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'[OT]:modem back to back.'
1998\09\11@024110 by Mike Keitz

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On Thu, 10 Sep 1998 20:27:09 -0500 David VanHorn <spam_OUTdvanhornTakeThisOuTspamCEDAR.NET>
writes:
[Paul Webster wrote]
>>  Aw come on, do a *decent* job!  For phone circuits, use a
>regulator,
>>e.g. 78L08 (as well as a couple of electrolytics).

Not a good idea, phones work on current rather than voltage.  Some won't
like having 8V forced into them.  Others won't work on only 8V.  The
simulated "phone company" you supply should look like a 50 V source with
about 1.2K ohm DC resistance, and 600 ohm AC impedance.  It can be
difficult to supply the full 50 V, so another workable model is a
constant current source of 25 mA or so with a compliance to 20 V.

{Quote hidden}

Especially at higher speeds, it's a good idea to not run the loop current
through the transformer, since the DC bias will cause the iron core to
saturate early and increase distortion.  Many that I have seen use a
transistor active load to pass the line current, then couple to the
transformer through a capacitor.  Since the active load needs a proper
positive DC bias to work, there is a diode bridge in line to the whole
affair.  I suppose the bridge could just be in line to the active load,
but then a non-polarized capacitor would be needed to couple to the
transformer.  Some designs switch the load on and off to control on/off
hook, but relays are still popular.  In external modems, I suppose it is
possible to use a larger transformer that would be more immune from
saturation.  The early external modems just had a transformer and a relay
at the input, so they could be connected directly without needing to
supply DC bias.


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1998\09\11@163752 by paulb

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face
Mike Keitz wrote:

> Not a good idea, phones work on current rather than voltage.

 I know that.  Use a lamp.  Use a regulated supply *and* a lamp.

> Some won't like having 8V forced into them.  Others won't work on only
> 8V.

 The latter won't pass FCC then!

> The simulated "phone company" you supply should look like a 50 V
> source with about 1.2K ohm DC resistance, and 600 ohm AC impedance.

 Obvious error there.  Firstly, if there is more than 600 ohms in the
line, and there often is, then the AC impedance the modem sees is going
to be *much* more than 600 or 900 ohms (which goose left omega out of
the Windoze character set eh?).  Secondly, you are using the modems in
parallel, are you not?  In which case, you want the source to have
*infinite*, or thereabouts, AC impedance.

 My concern however was simply keeping AC hum and rectifier harmonics
out of it.

>  It can be difficult to supply the full 50 V, so another workable
> model is a constant current source of 25 mA or so with a compliance to
> 20 V.

 Again, since the spec for line equipment is about 8V, this is mucho
overkill.  I could be wrong, but 8V has always done in the past.

> Especially at higher speeds, it's a good idea to not run the loop
> current through the transformer, ...  Many that I have seen
> use a transistor active load to pass the line current, then couple to
> the transformer through a capacitor.

 So you want to minimise the capacitor eh?  You're using an undersize
transformer by avoiding DC, and ...

>  Since the active load needs a proper positive DC bias to work, there
> is a diode bridge in line to the whole affair.  I suppose the bridge
> could just be in line to the active load, but then a non-polarized
> capacitor would be needed to couple to the transformer.

 ... you're worried about introducing distortion but you want to use
an electrolytic capacitor for compactness ...

> The early external modems just had a transformer and a relay at the
> input, so they could be connected directly without needing to
> supply DC bias.

 I'd be looking out for them for this project!  I can now see *why* you
might need DC bias for the "mini" modems, though I still can't see why
you'd need more than 6V or much more than 40mA.  My specification of an
8V regulator was actually a convenient value to use with a 12V wall-
wart rather than anything else, regulated 12V to a 12V lamp would seem
to be pretty good though a nice DC-rated choke as well would be great.

 I personally would be really tempted to "bodgie" the offending
components out of the modem. and connect through a (plastic) capacitor
to the transformer.
--
 Cheers,
       Paul B.

1998\09\11@171043 by David VanHorn

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face
>> Some won't like having 8V forced into them.  Others won't work on only
>> 8V.
>
>  The latter won't pass FCC then!


Debatable, The CO current limits out at 100mA, whatever voltage develops
across the phone is mostly irrelevant. I've seen as low as 4V, and as high
as 18-20V.

>> The simulated "phone company" you supply should look like a 50 V
>> source with about 1.2K ohm DC resistance, and 600 ohm AC impedance.


That's correct.

>  Obvious error there.  Firstly, if there is more than 600 ohms in the
>line, and there often is, then the AC impedance the modem sees is going
>to be *much* more than 600 or 900 ohms (which goose left omega out of
>the Windoze character set eh?).  Secondly, you are using the modems in
>parallel, are you not?  In which case, you want the source to have
>*infinite*, or thereabouts, AC impedance.


Series. The current source is in series with both modems, and should
therefore be as close to zero ohms as possible. And, each modem sees the
other modem as it's terminating impedance, plus wire Z, which shouldn't be
too bad.


>  Again, since the spec for line equipment is about 8V, this is mucho
>overkill.  I could be wrong, but 8V has always done in the past.


It's not voltage. You'll never see a spec that tells you you'll get XX volts
in the looped state. Only that you'll get not less than 20mA and not more
than 100mA (iirc)

>> Especially at higher speeds, it's a good idea to not run the loop
>> current through the transformer, ...  Many that I have seen
>> use a transistor active load to pass the line current, then couple to
>> the transformer through a capacitor.


That's a modem design issue, and if the transformer is adequately designed,
it's perfectly fine to run current through it. It's just that they are
usually rather large when designed that way.



>> The early external modems just had a transformer and a relay at the
>> input, so they could be connected directly without needing to
>> supply DC bias.


Yes, that's a wet transformer design.

>  I personally would be really tempted to "bodgie" the offending
>components out of the modem. and connect through a (plastic) capacitor
>to the transformer.


You could do that.

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