Differential filter design...

[bitrex]

There\'s a good guide here:

<https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html>

Two questions I have about applying filter-design software to this process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common core?

Single-ended filter design software you usually have to specify a source
and termination impedance. How would you work this if what you\'re after
isn\'t maximum power transfer between specified terminations but maximum
power available into arbitrary load, like a class D audio amplifier, say.

 

[Rich S]

On Wednesday, August 26, 2020 at 6:36:26 PM UTC, bitrex wrote:

There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html

Two questions I have about applying filter-design software to this process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common core?

Single-ended filter design software you usually have to specify a source
and termination impedance. How would you work this if what you\'re after
isn\'t maximum power transfer between specified terminations but maximum
power available into arbitrary load, like a class D audio amplifier, say.

Re Class D amps, I only have general response. My advice may be obvious, or
out of date now. But I recall my former colleagues who designed amps would:
(1) design around a known *range* of speaker impedance and parameters.
(2) simulate on computer the circuit using well-known speaker models.
(3) bench-test circuit using a \"loudspeaker simulator\" (passive RLC network)
so you don\'t have to blow up a lot of expensive drivers. Have multiple simulators, e.g. \"2-ohm\", \"4-ohm\", etc., to suit the application.
(4) finally bench test on actual loudspeakers, long term, with a suite of
\"stress test\" signals.
Cheers, Rich S.

 

[Jeroen Belleman]

On 2020-08-26 20:36, bitrex wrote:

There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html

Two questions I have about applying filter-design software to this
process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common
core?

Yes, of course. In case of full coupling, you must halve the inductances
/again/ to maintain the same response. This is because the effective
impedance in one branch is now the sum of the single-side inductance and
the mutual inductance, which has to remain the same.

Single-ended filter design software you usually have to specify a
source and termination impedance. How would you work this if what
you\'re after isn\'t maximum power transfer between specified
terminations but maximum power available into arbitrary load, like a
class D audio amplifier, say.

Filter responses are calculated for a given source and load impedance.
If you drive a filter from the wrong impedance, or terminate it into
a value for which it wasn\'t designed, the shape of the response will
change. This has nothing to do with maximum power transfer.

I suppose a class D amplifier is essentially a zero-impedance source,
so the filter should be designed for that. Such filters always start
with a series component, obviously.

Jeroen Belleman

 

[bitrex]

On 8/26/2020 3:26 PM, Jeroen Belleman wrote:

On 2020-08-26 20:36, bitrex wrote:
There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html


 Two questions I have about applying filter-design software to this
process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common
core?


Yes, of course. In case of full coupling, you must halve the inductances
/again/ to maintain the same response. This is because the effective
impedance in one branch is now the sum of the single-side inductance and
the mutual inductance, which has to remain the same.

Got it, thought it might be something like that.

Single-ended filter design software you usually have to specify a
source and termination impedance. How would you work this if what
you\'re after isn\'t maximum power transfer between specified
terminations but maximum power available into arbitrary load, like a
class D audio amplifier, say.

Filter responses are calculated for a given source and load impedance.
If you drive a filter from the wrong impedance, or terminate it into
a value for which it wasn\'t designed, the shape of the response will
change. This has nothing to do with maximum power transfer.

Right, completely arbitrary impedance would be silly, speaker impedance
aren\'t completely arbitrary.

I suppose a class D amplifier is essentially a zero-impedance source,
so the filter should be designed for that. Such filters always start
with a series component, obviously.

Jeroen Belleman

I think that may have been my sticking point, have to well-define
something. a parallel initial element doesn\'t make sense as while the
output stage has some low output impedance it\'s not well-defined.

 

[John Larkin]

On Wed, 26 Aug 2020 21:26:49 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

On 2020-08-26 20:36, bitrex wrote:
There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html

Two questions I have about applying filter-design software to this
process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common
core?


Yes, of course. In case of full coupling, you must halve the inductances
/again/ to maintain the same response. This is because the effective
impedance in one branch is now the sum of the single-side inductance and
the mutual inductance, which has to remain the same.

Single-ended filter design software you usually have to specify a
source and termination impedance. How would you work this if what
you\'re after isn\'t maximum power transfer between specified
terminations but maximum power available into arbitrary load, like a
class D audio amplifier, say.

Filter responses are calculated for a given source and load impedance.
If you drive a filter from the wrong impedance, or terminate it into
a value for which it wasn\'t designed, the shape of the response will
change. This has nothing to do with maximum power transfer.

I suppose a class D amplifier is essentially a zero-impedance source,
so the filter should be designed for that. Such filters always start
with a series component, obviously.

Jeroen Belleman

There\'s no reason a class-D audio amp should have a differential
output filter, and lots of reasons why it shouldn\'t.

 

[bitrex]

On 8/26/2020 3:52 PM, John Larkin wrote:

On Wed, 26 Aug 2020 21:26:49 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

On 2020-08-26 20:36, bitrex wrote:
There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html

Two questions I have about applying filter-design software to this
process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common
core?


