Discussion in 'Electronic Design' started by Myauk, Nov 10, 2008.

1. ### MyaukGuest

Dear All,

I need to design an atteuator to simulate 9000ft of ADSL cable loss.
Could anybody please let me know what are are the do's and don't's on
practical design of it. Based on my studies, it should give 22.76 dB
of attenuation at 150 kHz and 50.7455dB attenuation at 772 kHz.
What else should i considered more?

Best Regards

2. ### Phil AllisonGuest

"Myauk"

** Is this a REAL " flesh and blood": attenuator ??

Or a ** software simulation ** of one ??

- you PITA twit.

BTW:

" 50.7455 dB " ??????????

Wot fucking planet is this prick on.

....... Phil

3. ### Guest

test -------------------------------------------

4. ### David L. JonesGuest

I assume you are talking about an actual hardware attenuator, and not just a
software simulation?

With practical transmission line simulation like this I've always found it's
prudent to actually use the real cable in a real system test. It would not
be out of the bounds of practicality to actually use 9000ft of cable (I've
done this sort of length before a few times), or test a shorter length and
extrapolate your results. You can also test the performance of a real cable
and match it with any "simulated" attenuator.

Dave.

5. ### Allan HerrimanGuest

This reminds me of something I made in the late '90s. I had to test HDSL
cards and we didn't like the idea of heavy cable drums in our
manufacturing test setup. So I made some equivalent circuits out of
discrete R,L and C components on a board that could plug into our racks.

Steps:
1. Get the RLGC parameters for the line you want to simulate. These
will be a function of frequency. Bellcore used to publish technical
reports with cable parameters measured on a representative sample of
actual subscriber lines.

2. Work out how accurate your simulation needs to be, in terms of dB
error as a function of frequency. There will probably be some upper
frequency, beyond which you don't care about the accuracy at all (which
is good, as most simulations based on lumped elements have an upper
frequency limit beyond which the accuracy gets really bad).
BTW, your use of so many decimal places in 22.76 and 50.7455 dB is rather
naive; you are unlikely to get an accuracy of better than +/- a few dB
over any non-trivial frequency range.

3. Create a lumped model of the line (see the links (below) for ideas),
and check it in your preferred version of Spice. Increase the number of
sections (which reduces the ripple and increses the upper frequency
limit) until you get the required accuracy with practical component
values. Check both the attenuation and impedance.
The inductors and (to a lesser extent) the capacitors will not behave
like their ideal models. You will need to include this in your
simulation. BTW, avoid ceramic caps here, unless they are NP0.

The loss in these reactive components actually helps with the simulation.
Remember, we are trying to recreate the behaviour of something that has
both skin effect and dielectric losses.

4. Lay it out on a PCB. There will probably be a few hundred sections,
so a PCB is the only practical way. You must be very careful with the
layout, as crosstalk will ruin the attenuation characteristics. (BTW,
crosstalk is also a problem for cable drums.)

5. Build and test.

d/ec84b4ce3d5a07c3

fb52/

Regards,
Allan

What are you trying to prove by this simulation?
The main problem is the direct capacitive coupling between the input and
the output. Because of that, the frequency/phase response can be very
peculiar albeit have nothing to do with the reality. Do not use a piece
of actual cable cable winded on a bobbin; you will have to unwind it.

DSP and Mixed Signal Design Consultant
http://www.abvolt.com

7. ### BobGuest

My thoughts on this may be hopelessly naieve but I'd
think about doing it as two 10dB H-pads and a low pass
filter that rolls off at about 13dB per octave.

I suspect that it needs to be more or less symetrical
as you want to communicate both ways.

Inductor and capacitor filter in the middle of the
the ends will make the source impedance look more like
the characteristic impedance of the line which is
probably around 110ohms but you need to look that
up yourself.

There are a few h-pad calculators on the web eg

I'd exepct you could come up with a filter design with
about eight elements that would give a suitable rolloff.
Might need quite large inductors due to the low frequency.

