[RFI] Low pass filter opinions

Thu Jan 13 03:43:38 PST 2011

Hello Dale, 

Thanks for that info.  I have a Behringer MX882 used in a splitter configuration so that I can feed three rigs with one microphone.  Anyway this morning I used my Fluke 77 and checked the input pin one to chassis and it showed zero ohms all looks good so far.  On the output I check back pin one to ground and I get roughly 500 ohms, strange.  I also see there is two channels which have TRS instead of the XLR, the barrel/shield goes directly to chassis, all ok.  Checking the other channels with XLR barrels or ground shield they do not go to ground.  Right now I have no RFI running close to 1 KW, but I have had problems in the past.  I do use many #31 and 43 mix snap-on's all over the place.  The XLR and the output pin one needs some extra investigation.  I might pull that box out today and look inside to figure out why pin one on the output side is not grounded to chassis.  There is no schematic unfortunately.  

73
Dale, k9vuj
Bloomington, MN.  
Not too far from you.  

On 12, Jan 2011, at 22:26, Dale Svetanoff wrote:

> Dale,
> 
> You have touched upon one of the key issues with respect to curing RFI
> problems in most consumer and commercial equipment.  One of the key tenants
> in minimizing and controlling RFI into or out of equipment is that you
> NEVER carry chassis ground on a connector pin.  In aerospace design, most
> cables that carry sensitive leads or leads with high level signals that
> could radiate are fitted with overbraid shields.  Those shields terminate
> to chassis ground via the backshell of the associated connectors.  Thus,
> contaminating signals from the outside world are carried on the outside of
> the shield and are carried to chassis ground without ever coming into the
> interior of the equipment. Unfortunately, in the world of audio, computers,
> and so forth, these types of shield terminations are not generally used. 
> The "Pin 1" issue refers to the fact that Pin 1 of an XLR audio connector
> is the chassis ground pin.  (This same designation applies to the common
> 1/4" plug with either balanced ["TRS"] connections or unbalanced ["TS"
> only] connections.)   Because it is one of the integral pins of the
> connector, whatever is riding on the shield of the associated cable gets
> brought into the piece of equipment.  
> 
> The trick here is twofold: 1, get Pin 1 tied back to chassis ground via the
> shortest possible route; 2, apply common mode chokes (clamp-on beads, or
> similar devices) to eliminate or reduce the currents of unwanted signals
> flowing on the shield.  Some XLR connectors have the option to tie Pin 1 to
> the connector shell, a process that may help to reduce the coupling of
> external RFI into the equipment.  However, as I have found recently when
> doing some audio work on PA systems, not all XLR connectors have conductive
> shells.
> 
> In military and aerospace systems that employ overbraids and/or the use of
> conductive connector backshells for shield termination, chassis ground is
> carried on the shield, and any internal cables or wires that have "ground"
> on them are actually carrying "circuit common".  These circuit common
> connections do run to the internal ciruitry of the equipment, buit those
> leads are well-protected from  external fields are generally are not a
> problem for conducting RFI into the units.
> 
> 73, Dale
> WA9ENA
> 
> 
> 
> 
>> [Original Message]
>> From: dalej <dj2001x at comcast.net>
>> To: RFI RFI <rfi at contesting.com>
>> Date: 1/12/2011 4:49:32
>> Subject: Re: [RFI] Low pass filter opinions
>> 
>> How would I cure a pin one problem in a particular piece of equipment. 
> Would I look at the input XLR or TRS and put that pin one directly to
> ground or the chassis somewhere close to the receptacle?  What about the
> PCB?  Some of those doggone connectors are mounted to PCB and not that easy
> to find the pin one connection.  I like the theory, but implementation is
> another matter, in some cases.  
>> 
>> Thanks
>> Dale, K9VUJ
>> 
>> 
>> 
>> On 12, Jan 2011, at 13:27, Jim Brown wrote:
>> 
>>> On 1/12/2011 9:22 AM, Scott Holisky wrote:
>>>> Lets not throw out those LPF yet. I live a few miles from the local
>>>> broadcast towers here in the Twin Cities. The ERP from the three
> towers is
>>>> ~30 Megawatts, give or take.
>>> 
>>> I was similarly located in Chicago for nearly 20 years. For a research 
>>> project on RFI to audio systems, I did both calculations and 
>>> measurements (HP Spectrum Analyzer). At close in distances, field 
>>> strength is considerably less than a simple ERP calculation suggests, 
>>> because all those TX antennas have a LOT of gain in the vertical plane, 
>>> with VERY narrow beamwidth. Indeed, the greatest field strengths were 
>>> from low band VHF and FM stations, all of which were using one bay or 
>>> two-bay antennas and a lot less ERP.
>>>> In the days of analog TV you could tune across
>>>> the bands and hear the occasional sync buzz of a TV station.
>>> 
>>> I'd bet that most, if not all, of what you heard was low band VHF (Ch 
>>> 2-6). BTW -- video buzz is a better word than sync buzz -- the 
>>> repetition rate of NTSC video is 59.97 Hz, video is 75% of that 
>>> waveform, sync is only 25%, and you can (could) clearly hear the 
>>> character of the buzz vary as the video content changes.
>>> 
>>>> I have used a LPF (the Drake version) for years. It eliminated the
> mixing or
>>>> overload in my radio's front end.
>>> 
>>> While that's certainly a good solution, but the fact that it's needed 
>>> indicates it's a poor radio. If I had that sort of problem with a radio 
>>> today, I'd sell it and buy a better one. :)
>>> 
>>> I strongly agree with others who say that an outboard LPF is rarely 
>>> needed in today's world.  Virtually all GOOD rigs and amps have very 
>>> good bandpass filters built in, and for all the reasons noted, VERY 
>>> little TVI is caused by harmonics of the TX unless something is badly 
>>> broken or badly mistuned. Antenna tuners also tend to minimize VHF/UHF 
>>> harmonics, and HF antennas don't radiate VHF/UHF very well. 174 MHz is 
>>> the lowest harmonic we need to worry about, because there are VERY few 
>>> US TV stations below Ch 7 (174-180 MHz). Most that were on Ch 2-6
> (54-88 
>>> MHz) have moved to Ch 7 or above. Study 
>>> http://www.fcc.gov/dtv/markets/   to see who's on what RF channel in 
>>> your part of the country, or use 
>>> http://www.fcc.gov/fcc-bin/audio/tvq.html to search channel by channel. 
>>> (To avoid confusing non-techie viewers, stations that moved from one 
>>> channel to another continue to be identified by their old analog 
>>> designations.  For example, a station that was on Ch 9 in analog days
> is 
>>> still identified as Ch 9, even though they might actually be 
>>> transmitting on Ch 22.)
>>> 
>>> Rather, the primary causes are poor shield connections in the RF system 
>>> (rooftop antenna, cable TV, and interconnections of those systems) and 
>>> pin 1 problems in equipment connected to the TV (like stereo rigs and 
>>> video recorders of various sorts), and an LPF does NOTHING to solve 
>>> these problems. Money would be far better spent on a good choke to keep 
>>> RF off the feedline, getting antennas a bit higher (thus further from 
>>> the victim equipment, and on chokes to kill RF current on wires 
>>> connected to the victim equipment.
>>> 
>>> There's an RFI tutorial on my website.  
>>> http://audiosystemsgroup.com/RFI-Ham.pdf
>>> 
>>> 73, Jim Brown K9YC
>>> _______________________________________________
>>> RFI mailing list
>>> RFI at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/rfi
>> 
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