Yes, of course. In case of full coupling, you must halve the inductances
/again/ to maintain the same response. This is because the effective
impedance in one branch is now the sum of the single-side inductance and
the mutual inductance, which has to remain the same.

Single-ended filter design software you usually have to specify a
source and termination impedance. How would you work this if what
you\'re after isn\'t maximum power transfer between specified
terminations but maximum power available into arbitrary load, like a
class D audio amplifier, say.

Filter responses are calculated for a given source and load impedance.
If you drive a filter from the wrong impedance, or terminate it into
a value for which it wasn\'t designed, the shape of the response will
change. This has nothing to do with maximum power transfer.

I suppose a class D amplifier is essentially a zero-impedance source,
so the filter should be designed for that. Such filters always start
with a series component, obviously.

Jeroen Belleman

There\'s no reason a class-D audio amp should have a differential
output filter, and lots of reasons why it shouldn\'t.

Just a motivational example, man. Motivational example. this is an
educational exercise

 

[bitrex]

On 8/26/2020 3:06 PM, Rich S wrote:

On Wednesday, August 26, 2020 at 6:36:26 PM UTC, bitrex wrote:
There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html

Two questions I have about applying filter-design software to this process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common core?

Single-ended filter design software you usually have to specify a source
and termination impedance. How would you work this if what you\'re after
isn\'t maximum power transfer between specified terminations but maximum
power available into arbitrary load, like a class D audio amplifier, say.

Re Class D amps, I only have general response. My advice may be obvious, or
out of date now. But I recall my former colleagues who designed amps would:
(1) design around a known *range* of speaker impedance and parameters.
(2) simulate on computer the circuit using well-known speaker models.
(3) bench-test circuit using a \"loudspeaker simulator\" (passive RLC network)
so you don\'t have to blow up a lot of expensive drivers. Have multiple simulators, e.g. \"2-ohm\", \"4-ohm\", etc., to suit the application.
(4) finally bench test on actual loudspeakers, long term, with a suite of
\"stress test\" signals.
Cheers, Rich S.

Eh, it was just a motivational example. I don\'t have much interest in
building audio amps myself but just had questions about the differential
topology and how it works when it\'s done that way.

There is no way to \"match\" into an arbitrary load impedance makes sense,
the termination is in a range.

 

[bitrex]

On 8/26/2020 3:06 PM, Rich S wrote:

On Wednesday, August 26, 2020 at 6:36:26 PM UTC, bitrex wrote:
There\'s a good guide here:

https://www.analog.com/en/technical-articles/understanding-and-designing-differential-filters-for-communications-systems.html

Two questions I have about applying filter-design software to this process:

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common core?

Single-ended filter design software you usually have to specify a source
and termination impedance. How would you work this if what you\'re after
isn\'t maximum power transfer between specified terminations but maximum
power available into arbitrary load, like a class D audio amplifier, say.

Re Class D amps, I only have general response. My advice may be obvious, or
out of date now. But I recall my former colleagues who designed amps would:
(1) design around a known *range* of speaker impedance and parameters.
(2) simulate on computer the circuit using well-known speaker models.
(3) bench-test circuit using a \"loudspeaker simulator\" (passive RLC network)
so you don\'t have to blow up a lot of expensive drivers. Have multiple simulators, e.g. \"2-ohm\", \"4-ohm\", etc., to suit the application.
(4) finally bench test on actual loudspeakers, long term, with a suite of
\"stress test\" signals.
Cheers, Rich S.

I read the best way to break into the high-end audio biz is you gotta
take out your competition. Like you got to literally shoot a
motherfucka. Too high-stakes for me.

 

[Tim Williams]

\"bitrex\" <user@example.net> wrote in message
news:BYx1H.48296$Wl7.43034@fx26.iad...

The series elements are scaled by 1/2 vs the single ended topology.
Does this change at all if the inductors are coupled on a common core?

Single-ended filter design software you usually have to specify a source
and termination impedance. How would you work this if what you\'re after
isn\'t maximum power transfer between specified terminations but maximum
power available into arbitrary load, like a class D audio amplifier, say.

Coupling is covered by others.

So, in the article, they completely ignore common mode, which is peculiar.
Their motivating examples seem to have low CMV at the source, maybe they\'re
ignoring it for that reason.

So, first of all -- there are many reasons for choosing differential mode
signaling. They do not give much motivation here. They do mention
susceptibility and emissions, and cancellation of even distortion.

They also mention \"double the signal level\", and better linearity and SNR,
which are rather weak to me: you\'re also doubling the output stages, so you
get as much noise from them regardless. I suppose you aren\'t doubling the
input stages in comparison to regular op-amps, which use differential input
stages pervasively, whether or not you\'re using them single-ended. But
that\'s very easily solved by not using op-amps at all (e.g., use an MMIC),
or making a single-ended one (feedback around a common-emitter/source
transistor, say).

Some stronger reasons include:

Cost of the media. Losses. 100 ohm twisted pair, and 300-600 ohm twin
lead, typically are cheaper, and have lower losses than coax.

