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

[Clint Turner]

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