Reply by glen herrmannsfeldt●July 21, 20062006-07-21
Jerry Avins wrote:
(snip, I wrote)
>> I do agree that transmission lines, with or without transformers,
>> don't make good low pass filters.
> I assumed we were in the realm of lumped constants. After all, the
> transformer analogy was intended to be an idealization, a simplification
> of the "rigid body" sort of the actual problem. I'm rather sure that
> reflection coefficients were far from the OP's thoughts when he wrote,
> "It defines a high pass system as it's two transformers with a piece of
> cable in between so when the cable is short transformers block all DC."
I agree, but that was the only case I could think of where the
length would matter.
-- glen
Reply by Jerry Avins●July 19, 20062006-07-19
glen herrmannsfeldt wrote:
> Jerry Avins wrote:
>
> (snip)
>
>> The OP used the model of a pair of (presumably ideal) transformers
>> connected back-to-back by a (short) cable as the archetype of a DC
>> blocker. I fell into a mental trap I laid for myself. That's
>> embarrassing, but I'll get over it. Ideal transformers, having
>> infinite shunt inductance, block only DC, so the contrary argument
>> that the rolloff is 20 dB/decade is a reductio-ad-absurdam proof (if
>> any proof is needed) that ideal transformers don't exist. The OP
>> claims that he doesn't want just a DC blocker, and seems to assume
>> that the length of the connecting cable matters.
>
> If the cable is not properly impedance matched then its length
> matters, depending on the high frequencies in the signal.
>
> I think it takes most people a while to understand the effects
> of propagation delay through a cable, and then after they do,
> to understand that signals can be sent through a cable with
> wavelengths shorter than the cable.
>
> I do agree that transmission lines, with or without transformers,
> don't make good low pass filters.
I assumed we were in the realm of lumped constants. After all, the
transformer analogy was intended to be an idealization, a simplification
of the "rigid body" sort of the actual problem. I'm rather sure that
reflection coefficients were far from the OP's thoughts when he wrote,
"It defines a high pass system as it's two transformers with a piece of
cable in between so when the cable is short transformers block all DC."
Jerry
--
Engineering is the art of making what you want from things you can get.
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Reply by glen herrmannsfeldt●July 19, 20062006-07-19
Jerry Avins wrote:
(snip)
> The OP used the model of a pair of (presumably ideal) transformers
> connected back-to-back by a (short) cable as the archetype of a DC
> blocker. I fell into a mental trap I laid for myself. That's
> embarrassing, but I'll get over it. Ideal transformers, having infinite
> shunt inductance, block only DC, so the contrary argument that the
> rolloff is 20 dB/decade is a reductio-ad-absurdam proof (if any proof is
> needed) that ideal transformers don't exist. The OP claims that he
> doesn't want just a DC blocker, and seems to assume that the length of
> the connecting cable matters.
If the cable is not properly impedance matched then its length
matters, depending on the high frequencies in the signal.
I think it takes most people a while to understand the effects
of propagation delay through a cable, and then after they do,
to understand that signals can be sent through a cable with
wavelengths shorter than the cable.
I do agree that transmission lines, with or without transformers,
don't make good low pass filters.
-- glen
Reply by Jerry Avins●July 16, 20062006-07-16
glen herrmannsfeldt wrote:
> Jerry Avins wrote:
>> mk wrote:
>
>>> ... a high pass system as it's two
>>> transformers with a piece of cable in between so when the cable is
>>> short transformers block all DC. ...
>
>> When the cable is long, transformers pass DC?
>
> Maybe not, but it takes longer for you to notice.
>
> I think that is related to a joke from somewhere, but I can't
> remember where.
About a dachshund?
Jerry
--
Engineering is the art of making what you want from things you can get.
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Reply by Jerry Avins●July 16, 20062006-07-16
glen herrmannsfeldt wrote:
> Jerry Avins wrote:
>
> (snip)
>
>> You're right; I wasn't thinking straight. Each transformer is a single
>> L/r time constant, the L being the shunt inductance. In fact,
>> connecting two transformers back to back merely parallels the Ls,
>> halving it and raising the cutoff an octave. (The R, of course, is the
>> source impedance. I'm ignoring the leakage inductance and shunt
>> capacitance.)
>
>> So the rolloff to DC is 20 dB/decade; not a hard response to produce.
>> The high-frequency rolloff is probably not intended to be part of the
>> model. Thanks for picking up on my slip. I should do a bit more math.
>
> I haven't thought about this one for a while now.
>
> Isn't there also a term related to the core saturating?
>
> That is, what limits the low frequency response for power
> transformers? I always thought it was from core saturation.
>
> I missed the beginning of this discussion, though.
