Today's Message Index:
----------------------
1. 04:34 AM - RF interference fix? (Robert L. Nuckolls, III)
2. 04:41 AM - Re: RF interference fix? (Robert L. Nuckolls, III)
3. 05:32 AM - Stero to Mono (Robert L. Nuckolls, III)
4. 06:37 AM - Re: Stero to Mono (j. davis)
5. 07:25 AM - Re: Stero to Mono (Robert L. Nuckolls, III)
6. 07:26 AM - Re: xponder ant mounting (Robert L. Nuckolls, III)
7. 08:42 AM - RF interference fix (Erich_Weaver@URSCorp.com)
Message 1
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| Subject: | RF interference fix? |
At 11:27 AM 6/7/2009, you wrote:
From first flight, the GRT EFIS and EIS on my RV have indicated
momentary, anomalously high fuel pressure readings, typically just
before take-off and also when entering and in the pattern. Stewed
over that for over a year before asking myself the question - how
does the system KNOW Im going to takeoff or land? Asking that
question led me to the only possible answer: it knows because I get
on comm and announce it! A couple of weeks ago I confirmed this -
every comm transmission resulted in a sharp rise in fuel
pressure. Yesterday I took it a step further, seeking to determine
what the propogation mode was, and reasoned it was either RF
interference or some sort of magnetic coupling associated with the
increased power requirements during comm transmissions. The easy
test was to get out my battery operated handheld transceiver and
transmit from inside the cockpit with the engine running and the
EFIS/EIS powered up, so I did that first. Bingo - a similar increase
in the fuel pressure reading, although to a lesser degree than with
ship's comm.
This is not an uncommon problem with products
offered to the OBAM aircraft community. In the
TC aircraft world, we have to demonstrate immunity
from commonly encountered radio frquency stresses
in the lab during qualification tests.
Some of the instruments offere by Van's have
demonstrated sensitivity to VHF comm signals
from ship's transmitters.
Talk to GRT first. They have certainly
encountered this before. They know their product
better than anyone else and should be able to
recommend some filtering . . . probably compoents
added at the connector where the pressure transducer
wires come into the indicator.
Don't be surprised that there isn't a fix either.
These issues are realtively easy to fix during
prouduct development and qualification . . . but
MUCH more difficult later.
Also noticed a lot of noise on my handheld when receiving from
inside the cockpit - noise that isnt noticeable on ship's comm, nor
on the handheld normally.So, now Im feeling pretty high and mighty
with my diagnostic skills, but the problem still remains - where do I
go from here to fix this?
Igntion noise? The best way to enhance the performance
of a hand held transceiver is to provide connections to
the ship's regulator comm antenna. I used to recommend
a coiled loop of coax within reach of pilot in the
cockpit that could be opened with a pair of in-line
connectors.
http://www.aeroelectric.com/Pictures/Connectors/Coax/BNC_Cable_Female_1.jpg
http://www.aeroelectric.com/Pictures/Connectors/Coax/BNC_Cable_Male.jpg
Put the cable-male on the feeder that goes to the the
antenna and leave enough slack in it to allow connection
to the hand-held. A pair of right-angle coax adapers
http://www.aeroelectric.com/Pictures/Connectors/Coax/BNC_Rt-Angle_Adapt_2.jpg
can be used to do a small radius 180 degree turn in
the coax so that it will lay against the back of the
hand-held and exit downward.
Bob . . .
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
Message 2
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| Subject: | Re: RF interference fix? |
At 06:31 AM 6/8/2009, you wrote:
><nuckolls.bob@aeroelectric.com>
>
>At 11:27 AM 6/7/2009, you wrote:
> The easy test was to get out my battery operated handheld
> transceiver and transmit from inside the cockpit with the engine
> running and the EFIS/EIS powered up, so I did that first. Bingo -
> a similar increase in the fuel pressure reading, although to a
> lesser degree than with ship's comm.
P.S. You may find it more practical to live with it.
After one understands how the problem manifests itself
then it may be easier to just let it go. I've had several
customers who've elected to not pursue a best-we-know-
how-to-do fix when the $time$ associated with the
fix was going to be pretty high.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
Message 3
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At 06:40 AM 6/4/2009, you wrote:
Greetings... I'm a bit confused on this subject after reading this
thread and similar threads in other lists. I have the same objective:
plug my iPod into my Xcom 760's audio in. The Xcom's documentation
says to use a 3.5mm mono jack in combination with a stereo-to-mono
adapter plug. My thought was to eliminate the adapter plug by simply
jumping the left/right channel pins on the jack itself, and this does
indeed give [seemingly] good results with both channels clearly
heard. But lately I've read that this may damage the iPod, and I
should be using resistors/capacitors and/or the adaptor, as though
the adaptor had some components built into it's small, moulded little
case. I'm relatively sure that this adaptor plug is doing nothing but
physically joining the left and right channels from the stereo plug
into a single tip on the mono plug, and is in no way electronically 'magic'.
Not wanting to damage my iPod, I'd be interested to know the science
behind all the posts saying 'don't do it' (wire the left and right
together), use the adaptor plug. Thanks!
