Help me with RF interference - Page 2 - Videokarma.org TV - Video

Dangler · #16 03-09-2015, 01:29 PM

Quote:

Originally Posted by Electronic M

I did before asking, but I got a bunch of non-electronic related crap that was clearly not the same acronym being used here....

You didn't look very hard, Wikipedia, the third listing down explains it.

TinCanAlley · #17 03-09-2015, 03:48 PM

I wrote too soon. The problem isn't solved.

It seems the interference is coming for a switching DC power supply I'm using to power all the components in the rack. I got it so I could get rid of several wallwarts. The unit provides power to an FM transmitter, Amped Wireless access point and Roku unit. It is supposed to be filtered, etc. When I unplug it, the static is completely gone.

Is there any way to clean it up?

This is the unit I got from Amazon:

http://www.amazon.com/Universal-Regu...g+power+supply

Electronic M · #18 03-09-2015, 04:53 PM

It is almost certainly a switch mode power supply. You can try plugging that supply in through a power line noise suppressor (keep the cord between the power supply and suppressor as short as you can so it don't act as an antenna), add extra filter capacitance to the output, and extra metal shielding around it, but the benefits of that are limited. Switch mode supplies are basically oscillators controlling power circuitry, and are MASSIVE noise generators. If you can't suppress the noise from it, then look into going back to the wall warts, or get a 120V-12V line step down transformer, a diode bridge rectifier, two good size 25V caps, a 12V regulator chip (if you are using it as a 12VDC supply), and build your own noise-less linear power supply (it is fairly easy).

TinCanAlley · #19 03-09-2015, 05:14 PM

How about moving the switching supply about 10 feet away? I could then run a lead from the DC output to the connections for the devices. Isn't the noise radiating from the supply? If so, moving it should help, right?

These wallwarts are still a bit of a mystery to me in how they provide their power. The FM Transmitter had a 12V .5A wall wart, but setting the switching supply to 12V created a problem with the transmitter locking on the frequency. I had to bump up the output to 13.8V to make it work properly.

I'd like to keep away from the wallwarts as they take up way too much space and create a wiring nightmare. I'm back to them, temporarily and now have 4 of them and three standard AC plugs. It looks just like the electrical octopus the fireman told me never to create back in elementary school.

Electronic M · #20 03-09-2015, 05:28 PM

Switching supplies often radiate their noise thru the AC mains wiring and sometimes the DC output too. You may or may not get a noticeable noise drop if you move it 10' or half a block...

The electric octopus is not a danger as long as you don't overload anything (most are too dumb to check so they advise not to do it, assuming everybody is dumb). Electricity can handle as complex and messy a path as you can give it (the CPU chip in your computer is billions of times as complex as your little rats nest), and remain well behaved as long as you observe ratings and rules.

TinCanAlley · #21 03-09-2015, 06:20 PM

I found that combining all 4 in the little "nest" causes a buzzing as one of them is vibrating just enough to resonate when bundled.

I'm going to try and move the supply and lengthen the DC output line. Do you think a cap across the DC leads might help if there is noise there? Also, I don't think it's going back along the AC as it's being radiated to the antenna input of the TV.

Thanks

Electronic M · #22 03-09-2015, 07:16 PM

The cap on the DC may help. I'd try a lytic in the 1000uF range, and a parallel ceramic or film cap in the 0.01uF range, and maybe a series resistor in the 5-10ohm range between the caps and a terminal of the supply.

It don't have to travel the whole way through the AC lines. 6-10' of travel = a decent tuned VHF TV antenna (for it to transmit to the air), and 3-6" a decent antenna for UHF....

If you can grab a newer SS parts TV, video monitor, or computer powersupply most of them have a line noise suppressor built in. The new video gear used switch mode supplies too, but they build them properly so they don't make too much noise. Find the big 200V cap nearest the power cord on the parts board, it is fed by a diode bridge, everything between the bridge and line cord is the filter (usually consists of some ceramic caps and what looks like a small weirdly connected line transformer which is actually a dual line choke). Harvest that filter (or buy a more expensive one from the store if you don't have faith in your skills) and add it inline with your supply as close to the supply as you can.

zeno · #23 03-09-2015, 07:28 PM

Quote:

Originally Posted by Electronic M

Zeno, what is QRM?