Bob

9. ### MyaukGuest

Dear all,,

Now I have better confidence for starting up.

By the way... Mr Allan...I cannot reach the links you described
there..somehow they are not complete...could you please provide the
title of the discussions??

Best Regards..

10. ### MyaukGuest

Yes Mr Dave..

I am talking about an actual hardware attenuator.

From practical measurement I found out that the series resistance
value is 250 ohms in each wire. So I am thinking of adding series
resistance of 250 ohms to simulate the resistance of the wire.

---/\/\/\-----

.../\/\/\....

This will give an attenuation of -3dB for all frequencies.

Then I am thinking of creating a T network with two series inductors
and one parallel capacitor which will give attenuation of around -20dB
at 150kHz.

Then I will addjust values acordingly to have the attenutaion around
-51dB at 772 kHz.

Hopefully, this can be used instead of actual 9000 ft cable drum for

Please correct me if you find any practically impossible things in my
idea...

I am new in this kind of things..

Thank you so much for everything..

Regards

11. ### Allan HerrimanGuest

Think of it as an aptitude test.

Hint: find my post at groups.google.com, click on "more options" then click
on "show original".

Allan

12. ### MyaukGuest

Thank you so much.

Now I am able to find it.

Regards

13. ### Bob SmitherGuest

One way to design such a simulator is to use optimization. The program
CCICAP has a genetic algorithm based optimization feature that is well
suited to this type of design.

I have included Myauk's specifications in an example that can be studied
here:

http://www.c-c-i.com/node/24

I used two identical sections of RLC low pass filters. Using guessed
values for the RLC components results in an initial error at the
specified frequencies of 37.6 dB. After optimization the error is
reduced to .08 dB.

The example could be improved upon with more sections if additional
attenuation specifications are available.

Regards,
Bob Smither

14. ### SteveGuest

They do sell DSL loop simulators by the way.
Different models, some with and without noise sources and
programmable loop lengths to allow automated testing.

15. ### MyaukGuest

I think they have more features than it is necessary for us.
Do you know any good source where I could check the price?
If the cost is less than the cost of developing it myself, I'd better

16. ### MyaukGuest

Dear Allan..

Bellcore used to publish technical
Is there any link to this? I could not find it.

BTW, avoid ceramic caps here, unless they are NP0.

Could you please explain to me why I should avoid using ceramic caps?

Regards

17. ### Allan HerrimanGuest

In one of the posts I directed you to, I mentioned these standards:

ETSI ETR-152 (HDSL standard)
ETSI ETR-080 (ISDN U interface standard)
ANSI T1.601 (ISDN U interface standard)

Ceramic caps have a "voltage coefficient" that describes their change in
capacitance with the applied voltage. This will produce non-linear
The wires you are trying to model typically use some sort of plastic
insulation (e.g. polyethylene) which does not have this effect.

BTW, you should also make sure the inductors are used well below
saturation, for similar reasons.

Regards,
Allan

18. ### Ross HerbertGuest

ear All,
:
:I need to design an atteuator to simulate 9000ft of ADSL cable loss.
:Could anybody please let me know what are are the do's and don't's on
ractical design of it. Based on my studies, it should give 22.76 dB
f attenuation at 150 kHz and 50.7455dB attenuation at 772 kHz.
:What else should i considered more?
:
:Best Regards

I remembered seeing some artificial line section values in a document I was
researching recently. You can ind this document here
http://www.commsalliance.com.au/documents/standards/S002_2005

See Fig. 26 on p.63

19. ### MyaukGuest

Thank you so much

That one also is of some help to my work.

Best Regards

20. ### JosephKKGuest

I once built one for pe-39 6 pair 22 and found for a frequency range
of 200 to 20,000 Hz i had to use "segments" of not much more than
1000 feet. At your frequency range you may need to do 100 foot
lengths or less. You should probably do 1000 foot groups in separate
boards on shielded boxes.