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Experimentonomen quits class d

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LAN_403

Steve Conner

Mon Jun 24 2013, 10:43AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hard to say if it's worse or better than the previous one, since you deleted the previous one and I can't remember what it looked like.

Putting all the terminals together on one side is good, it reduces the common-mode ground impedance.

To get the full benefit of this low ground impedance, add some filters to the power rails and speaker output, placed right next to the terminals, with their grounds close together. I like the little 3-terminal disc capacitors with built-in ferrite beads, for example the DSS6 series from Murata.

Surface mount versions are available with even lower stray inductance.

If you don't like the thought of a ceramic disc cap on the speaker output, you could make a discrete LC filter with a film capacitor.

Experimentonomen

Mon Jun 24 2013, 11:15AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Something like this ?

Experimentonomen

Mon Jun 24 2013, 12:54PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Added more caps everywhere:

Steve Conner

Mon Jun 24 2013, 01:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Try to get the ground terminals of your little SMT capacitors closer to the ground spade terminals (X1 and X5). They are on a "spur" of groundplane that will have a fair amount of inductance. Maybe add some groundplane to the top layer and stitch it down to the caps with vias.

Also consider adding a second inductor to the speaker output line, to help the filter cap do its job. A ferrite bead like L2 and L3 would do. If you're worried about distortion from the ferrite, maybe even use an air cored inductor of a few turns. You may have to relocate the feedback tap before this extra inductor, or it may work fine since the corner frequency of the second filter will be so much higher than the first one, it probably won't add much excess phase to the loop.

Finally I suggest making room for a series RC snubber from the switching node to ground. This will help suppress the VHF ringing you usually see here, which is probably what clobbers FM reception. You don't have to populate it. Use a SMT capacitor and 1210 size SMT resistor to minimise inductance.

Experimentonomen

Mon Jun 24 2013, 01:32PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

I had no problems what so ever on the FM band , all stations i can get here normally showed up unchanged with the amp modules running right next to the tuner.

My problems are on the AM band at second and third harmonic of the switching frequency(around 600-700kHz and 1-1.2MHz) The tuner does not see the fundamental at 300-400kHz, only the first few harmonics which show up as full strenght signal AM stations filled with loud buzz and noise and slight very distorted hint of the music played.

This within a one meter radius, after that the signal meter on the tuner drops to zero and goes quiet within a few centimeters.

Experimentonomen

Mon Jun 24 2013, 02:03PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Redid the caps at the power inputs alittle:

Steve Conner

Mon Jun 24 2013, 02:58PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Looks good

Experimentonomen

Mon Jun 24 2013, 03:19PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Trying alittle more:

But im fairly sure this will not be any better than the previous version that interfered with AM reception.

TBH im not sure im actually willing to sink any more money into further boards and components only to find out this board produces as much AM garbage as the previous version, however i have a hunch that maybe its the fact that i ran the power traces on eather side of the board that could maybe be a reason for the EMI that that could possibly maybe create some form of a antenna that having the power traces run alongside each other on the same side of the board doesent.

I wish i had any kinda clue what a low impedance ground plane would look like.

Steve Conner

Mon Jun 24 2013, 03:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A low impedance ground plane looks like an unbroken sea of copper with as few gaps in it for tracking as possible.

AM radio really is a hard test to pass. Even if your PCB layout was perfect, the inductor could be radiating enough leakage flux for an AM radio to pick up at 1m. Mounting the module inside a metal box would probably help even without changing the PCB.

Running both power traces round the same side would certainly help.

Experimentonomen

Mon Jun 24 2013, 03:45PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

I think im gonna take all of the yet unpopulated boards, pack them in a cardboard box with some crumpled up news paper, fo out somewhere where theres no ppl around and burn the crap.

The boards for the all discrete version arrived today but as they have the power traces running on either side of the board, they will surely be the same transmitters as the ic based version i have the EMI problems with, so im not even gonna bother wasting components to try one out.

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