Any advice for Model F RFI Sensitivity Issues?

[ekeppel]

Hi All,

I've had my Model F XT keyboard in the rotation recently, and this morning I noticed that I've run into RFI issues with it.  I'm a ham radio operator and use the radio many mornings for about 30 minutes, putting out around 1200 watts while transmitting.  Due to the proximity of my antenna system directly above the house, there is of course a good deal of RFI that gets into my office when transmitting.

It's been perfectly manageable with more modern non-capacitive keyboards, but with the Model F, key presses are triggered randomly when I transmit.  I fired up the keyboard testing utility and noticed that the issue seems to affect primarily the top row keys on the keyboard, but I haven't done extensive testing.

This keyboard has a Pro Micro installed internally running Soarer's Converter with a USB cable replacing the old curly cable. 

No matter what I try, I seem to still get RFI triggering keys when transmitting.  Ordinarily, my PC is off while I'm on the radio, so it shouldn't be a problem, except for the fact that I like to have my PC set to wake on keypress, lol.  Unfortunately, this means that the moment I start transmitting, the PC turns on, which I really would like to prevent.

I've done the following so far:

--Various combinations of ferrite chokes on the USB cable
--Added a grounding wire from the XT PCB test pad over to the ground lug on the metal case back
--Wrapped the Soarer's Converter in foil

Nothing seems to be really working, so I thought I would throw this out there and see if anyone else in the community has had a similar issue and found a reliable solution.  One thing I have not done yet is to add a ground wire from the Pro Micro USB shield to the metal case back, so I may try that next unless anyone has some other suggestions.

Thanks in advance for any ideas!

[dfj]

Heyo:

It's not the converter, as your problems on the top row give away what is happening.

 Unfortunately this is particular to the XT era boards as previous beam-spring and subsequent F used separate controllers. The XT's controller is on the same pcb as the main matrix, and while it is digital and not particularly vulnerable to EMI - it can emit it, so you'll see it fully shielded with copper foil for EMS applications, say, for use near an old EKG in a hospital. (One of the hospitals I had a scan at still used the old faraday cage room that was needed in the early days.) Such shielding is no longer needed, but it was around 1980.
  Point here was that just by taking a look at how those are shielded in some of the posts here will give you ideas how to wrap up your beasty, without worrying about details... or so I thought.

IMO the reason this is particular to the top row on the XT is that the top sense line is exposed to a ground plane by running parallel to it for 14" - other F don't treat ground planes as magical or safe.

I don't use a pro-micro, you'll need to look and see if that isn't connected to the logical ground already - if were, I might have suggested you separate them - but it doesn't matter - the XT ties them together. This takes place at the screw on the controller, which is visible when you open up your XT.

  Rather than trying to separate the logical and case gnd all along the chain, I'd just release that one panel from the local gnd - since I expect it the interference is coming along the gnd, via the shielding. The panel will be exposed - but only for 15cm (6"), so we're good.
  So - the screw head will partly disconnect the case (meaning the steel plate on the bottom of the keyboard unit, and the plate-with-holes overtop, not the plastic top-shell and bottom steel base full of crumbs and spiders.)
  You could fully disconnect the case from everything else, by placing something under the pad under the screw hole - between the pcb and the backplate of the case. This might help, if the interference is being picked up by the case mostly - but I expect it is coming along the USB shield and ground lines.

  I am confident that the sketchy row will behave as well or as poorly as the others if you free that problem section of ground plane from the rest of the system. To do this, the screw needs to be removed, though it's up to you whether to leave the bottom pad connected, as it connects to the other side of the pcb... and, the two traces from the problem plane directly above the screw, connecting it to the logical (and thus USB) gnd need to be cut, as well as the two vias connecting the section of plane to gnd on the other side of the pcb. The two vias are to the right, about a half inch apart, just before the 8 sense lines drop down under the top of the case. This is all on the screw-head side of the case, i.e. the top side of it, which is the solder-side of the pcb, not the component-side. Oops - and dont forget to get the little trace running up from the second via beside the 8 lines - it runs up to pin 20 on the IC, needs to be cut as well.

  Great thing about this, is you can do it without opening the irritating part of the case and getting keys and springs everywhere.