QRM is man made noise like computers, arc welders etc.
QRN is natural noise like lightning crashes etc.

73 es cul de Tom KA1YHI sk

zeno · #24 03-09-2015, 07:48 PM

It also may be one of the things you got hooked up
is radiating off all the extra wires involved. Try unhooking
one thing at a time ( power) if you havnt. As far as well
filtered goes take that with a grain of salt. Switching supplies
are wickid noisy. They are no different than the hoz
sweep in a TV. Both noise generators.

73 Zeno

TinCanAlley · #25 03-09-2015, 08:21 PM

Quote:

Originally Posted by zeno

It also may be one of the things you got hooked up
is radiating off all the extra wires involved. Try unhooking
one thing at a time ( power) if you havnt. As far as well
filtered goes take that with a grain of salt. Switching supplies
are wickid noisy. They are no different than the hoz
sweep in a TV. Both noise generators.

73 Zeno

I disconnected all devices from it and the interference was still there. The only thing that got rid of it was to unplug the PS.

I have a small access hole to the closet with all the gear. I'm going to run a lead though it to the harness supplying the devices. The PS will be outside (other side of wall) in a different AC outlet. Hopefully this will take care of it. It's amazing how much space is lost with 4 wallwarts, extension cords and associated wires.

Adam · #26 03-09-2015, 08:38 PM

I've had the same problem. What fixed it for me was swapping the adaptor going to the antenna terminals, some of them seem more susceptible to receiving the interference than others. I tested several, and the vhf/uhf type seemed to pick up on the interference the most, and the shorter the length of twin lead on the tv end of the adapter the better.

john f · #27 03-10-2015, 08:14 AM

I had similar interference which turned out to be a bad 75-300 ohm matching transformer. Switching to one that was about 20 years old cured it. Just reinforces how new stuff is made out of crap.

Dude111 · #28 03-11-2015, 12:21 AM

Quote:

Originally Posted by zeno

QRM is man made noise like computers, arc welders etc.
QRN is natural noise like lightning crashes etc.

Gracious..... I thought it was the other way round!!

powerking · #29 03-11-2015, 08:16 AM

I would vote for building/sourcing a linear 12v power supply which would not have any power line induced or more importantly RF emitted noise and using that as your central 12v power distribution source. Probably lots of candidates out the on fleaBay to choose from as a building block. You could use a small car battery charger, whack in a full-wave bridge, add some 10Kuf filter caps, regulate it with an LM-338K (or a low dropout voltage LM1084IT-ADJ ) and there's a solution for a pretty cheap and plenty of +12vDC power supply for your needs (and output voltage adjustable with a pot, if you wanted). Total cost for the mod parts mentioned is about $10 or so.

http://www.ebay.com/itm/Schumacher-8..._Tools&vxp=mtr

Tom (PK)

Electronic M · #30 03-11-2015, 11:29 AM

Quote:

Originally Posted by powerking

I would vote for building/sourcing a linear 12v power supply which would not have any power line induced or more importantly RF emitted noise and using that as your central 12v power distribution source. Probably lots of candidates out the on fleaBay to choose from as a building block. You could use a small car battery charger, whack in a full-wave bridge, add some 10Kuf filter caps, regulate it with an LM-338K (or a low dropout voltage LM1084IT-ADJ ) and there's a solution for a pretty cheap and plenty of +12vDC power supply for your needs (and output voltage adjustable with a pot, if you wanted). Total cost for the mod parts mentioned is about $10 or so.

http://www.ebay.com/itm/Schumacher-8..._Tools&vxp=mtr

Tom (PK)

I agree. But one thing to beware of is that some car chargers now are switch mode supply based. Make sure there is not switch mode supply in any car charger you buy lest you replace the devil with the same devil in disguise...