Availability. You can\'t construct coax on a PCB very easily, but diff is
free. Coax requires special connectors; diff can be transmitted on any old
headers.

Noise rejection. For CM noise within the CM range of the receiver,
rejection is very good. Beyond there, signal quality degrades aggressively
(typically, CM current forward-biases the input ESD clamp diodes, shorting
out the diff signal in part or whole).

Note that coax can be connected differentially, too, to the same benefit!
But this is rarely done. Probably because designers are creatures of habit?

Isolation. When the signal can be transformer coupled, single-ended signals
must use shielded transformers: expensive, and poorer performing.
Differential signals can use simple twisted-pair windings. (Both can be
improved with CMCs, of course.)

Alternative functions. We can put whatever we want on the common mode,
assuming that we have a return path for it, and that we don\'t mind it may be
noisy. PoE is an excellent example: we don\'t care that it\'s noisy because
we only want DC, and we have four pairs in a CAT5+ cable to choose from.

This could be done with pairs of coax as well, but it\'s probably a bit rude
to put \"hot\" voltages on shields that are ordinarily assumed to be grounded?



As for the filter circuit:

The full circuit includes stray capacitances to ground (which affect the
differential values in the filter, too!). The parallel equivalent of those
capacitances, acting against the leakage inductance of the coupled (diff
mode) inductors, give a CM filter of some cutoff and impedance.

Note that, if the inductors are inverse coupled, the leakage becomes the
critical diff-mode spec, and CM inductance instead can be very high. So you
can, very effectively, craft very different responses and impedances for DM
and CM.

If you have much CM noise to filter, and it\'s present at frequencies
feasible to filter, well there you go, that\'s part of your filter spec! The
CM filter prototype will probably be different, reflecting the different
spectrum, and because you can\'t make accurate assumptions on the source port
impedance. Namely, a lossy network is preferred, to act to stabilize the
source impedance.

For loose cables, you can broadly assume a CM impedance of 100-200 ohms, but
with peaks and valleys due to random resonances (because it\'s a transmission
line in somewhat-free space). Ferrite beads help flatten out those
resonances, and further stabilization (R+C to ground and R||L in series)
furnishes something you can confidently filter against.


Which by the way, relates to the class D amp example: the two sources are
both nearly zero ohms. Note that we don\'t need to model this
differentially, because this is the same whether expressed as normal mode,
CM or DM.

We can possibly save a little cost by using a weak DM filter: the speaker
won\'t care if it\'s eating a few volts of 200kHz or whatever. But we still
need a strong CM filter to meet emissions (assuming this is an amplifier
block).

Or conversely, if this is an integrated speaker, the wiring can be much
shorter, and the speaker frame can be grounded, allowing much tighter
control over emissions. The CM filter might be much simpler, or perhaps
even nothing at all. Yes, they actually get away with that sometimes!

Anyway, the fact that the source impedance is effectively zero, means we
must design with a one-port-shorted prototype. And really, the other port
isn\'t much better (the speaker impedance is... well, anywhere on the right
half-plane, there\'s that :^) ), so we probably want a lossy network to
stabilize the impedance.

So, the basic form of this filter might be:

.. +--/\\/\\--+ R1
.. L1 | |
.. Amp o---ccc--+------+-+--ccc---+---o Speaker
.. | | L2
.. | === C2
.. === C1 |
.. | > R2
.. | >
.. GND _|_ _|_

The L1-C1 section can be as many LC(LC..) as needed to get desired
attenuation, and can end with a series branch (L) too. The R+C and R||L
sections provide termination even when the load is open or shorted. (Open
is the higher priority, as speakers are generally inductive at high
frequencies. However, beware of the piezo tweeter!)

(Note that, when there\'s an R+C in parallel with a C, the R+C\'s capacitance
needs to be several times larger, so that the R is dominant in the
transition band.)

We can apply this with as many CM and DM stages as needed to meet the
above-discussed attenuation requirements.

Oh and, note that the CM filter in this case, is shorted one side and /open/
on the other (the speaker isn\'t grounded or anything) -- what fresh hell is
this? Obviously, filtering is _meaningless_ without CM termination, and
an R+C is obligatory, if you\'re going to do a CM-DM type filter here.

But also, yeah, more reasonable to take the simpler case -- an H-bridge is
nothing more and nothing less than two half-bridge stages, and you would
filter those with normal-mode filters. Why make it more complicated by
imagining it any other way?

Which does assume the usual case -- the H bridge is just driven balanced, in
the usual \"bridge tied\" way.

When this isn\'t the case, it can be different. Consider phase shift PWM:
this is used in SMPS, and delivers a CM voltage of full rail-to-rail
switching, almost all the time! This is quite an EMI burden, and some heavy
filtering will be desirable. The filtering can be done on the AC or DC
side, preferably not much on AC (direct inverter output) due to the added
leakage inductance reducing power output. But given the magnitude, it might
be required on the AC side, just for basic functional operation (i.e., 400V
of switching noise is likely to confuse the controller!).

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/