I'll add a pic in a bit, I expect - but my pcb at hand has been slightly modded - be warned.

Hope to be at least entertaining,
dfj

[ekeppel]

Thanks for the very detailed and well thought out response. :-)

I'm not sure if I want to cut any traces on this particular keyboard, but if it won't cause any ill effects, then I may go ahead with the modifications anyway. 

Just for kicks, though, I ran a quick test, and it appears that the keys I'm having trouble with are not only top row, but alternating.  For example my keyboard test utility showed keys 2, 4, 8, and Backspace were triggered, but not odd number keys like 1 3 5 7 9, etc.  I noticed the same thing this morning -- no odd keys being triggered. 

When I removed my earlier grounding modifications and also removed the pcb/backplate screw that you mentioned, the keyboard seemed much more resilient to the RFI, but I still managed to trigger keys once I got some good voice peaks going on my transmitter.

I'll take your advice and have a look at the RFI-hardened versions and see what they look like inside.

[dfj]

yeah - I'm taking a pic - it'll be obvious why only ~24680... can trigger.

[dfj]

i.e.


and yeah - I may have added some jumpers to the beast, in a fun experiment on account of it having the nasty tear above, and thus making good test material. ;>

The sketchy trace is the very top one as it is vulnerable to any moderately high frequency ground noise - worse that trace is also weak, as some of the signal is being lost to ground as well.
  The line carries unamplified signal from the capacitive pads, it's biased at around a volt, iirc, with a difference of a few tens of mV needed to distinguish between pressed and unpressed states.

enjoy.

[ekeppel]

Oh wow, you are right about that ground plane.  Could the trace be any closer?!

I'm going to go ahead and do that modification after dinner tonight, lol.

Thanks again.  I'll let you know how it goes once it's done.   

[y11971alex]

Ripster had a guide for installing lead weights, which I think could be useful as a form of radiation shielding.

[ekeppel]

Lead weights?  Hmm, I think this Model F is heavy enough already! :-)

An update:

dfj, I followed your advice and cut the traces shown on the photo you posted.  I also vacuum desoldered the two vias you mentioned so they do not connect to the back of the PCB. 

After reassembling and testing, I found that the keyboard still picked up RFI, so I opened it back up and added back the screw that connects the two sides of the PCB.  I'm at a loss as to why it's working now, but it seems to be rejecting RFI really well.  Preliminary tests show zero false triggering of key presses when I made a few brief radio transmissions.  I'll give it a proper try in the morning when the frequencies are clear.  Right now there is a lot of radio activity!

Thank you!   

[dfj]

  I was super tired the other day... glad it helped. I suspect the 'alternate' cut wanted the screw back, but I wasn't sure about it otherwise - if you have it in, and it's helping, then sweet stuff.

btw - this implies you have some horror emi on yer grounds, but - I suppose this is no shock. ;P

  The different ways the XT was modded aftermarket serve different functions - some to shield the XT from interference, and some to reduce the amount it emitted, for different reasons. The ones with a mess of caps and resistors added are implementing filters to round the edges of the logic signals, reducing the emissions of the F, presumably for medical, laboratory or security emission reasons? The aggressive foil wrapping could be either.

  It might have been possible to do a near fully shielded case, as on the 4704 beasties, but the shell would then still need to be separated from the gnd - so not trace cuts, but a fair bit of work, still.

Note : that panel actually carried some of the gnd return current back from the processor, so next time you are in there you might want to jump from the pin 20 that was cut, directly to the gnd input (thick trace from middle pin). This will re-introduce the loop described next, though...

 The XT has (though not yours anymore) an aggressive ground loop on the controller exposed. I'm not up for frequency analysis, against emi - it could even be a ground loop in the building, or the power draw of the transmitter varying - none of which matters anymore for the keyboard or me.
  Ground loops are exciting and you likely care if you have one, but I'm going to assume you have looked into it already and done what you can. You likely want to google up a bit if not, as it's in mains safety-land: well out of my sphere. ;]

I'm just a keyboard (F in particular) keyboard geek, not an ee or similar; I'm so very glad it worked out!

yup,
dfj

[ekeppel]

Since it's been a couple of days, I wanted to give an update on things...

