RFI protection concepts

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
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JR.
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Re: RFI protection concepts

Post by JR. » Sun Jun 01, 2014 4:43 pm

One subtle point. Since it appears you are routing the audio single-ended between the mic pre and output driver, those two 0V local grounds should be intimately connected to each other. In other words they should share one robust local 0V reference trace, not two local 0V references that link back to a star ground.

The integrity of the 0V reference between those two ICs will directly impact audio quality transferred between them.

I do not care if you grab input or output side chassis ground, perhaps you need a third drawing with chassis connected to your star ground node.

JR

ricardo
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Re: RFI protection concepts

Post by ricardo » Sun Jun 01, 2014 5:14 pm

Gertjan, I like your last set of schematics which are much like what I would do.

My only caveat is like JR's. The signal 'GNDs' should share a nice solid net which I call 'CLEAN'. What you must ensure is that no sewage is dumped onto this CLEAN reference.

The main culprit is supply decoupling caps. I take these to a separate 'DIRTY GND'. 'CLEAN' & 'DIRTY' are joined only at the STAR point.

Sometimes, there are special considerations. eg THAT 1646. It's pin 3 (which is also its CLEAN reference) needs to be decoupled to Vee by at least 1nF directly AT the device. ie this is most conveniently the Star point or very close to it. Another reason for having the Star point at the Outputs :D


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Gertjan
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Re: RFI protection concepts

Post by Gertjan » Mon Jun 02, 2014 10:55 am

You’re both right of course.
The separation between clean audio ground, keeping supply currents out is always on my mind when doing a PCB layout.
(Often it is the hardest part to get right).
But I never draw it in my Schematics.

For a Concept, this is a better representation:
Image

I also left the THAT part numbers out. So it is more about generic concepts.
(Although I do appreciate the details about the implementation of the THAT chips)

Regards, Gertjan
Last edited by Gertjan on Mon Jun 02, 2014 1:30 pm, edited 2 times in total.

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Gertjan
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Re: RFI protection concepts

Post by Gertjan » Mon Jun 02, 2014 11:58 am

Back to the topic of RFI protection.

I took a hard look at the THAT recommended Y configuration for the RFI C's:

From THAT 1240 datasheet, and from my concept:
Image Image

I appreciate that the THAT Y configuration is intended to minimize the unbalancing impact of differences in
the values of C4 and C5. (capacitor tolerances)
Maybe an important factor when designing for production.

But these filters are intended to protect against signals with frequencies of 100MHz....2.5GHz
How important is keeping perfect balance beyond the bandwidth of the chips?
(All THAT I/O chips have around 10MHz small signal bandwidth)

I don’t like that the capacity across the signal pair is double, meaning 50% lower cut off frequency for audio.
And the capacity from one signal wire to Chassis is roughly a quarter, meaning for RFI 400% higher cut-off frequency.

Am I missing something?

Regards, Gertjan.

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mediatechnology
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Re: RFI protection concepts

Post by mediatechnology » Mon Jun 02, 2014 2:51 pm

I appreciate that the THAT Y configuration is intended to minimize the unbalancing impact of differences in
the values of C4 and C5. (capacitor tolerances)
Maybe an important factor when designing for production.

But these filters are intended to protect against signals with frequencies of 100MHz....2.5GHz
How important is keeping perfect balance beyond the bandwidth of the chips?
(All THAT I/O chips have around 10MHz small signal bandwidth)

I don’t like that the capacity across the signal pair is double, meaning 50% lower cut off frequency for audio.
And the capacity from one signal wire to Chassis is roughly a quarter, meaning for RFI 400% higher cut-off frequency.
Reduced capacitive unbalancing of the signal within the audio band was the original motivation for the "wye" connection. I recall being told by THAT (after I followed their published suggestion in a design for them) that the series inductance of the 47 pF was thought to be less effective at shunting common mode RFI than simply two capacitors to pin 1 on each leg. The jury may still be out on that one...

FWIW the effective capacitance for common mode signals of the 47 pF in the wye configuration is actually approx 90 pF because by definition both inputs are driven for common mode inputs. Thus, the current in the 47 pF is twice what it would be if one leg were driven making it look like 90 pF. (The actual value is 90 pF rather than 94 pF for common mode signals because it has a 470pFx2 in series.)

For equal and opposite differential signals the 47 pF is effectively out of circuit because the center point of the "wye" is driven to 0V.

As to increased differential capacitive loading of the source based on an added 125 pF of input capacitance (e.g. 110 pF vs. 235 pF) we're talking, from perspective capacitive loading of the source, a difference of about 4 feet of added interconnect cable. 30 feet of cable is about 1 nF differential load. The effect of an additional 125 pF of loading is de minimus.

I honestly don't know which is best: Delta or Wye.
For mic preamps I've had pretty good with a Delta connection but I've used both.

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Gertjan
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Re: RFI protection concepts

Post by Gertjan » Thu Jun 05, 2014 12:58 pm

Wayne, thanks for your insights.

It is a privilege to be able to discuss matters with much more experienced people.
Their level of expertise goes far beyond textbooks & application notes, gained by decades of hands-on design.

Most appreciated!

Regards, Gertjan.

miraphonic
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Re: RFI protection concepts

Post by miraphonic » Tue Jul 15, 2014 2:27 pm

Great thread, and as has previously been mentioned it is very useful to hear from people with so much experience :)

Has anyone tried the Harting Filter connectors:

https://b2b.harting.com/ebusiness/app/d ... gory=)/.do

Particularly this model I was looking at:

https://b2b.harting.com/ebusiness/app/d ... )/.do?rf=y

Some models have caps, whilst some have ferrites. Strangely it seems like all the 'standard footprint' models have caps, whilst the 'US footprint' models have ferrites.

They are pretty expensive (~£20 each), do you think they are worth it?

Thanks

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JR.
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Re: RFI protection concepts

Post by JR. » Tue Jul 15, 2014 2:57 pm

Looks pricey to me... Looks like something a customer would pay that much for to pass emissions testing without a PCB redesign.

JR

ricardo
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Re: RFI protection concepts

Post by ricardo » Tue Jul 15, 2014 9:30 pm

JR. wrote:Looks pricey to me... Looks like something a customer would pay that much for to pass emissions testing without a PCB redesign.
Yes. But I might use these for a one-off just to avoid having to deal with SMT caps & layout. :o I tend to over agonise over PCB layout for LN & EMI :D

Another quick trick is those Clip-on ferrites you find on Computer Cables. You can put them just where they are most effective.. at the ends of the cables. They would work well with the 'C' version of these sockets.

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