Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[burakkucat]

I can see an additional and quite unnecessary extra failure mode though - ie the transistors.

That was also my initial thought when I first read about the ADSL Nation filtered faceplate, some years ago.

The soldering on those pictures is appalling as well

I'm glad you've said that, so now I know that my eyesight is still reasonable! 

[guest]

I can't see the point of an active filter. The 3dB points on either active or passive filtering are going to be broadly similar and the guard band for ADSL is enough that filter rolloff is unlikely to be an issue. If you wanted to have a nice sharp response then simply add more poles - a four-pole passive filter is cheap enough to make and will do the job fine.

You only need an active component for notch filters operating below a few hundred MHz. The ADSLNation faceplate looks like overkill to me.

[burakkucat]

With you obvious knowledge on the subject, would you be willing to sketch out a few example schematic diagrams, please? 

[guest]

I'll knock something up later.

Having thought about this I don't believe the ADSLNation filter is actually "active" - ie its not a tuned circuit amp/switch.

I think the purpose of the two transistors is to switch a secondary filter in when the phone is in use, then switch it out when the phone is not in use.

The reasoning behind that would be that the default filter would filter everything out above say 50Hz (ring signal is 25Hz in the UK isn't it?), then when the phone is picked up the secondary filter switches in and allows frequencies up to 4kHz to pass through so voice works.

Quite an elegant solution as it effectively prevents the phone portion of the subscriber wiring passing any noise above 50Hz to the ADSL modem until the phone is actually in use.

On the page Eric linked, the second filter picture shows two reed relays in the top right hand corner. Assuming I'm right in my assumption, those relays will do the same job as the transistors - ie switch filters in and out. I think I'd rather have the relay solution than the transistor solution, less prone to failure.

[sevenlayermuddle]

I think the purpose of the two transistors is to switch a secondary filter in when the phone is in use, then switch it out when the phone is not in use.

That would address my earlier question, about the legality of drawing power (by/for the transistors) when the phone is not off-hook.

But I would have concerns about this approach, or the equivalent relay solution.  If it actually conferred a benefit (which I accept, it may do in some cases) then surely that benefit would evaporate as soon as a phone were lifted off hook?

[guest]

Not really.

You can design a Chebyshev passive filter with a nice sharp cutoff response for on-hook and then use a Butterworth filter for off-hook. The cutoff response of a Butterworth filter isn't as good as a Chebyshev but you don't end up with ripple on the passband, which isn't something you want being superimposed on the voice signal. That way you get the best of both worlds.

A quick scribble suggests that a 5 pole Butterworth lowpass filter with a cutoff frequency of 5kHz would have an insertion loss of 70dB @ 30kHz. The equivalent Chebyshev filter would have an insertion loss of near enough 100dB @ 30 kHz but it'd have 1dB ripple on the bandpass frequencies (0-5kHz).

http://en.wikipedia.org/wiki/Chebyshev_filter - look at the section entitled Comparison with other linear filters, specifically the Butterworth and Chebyshev type 1 graphs. That might help if you don't understand what I'm on about, which is quite likely as I'm no teacher

For any BT guys - is the termination impedance for a line 600 ohms? That figure rings bells so to speak

[sevenlayermuddle]

@Rizla,

Now I'm confused, I thought the suggestion was that the effective cut-off of the filter could be radically shifted,  from a few kHz to a few tens of Hz, when the phone is on-hook vs off-hook?

I must admit I'm way out of my depth in the level of detail you cited, and further detail would probably drown me. 

But can you at least confirm whether I'd misunderstood that point about shifting the cut-off?

[guest]

No you're quite right in your thinking.

You can (with sufficiently large chokes) have a Chebyshev filter designed to cutoff at 100Hz which will have an insertion loss of >90dB @ 500Hz. That means virtually no noise will pass between the voice port and the DSL.

I think I've worked out WHY you need to have a switch - its CLI. You have to be able to pass CLI info to the phone and that's in the voice band.

So when a ring signal is received you switch filters so that CLI can be passed to the phone. So for example you could switch from your 100Hz filter to a 5kHz filter when the ring signal is received.

Apparently (according to SIN346) +/- 1dB ripple is permissable on the voice signal so Chebyshev would be the filter of choice due to the nice sharp rolloff.