It looks like the situation is very much improved, but not completely eliminated.  Apparently, I have a few 'hot spots' here on my desk, lol.  If I push the keyboard back about a foot on the desk, it will still trigger while transmitting.  So long as it's sitting in the usual operating position, it's fine.  I'm really not surprised, because the antenna feed point is only about 30ft (10m) away, and almost directly above my office.  During transmissions, this area receives a pretty good dose of RF, though during my last site evaluation I found it to be within the FCC's recommended safety limits. :-)

Anyway, thank you for all of your help dfj.  I'm hoping to switch over to an F122 at some point, so I'm sure that will bring it's own issues.  Looking at the F122 pcb, I don't really see as many potential trouble areas though, so hopefully that keyboard will be even better in my RF-rich environment.

Thanks again, and enjoy the long weekend everyone!

[dfj]

interesting - if it responds to location on your desk, it suggest the problem is very near to the keyboard - so the AT cable itself, the USB cable, the exposed portion of a controller or the case... though without the screw... you did block the pad on the reverse side of the screwhole, right?

Enjoy. :}
dfj

[engicoder]

How about something like this


or this

[dfj]

Ha Ha - as you can plainly see - the digital signals have had small RCI  filters installed to ameliorate the sharp edges - so it's to reduce EMI *from* the keyboard, not shield it for use in noisy environments. It also has the XT error - in that a couple of large sections of the ground-plane are exposed to outside emissions as above in this thread. They are gnd, so they don't tend to transmit, though. After the XT, the other F didn't have that aspect - the entire matrix was covered - though how well the cable would appreciate TOP's environment, I have no clue.

Likely more useful near old EKG machines, and other sensitive electronics, rather than against spooks.

my $0.05
dfj

[ekeppel]

TL;DR:  I fixed the source of the RF.  It was coming in on the shield of the coax, and was mitigated by adding two RF chokes to the antenna feedline.



Hi All,

Just an update on this, since it's been a while.  I have been using an F122 recently, and ran into this same RF triggering issue with it.  Since I didn't want to modify every capacitive BS keyboard that I own, I thought it better to take care of the source of the problem, rather than put band-aids on the affected devices. 

After tearing things apart here at my desk, I realized what was happening.  I completely forgot that I don't have any RF choke on my antenna system at all, so I added a couple of them and the problem went away entirely.  As it turns out, there was a lot of RF coming back down to my office on the shield of the coax.  Adding a choke consisting of seven mix-43 ferrites took care of that.  The minimal amount of RF that remained was taken care of by adding another (smaller) choke on the coax between my amplifier and antenna tuner.

So, I guess it was just poor antenna system design on my part, really.  With the amount of RF I was getting near this keyboard, no amount of shielding would have taken care of it completely.   

I posted this most recent RF-hunt over on DT, but wanted to share my final results on this thread as well, in case anyone else ever runs into similar problems.

Thanks to everyone for the ideas and assistance!

[Tactile]

TL;DR:  I fixed the source of the RF.  It was coming in on the shield of the coax, and was mitigated by adding two RF chokes to the antenna feedline.



Hi All,

Just an update on this, since it's been a while.  I have been using an F122 recently, and ran into this same RF triggering issue with it.  Since I didn't want to modify every capacitive BS keyboard that I own, I thought it better to take care of the source of the problem, rather than put band-aids on the affected devices. 

After tearing things apart here at my desk, I realized what was happening.  I completely forgot that I don't have any RF choke on my antenna system at all, so I added a couple of them and the problem went away entirely.  As it turns out, there was a lot of RF coming back down to my office on the shield of the coax.  Adding a choke consisting of seven mix-43 ferrites took care of that.  The minimal amount of RF that remained was taken care of by adding another (smaller) choke on the coax between my amplifier and antenna tuner.

So, I guess it was just poor antenna system design on my part, really.  With the amount of RF I was getting near this keyboard, no amount of shielding would have taken care of it completely.   

I posted this most recent RF-hunt over on DT, but wanted to share my final results on this thread as well, in case anyone else ever runs into similar problems.

Thanks to everyone for the ideas and assistance!

I once had a similar problem. In my case the RF caused random triggering of my electronic keyer, which is problematic if you're working CW at the time.