[Lowfer] HDTV RFI, request recommendations for reduction of noise

[Douglas D. Williams]

Great post, Clint. I'm going to print that out for future reference.

Speaking of chokes, on the advice of Jack from Clifton labs I
purchased ten Steward
40T1417-10H toroid cores (DigiKey PN 240-2537-ND) for use as chokes. I
super glued 5 of them together and wrapped as many turns of RG-174 though
them as I could. This I placed between my DC power coupler and active whip
antenna as a common mode choke. I used the rest of them on various DC power
leads that power the devices in my shack. After reading what you said, I
may get some more of them for the various USB and CAT5 cables that are in
the shack.

73, Doug KB4OER

On Fri, Jan 4, 2013 at 1:55 PM, Clint Turner <turner at ussc.com> wrote:

> I know that you mentioned the antenna coax, but I take it that you've run
> the TV with nothing connected *other* than the AC line cord (e.g. no audio
> cables, HDMI cables to DVRs, etc.) and that it generates this noise
> "stand-alone", when displaying an on-screen menu?
>
> On many TVs they use a CCFL (Cold-Cathode Fluorescent Lamp) as the
> backlight and these operate at a fairly high frequency - but 600 kHz
> strikes me as a bit high, but I take it that you've checked the
> subharmonics of the strong 600 kHz component?  It's often the case that the
> high-voltage CCFL supplies use long conductors to get to the "other" side
> of the screen and these can couple energy into nearby leads - although the
> ones that I've seen tend to take great care in keeping these wires apart
> from everything else.  Often, the CCFL supplies are integrated on the main
> switcher that runs everything else on the set so the noise could easily be
> conducted back into the line from here.  Worst case, it could be the tubes
> (along two edges of the screen) themselves that are doing the radiating,
> but all the sets that I've seen seem to do a good job in shielding the
> tubes themselves, which makes sense since they use shiny metal to reflect
> as much light as possible into the plastic back-panel of the set.
>
> A lot of sets these days use high-power LEDs as backlights and the good
> news with this is that you don't have this long tube with RFI-charged ions
> along the entire edge of the screen.  If you TV uses these, there's at
> least some hope in adding internal filtering somewhere (but voiding a
> warrantee) to keep its switching supply noise out of the rest of the set
> and/or break up circulating currents.
>
> I presume that since you've been at it for a while, there's no "direct"
> connection between any of the three AC line connections (Hot, Neutral and
> Ground) and the "Plug side" of the filter you built - that is, there's a
> significant amount of inductance in each leg?  It might be worth
> temporarily trying a "ground lifter" on the "TV side" of your brute-force
> AC line filter to rule out grunge being conducted along the green-wire
> ground.
>
> Worst case is that the LCD panel itself is doing the radiating, but I've
> never seen an LCD set where this noise could really travel very far
> (near-field only) - and it wouldn't think that the "scan rate" of the LCD
> would change much with video source or content, anyway. This could be
> easily checked by temporarily taping a piece of aluminum foil to the front
> of the set to capacitively couple to the panel and then
> connecting/disconnecting a long piece of wire (to serve as an antenna) to
> see if this causes any obvious change in the same QRM.  (On a plasma TV,
> the panels themselves definitely do radiate a strong enough E-field to be
> detected from some distance!)
>
> My best bet would be that it's coupling from something other than the coax
> and power - maybe through an HDMI or A/V cable, possibly completing a loop
> into which circulating currents could be induced. If this is the case, the
> best way to kill these currents is to wrap the cable(s) on a core scavanged
> from a TV flyback transformer as these are both very lossy at high
> frequencies and very high permeability:  In my shack, I have the RG-58 from
> the LF antenna wrapped on one of these and it has enough inductance to
> choke out energy down into the 10's of kHz and I have a similar arrangement
> with the USB cable that connects my SDR-14 to the computer:  No "snap-on"
> anything comes even close to providing enough inductance to even make a
> dent in LF and low-HF currents that circulate through such cables!
>
> Let us know what you find and best of luck!
>
> 73,
>
> Clint
> KA7OEI/CT
>
>
>
>
> Andy - KU4XR wrote:
>
>> Greetings all:
>>
>> As you are all very aware.. The noise we deal with on MF / LF
>> frequencies can be terrible, and sometimes curable, sometimes not..
>> PC's, switching power supplies, especially cheap Wall-Warts, and
>> for sure the lovely HDTV's of today.. Here is my current delimna..
>> My old HDTV had a low frequency refresh rate, and just ripped to
>> pieces many frequencies in the 600, 630, 1750, and 2200 meters bands..
>> After replacing 3 defunct power supplies in it, I finally retired it..
>> I then went thru the process of finding a new " radio friendly " HDTV
>> for the family room.. My wife is the TV-aholic, as for me; I would
>> rarely have one on.. I purchased a Zenith 42 inch LCD with a
>> Refresh rate of 600 KHz.. As I expected, the interference to
>> frequencies below 500 KHz was either eliminated, or greatly reduced..
>> However... I did not at the time, consider the regular ham bands..
>> Well of course 600 KHz times 3 equals 1.8 MHz ... Yaaaayyy
>> The TV does cause interference at lower frequencies, but it is
>> tolerable.. 160 meters though is another story.. 20 dB over S-9
>> growl, and garbble.. My noise blanker will reduce, mask, or whatever
>> it does to it, and drop it down to S-9, but 160 meters is un-usable
>> except to transmit in beacon mode... I have thru the years built
>> heavy filters for power supplies, and been able to eliminate lots
>> of noise.. But, I am not sure that this type of noise has a remedy..
>> The refresh rate is fixed, and cannot be changed thru the setup menu..
>> I have tried putting a home-brew, heavy duty line filter on the TV
>> with no sucess at all.. I can disconnect the coax from the TV and the
>> noise is still there, but does not change in frequency until I
>> reconnect the cable and the picture comes back on.. The old Rat Shack
>> inline caox RFI filter does nothing.. SO; I'm stuck as to how to deal
>> with this type of interference.. We all know that hind sight is 20/20,
>> and had I thought it thru better, I would have saw this issue beforehand..
>> Yes, turning the TV off, it does go away, and a nice S-2 meter reading
>> on 160 meters.. Any thoughts, ideas, recommendations, etc. will be most
>> appreciated..
>>
>> 73 to all:
>>
>> Andy - KU4XR
>>
>
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