Do note that this is all speculation as to how the XF-1e and FDX100 filters work, although I'm buggered if I can work out what else relays would do in a passive filter

Edit - are there any chunky looking capacitors on the ADSLNation filter and if so what are the markings? There will be one 1.8µF cap for the ring circuit, just curious about the rest.

[guest]

Just had a look at the filters Sky send out.

Its a 4-pole filter, two chokes in series on both the A&B lines, couple of caps across A/B after each choke (so its a series filter rather than shunt filter, as you'd expect), ring cap across pins 4 & 5, couple of 82 ohm resistors after the second chokes for impedance matching purposes.

Datasheet is here : http://www.lea-networks.co.uk/upload/pdf/65_131583446053.pdf although it looks like they just copied the requirements of SIN346.

Looks perfectly adequate to me and if you stick a couple of them in series that will sharpen up the rolloff nicely.

I think the trannys/relays are overkill and just add another failure mode.

[Blackeagle]

are there any chunky looking capacitors on the ADSLNation filter and if so what are the markings? There will be one 1.8µF cap for the ring circuit, just curious about the rest.

Yep, 1.8µF electrolytic & a 1µF electrolytic (250v, 85°) next to the bridge reccy. Q2 is a 2N2222 and Q1 is an MPS2222A, so basically same transistor.

There are no values on the caps below the coils, but the mfg numbers are F393J (on the left) and F562J to the right of it.  No markings on the coils at all apart from a coloured dot !!

[burakkucat]

. . . (ring signal is 25Hz in the UK isn't it?) . . .

I'm not aware of any changes, so I'll answer with a tentative "Yes" and wait for Black Sheep's definitive answer. 

For any BT guys - is the termination impedance for a line 600 ohms? That figure rings bells so to speak

"Yes", says the b*cat, emphatically. I don't think we need the wooly-back's assistance for that query. 

[guest]

The 1µF cap will just be there for some rough smoothing on the bridge rectifier.

Looking at the board and trying to work out what's happening I think that there is only one filter and I've been overcomplicating things

It rather looks like the phone is electrically disconnected from A/B in the NTE while the phone is on-hook and no ring signal is being received.

If the phone is taken off the hook then Q2 is switched on, which in turn switches Q1 on and connects the phone to A/B via the filter.

If a ring signal is received then Q1 is switched on, which in turn switches Q2 on and connects the phone to A/B via the filter.

So basically its one filter with a switch which only connects the phone when its off-hook or a ring signal is received. At all other times you can effectively consider the phone - and all associated wiring - as unplugged.

[sevenlayermuddle]

So basically its one filter with a switch which only connects the phone when its off-hook or a ring signal is received. At all other times you can effectively consider the phone - and all associated wiring - as unplugged.

Which would bring us back, would it not, to my earlier criticism... that the benefits evaporate with ( possibly substantial) DSL degradation whenever the phone goes off hook, or ring tone received?

[guest]

Yes, once the phone is in use it will be dependent on the filter design which (going by the number of chokes) is a 4 pole series filter. As the chokes have no value on them we can safely assume they're custom wound for this particular application. That should make for a nice sharp filter.

I think its a rather elegant design though. I don't think the idea of disconnecting the voiceband cabling when not in use would have occurred to me.

The main failure mode would be lightning causing Q2 to go open collector, hence the spark gap just in case. Looks like a "one-shot" job which is fine as its secondary to the NTE spark gap. They've used bipolar trannies too so no worries about static damage, only likely lifespan reduction would be excess heat when soldering (which is pretty bad on that board).

I'm quite impressed with that filter - I can see why its recommended for people with "dodgy" internal wiring.

[sheddyian]

I use an ADSL Nation filtered faceplate, having previously had problems with ADSL noise on the voice telephone, despite using correct filter.  I assume there's something odd with my wiring, though it all works ok.

Anyhow, with the ADSL Nation filtered faceplate I get the same S/N margins and line attenuation with the extensons wired through the filter on the faceplate as I do if I plug my modem direct into the test socket with everything else disconnected.  The synch speed was about 250K lower with the filtered faceplate at the time I tested, but I reckon that's easily within margin of error.  (I've later had it a bit higher than the "test socket only" synch speeds)

So : I certainly like it.  Didn't study the soldering quality on my one, so can't comment - but maybe I'll open it up soon and take a few pictures.

Of course, I can't comment on how it compares to a standard BT issue filtered faceplate, or similar - I've not tried those.  Someone should do some independent tests

Ian