Author Topic: No OOK received unless earth grounded?  (Read 1550 times)

CapnBry

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No OOK received unless earth grounded?
« on: July 24, 2015, 08:11:45 AM »
I've use a MoteinoUSB that I use to monitor my home power usage which is setup to do OOK reception (Github: Powermon433). It works just fine plugged into my main linux computer but it is pretty far from the transmitter so I get dropouts. I decided to plug it into a Pi (still via USB) and have that closer but for some reason I just could not get it to pick up any signal at all.

I took the whole thing back into the lab and hooked it to the oscilloscope to check the pulses received. It worked perfectly. I moved it 6 feet to the left into an available outlet, it stopped working again. Hooked up the oscilloscope, worked perfectly. Then I moved it over to the spare monitor and keyboard I have for working with Pis in my lab. It worked great there, but if I unplugged the HDMI it would stop receiving again. That got me thinking, the chassis ground of the HDMI connection to the monitor is earth grounded. The oscilloscope probe ground is earth grounded...

Went back to my original Pi location and plugged it in, no reception at all as anticipated. Then I jammed a wire into the 3rd prong grounding hole in the electrical outlet and clipped it to the Moteino ground DO NOT STICK WIRES INTO ELECTRICAL OUTLETS UNLESS YOU ARE DUMB and it worked perfectly. Unclipped the extra ground, it stopped receiving. It is impossible to see what's going on because the second I hook the oscilloscope to it, it works fine thanks to the grounding.

So what the heck is happening here? How can the RFM69 receiver be completely incapable of receiving OOK unless it is earth grounded? The Pi is a Pi 2, and I've tried all sorts of different shielding setups wrapping both the Pi and the Moteino in aluminum foil then grounding the shield. Could it be the power adapters I have tried the regular 0V grounds on them aren't suitable for powering this? I'm really at a loss as to why it doesn't work at all when just plugged into a Pi and a 5V supply (nothing else connected).

Felix

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Re: No OOK received unless earth grounded?
« Reply #1 on: July 24, 2015, 11:26:28 AM »
I remember I've seen at least 1 or 2 previous threads reporting similar issues with older Pis. I can't recall the resolution but similar things were attempted, shielding wiht AL foil and such.
It may have to do with the antenna polarization with respect to grounding. A switching supply should not really affect it although it might have a different GND level with respect to earth. I have a Pi rev1 model B, and a Pi v2 and I can't find any similar issues, but it's hard to compare apples to apples, most variables would need to be the same to have some clear test case.

CapnBry

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Re: No OOK received unless earth grounded?
« Reply #2 on: July 26, 2015, 09:26:49 AM »
Yeah it seems highly unusual to me as well. I mean battery powered nodes aren't referenced to earth at all! I tried it with an old Pi A, Pi B, a newer Pi B (not a plus though), and compared it to the Pi 2 B original. All seemed to have the same issue. It had been working for 3 days with just a handful of missed packets while grounded through the HDMI jack to a monitor.

I then decided to take the Pi out of the equation all together and plugged the Mote directly into the USB power supply. The onboard LED lights when it is locked on a signal so I can tell if it is working even if I can't see the data. I tried some other chargers and found I have a few that work. I checked them with the multimeter just to verify that those that did work didn't have some sort of connectivity to the mains neutral leg and they did not (you never can tell with these cheap things right?). Running the Pi 2 on one of these but with no ground, I still am able to receive data however I don't think it works as well because I was seeing about 30-40% missing packets.

I've got one of those 5V USB battery packs at the office and I'm going to check it out on full battery power later this week. Not that I could run this 24/7 on that, but I am just curious. Definitely strange behavior to me, but RF is all f'ing magic to me anyway.

emjay

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Re: No OOK received unless earth grounded?
« Reply #3 on: July 26, 2015, 08:40:11 PM »
@CapnBry,

The multimeter test on the USB bricks is misleading - you are looking for an RF path to ground, not DC.
A brick designed for the UK (not just a 'universal' with the right prongs on) may have some low value cap between the DC side and the ground reference pin.  The cap value may not be low enough and/or there may be a common mode choke in series making this path ineffective at 0.5 Ghz.

CapnBry

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Re: No OOK received unless earth grounded?
« Reply #4 on: July 27, 2015, 03:04:18 PM »
Oh that's definitely interesting, emjay. That's the sort of RF magic that I don't understand and I came looking for. It has occurred to me that even my linux box I usually do all my RF stuff from isn't fully connected to earth ground. It has a PicoPSU DC power supply but it is also connected over an HDMI cable to an AV receiver so I am guessing the earth ground is going that way. I am wondering if I put a regular PC power supply with it's own ground connector in there, if it will get better reception. The problem there is the noise floor is super high so it makes me wonder if better grounding of the PC would make a difference. Definitely something to try, thanks for keeping my brain churning!