The OP used the model of a pair of (presumably ideal) transformers
connected back-to-back by a (short) cable as the archetype of a DC
blocker. I fell into a mental trap I laid for myself. That's
embarrassing, but I'll get over it. Ideal transformers, having infinite
shunt inductance, block only DC, so the contrary argument that the
rolloff is 20 dB/decade is a reductio-ad-absurdam proof (if any proof is
needed) that ideal transformers don't exist. The OP claims that he
doesn't want just a DC blocker, and seems to assume that the length of
the connecting cable matters.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by glen herrmannsfeldt●July 16, 20062006-07-16
Jerry Avins wrote:
(snip)
> You're right; I wasn't thinking straight. Each transformer is a single
> L/r time constant, the L being the shunt inductance. In fact, connecting
> two transformers back to back merely parallels the Ls, halving it and
> raising the cutoff an octave. (The R, of course, is the source
> impedance. I'm ignoring the leakage inductance and shunt capacitance.)
> So the rolloff to DC is 20 dB/decade; not a hard response to produce.
> The high-frequency rolloff is probably not intended to be part of the
> model. Thanks for picking up on my slip. I should do a bit more math.
I haven't thought about this one for a while now.
Isn't there also a term related to the core saturating?
That is, what limits the low frequency response for power
transformers? I always thought it was from core saturation.
I missed the beginning of this discussion, though.
-- glen
Reply by glen herrmannsfeldt●July 16, 20062006-07-16
Jerry Avins wrote:
> mk wrote:
>> ... a high pass system as it's two
>> transformers with a piece of cable in between so when the cable is
>> short transformers block all DC. ...
> When the cable is long, transformers pass DC?
Maybe not, but it takes longer for you to notice.
I think that is related to a joke from somewhere, but I can't
remember where.
-- glen
Reply by Jerry Avins●July 16, 20062006-07-16
Mark wrote:
> With
>> the two transformers you describe, the rolloff is 80 dB/decade.
>
> Why do you say that....... 20 dB / decade for each winding pri and sec
> for a tota; pof 80 dB/decade?.. I don't think that is true espcecially
> if the two xformers are connected back to back.
You're right; I wasn't thinking straight. Each transformer is a single
L/r time constant, the L being the shunt inductance. In fact, connecting
two transformers back to back merely parallels the Ls, halving it and
raising the cutoff an octave. (The R, of course, is the source
impedance. I'm ignoring the leakage inductance and shunt capacitance.)
So the rolloff to DC is 20 dB/decade; not a hard response to produce.
The high-frequency rolloff is probably not intended to be part of the
model. Thanks for picking up on my slip. I should do a bit more math.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Mark●July 16, 20062006-07-16
With
> the two transformers you describe, the rolloff is 80 dB/decade.
Why do you say that....... 20 dB / decade for each winding pri and sec
for a tota; pof 80 dB/decade?.. I don't think that is true espcecially
if the two xformers are connected back to back.
Mark
Reply by Jerry Avins●July 14, 20062006-07-14
mk wrote:
> On Fri, 14 Jul 2006 14:15:00 -0400, Jerry Avins <jya@ieee.org> wrote:
...
>> I can at best guess what you mean. All I really know is what you write.
>
> So you read through all of
> "I am trying to generate an IIR filter from an H(s) and having
> difficulty stabilizing it. The H(s) in question is documented here:
> ftp://ftp.iol.unh.edu/pub/ethernet/test_suites/CL25_PMD/PMD_Test_Suite_v3.4.pdf
> on page 44 equation 25.C-5. It defines a high pass system as it's two
> transformers with a piece of cable in between"
> and got confused by "when the cable is short transformers block all
> DC" ?
> Why did you have to guess ? Isn't "a high pass system as it's two
> transformers with a piece of cable in between" clear enough ? If you
> had to guess, couldn't you guess that "a high pass system as it's two
> transformers with a piece of cable in between" would also block DC
> when the cable is long ?
I have a dial-up connection. I don't ordinarily read an expression on
page 44 on a who-knows-how-long pdf in the course of following a news
group. Assuming that I would is arrogant.
>> You wrote "when the cable is short transformers block all DC." Of course
>> that makes no sense. It was incumbent on you to correct it, and you did.
>>
> Granted but there is way too much information in context which shows
> that that's not what's meant; unless of course you're purposefully
> ignoring the said context.
>
>> Now that you've made it clear that what you want is a DC blocker,
>
> I don't want a DC blocker per se. I know about R.B-J's code. What I am
> trying to do is to model a given H(s). I happen to know that it is a
> DC blocker but I have to model the given system so that it depends on
> the parameters (L1, L2, Rl etc) so any DC blocker wouldn't do alas,
> albeit a very nice one.
We come back to not knowing what you want, then. A pair of transformers
connected by a short cable -- or a single transformer, for that matter
-- is a DC blocker; a high-pass filter whose rolloff is well known. With
the two transformers you describe, the rolloff is 80 dB/decade. Pretty
simple: why the problem?
> I think Tim's suggestion makes sense at this point.
Tim's suggestions almost always make sense.
Jerry
--
Engineering is the art of making what you want from things you can get.
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