. The L consists of 2 windings on one core to retain line symmetry. They also include the 'ring capacitor' too, a UK thing AFAIK. The bell circuit runs as a 3rd wire inside the house here. Doing it that way avoided 'bell tinkle' in the old days of pulse dialling.
See above. The third is stuffed full of bits. 3 x 2-section inductors, 5 caps, and 2 Rs.
Not so sure about impedance matched. What impedance is a phone anyway, never mind 4 in parallel ( the max load for a line and a single filter is *supposed* to cope ) ?
Graham
p.s. *what impedance is a phone anyway* ? Please see thread title ! Off-hook, on-hook I need data !
'Create the impedance'. Can you elaborate on that ? I don't think it's that at all unless I'm very mistaken. In any case the impedance of a filter will be all over the place with frequency.
More like a different resistance actually. At *audio* frequencies the short run of cable from the exchange doesn't really impose any characteristic impedance value.
At the DSL frequencies the issue will come into play but the DSL filter's not in that path so the DSL modem / router has to deal with that one just for itself.
I've heard of them but know almost nothing more about them. I gather DSL won't work if there's one on the line. I suspect also they may have been there to help with the volume level on long lines.
OK. Easy to simulate again if so inclined.
Yup. Or several examples of same from 100m from the exchange to several km.
They are apparently.
I'm not making that decision. I have some examples here and they vary considerably. That's the objective of this exercise. To see just how much difference a DSL filter can make and in what ways.
Only when you're talking about a transmission line. POTS telephone local loop circuits are *NOT* transmision lines.
Can't take all that into account too. I'm interested in what's likely to have been around for say 30-40 yrs.
Although it may be a pass through, what is shunted across the line is important. Like a recent post here, where sync only occured when the phone was lifted. What happens on the phone side will be reflected onto the ADSL side.
Tlelephone twisted pair has a characteristic impedence, for high spec cables the mfr usually quotes it, along with figures for NEXT and other line characteristics. You can do it the hard way and start from the dielectric constant of the insulation, the wire spacing (thickness of the insulation), and the wire diameter. Old timers will remember the Royal Core of Signals bible The Handbook of Line Transmission, and the various BT books. There are also test sets for measuring line parameters.
You can measure anything, it's really a matter of determining what you need to measure, and how it relates to the real world.
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