The roots of this thread go back a few years. The
first adaptation I recall of a common commercial
stereo music product into an OBAM aircraft was for
a builder wanting to adapt a compact automotive
AM/FM radio to his airplane. In fact, he had pulled
a non-working ADF and was plugging the hold with the
automotive receiver. He missed the ADF's ability to
turn the AM radio stations.
This was before car radios featured low-level audio
output/input jacks. The only outputs were intended to
deliver WATTS of energy to speakers. Voltage levels
at the speaker output of such radios were much higher
than necessary to service the headphone level inputs
to intercom systems in tens of milliwatts.
The task then had several considerations. The output
stages of the radio were designed to deliver energy
to low resistance loads (4 ohm speakers). If not
similarly loaded in this new application, audio quality
at low output levels might suffer (cross-over distortion).
So the first thing we did was "load" the radio with
some handy but relatively low value resistor . . .
10 ohms, 1 watt was common.
After that, the too-high output voltage needed to
be attenuated to levels appropriate to headphone
input levels at the intercom. At the same time, the
right-left stereo signals needed to be mixed in a
manner that provided a summation of the two signals,
not a conflict. By conflict, I mean that simultaneous
positive and negative going transients in program
material do not cause large currents to circulate
between the right and left amplifier output stages.
The risk for damage to the system is low. The strongest
effect for these circulating currents is distortion
of the audio. It's common practice to first reduce
the potential for circulating currents between the
two outputs by artificially raising their output
impedances (series resistors) and then setting the
system losses with a downstream load (parallel
resistor) to set volume.
In this example
http://aeroelectric.com/Pictures/Schematics/WalkmanAdapter.gif
only mixing resistors are called for but if the
signals from the audio source proved too strong, a
resistor on the order of 10 to 100 ohms could be
paralleled with the output to bring the level
down to size.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
Message 4
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| Subject: | Re: Stero to Mono |
Robert L. Nuckolls, III wrote:
>
> The roots of this thread go back a few years. The
> first adaptation I recall of a common commercial
> stereo music product into an OBAM aircraft was for
> a builder wanting to adapt a compact automotive
> AM/FM radio to his airplane. In fact, he had pulled
> a non-working ADF and was plugging the hold with the
> automotive receiver. He missed the ADF's ability to
> turn the AM radio stations.
>
---------------snip -------------
Thanks so much for this inclusive explanation, Bob. In my particular case,
though, I *think* it's safe to assume that the modern Xcom 760 radio is designed
with iPod/CD player input in mind, and that any attenuation/gain/amplification
is done on the radio's c.b. The fact that the manufacturer states to simply use
a 3.5mm mono jack along with a stereo-to-mono adapter plug from the iPod
headphone out, enforces this assumption, in my mind.
My question, really, is if shorting the ring and tip pins on a stereo jack
installed on my panel is the electrical equivalent of using the adapter plug.
Which begs the question: do those adapter plugs contain any sort of electronic
components at all? Since the price of these critters ranges from $1.00 at the
Dollar store to $9.95 at the Source (Canadian Radio Shack, gold plated), perhaps
the high-end ones do, but I doubt it. The sound is good the way I've got it
(jack pins connected), hearing both channels mixed, good volume. Just don't want
to put my little iTouch at risk, as some have implied I may be doing...
Thanks, again!
--
Regards, J.
Sonex C-FJNJ, Jab 3300, Prince P-Tip
http://cleco.ca
+-------------------------------------------------------------+
| J. Davis, M.Sc. | (computer science) |
| *NIX consulting, SysAdmin | email: jd at lawsonimaging.ca |
+-------------------------------------------------------------+
and now... Deep Thought #35, by Jack Handy
When you go in for a job interview, I think a good thing to ask is if they
ever press charges.
Message 5
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| Subject: | Re: Stero to Mono |
>Thanks so much for this inclusive explanation, Bob. In my particular
>case, though, I *think* it's safe to assume that the modern Xcom 760
>radio is designed with iPod/CD player input in mind, and that any
>attenuation/gain/amplification is done on the radio's c.b. The fact
>that the manufacturer states to simply use a 3.5mm mono jack along
>with a stereo-to-mono adapter plug from the iPod headphone out,
>enforces this assumption, in my mind.
Yes, we're talking headphone levels for
signal strength in both the aviation and
the pocket entertainment world. These voltage
levels are on in the ballpark with votlages
that are exchanged between component systems
on their line-in/out jacks.
>My question, really, is if shorting the ring and tip pins on a
>stereo jack installed on my panel is the electrical equivalent of
>using the adapter plug. Which begs the question: do those adapter
>plugs contain any sort of electronic components at all? Since the
>price of these critters ranges from $1.00 at the Dollar store to
>$9.95 at the Source (Canadian Radio Shack, gold plated), perhaps the
>high-end ones do, but I doubt it. The sound is good the way I've got
>it (jack pins connected), hearing both channels mixed, good volume.
>Just don't want to put my little iTouch at risk, as some have
>implied I may be doing...
The only way to see if there are components
included inside the adapter is to research
it from outside with test equipment. I suspect
they have none. Modern integrated circuits
are quite tolerant of "circulating currents"
so if you're satisfied with the audio quality
then no further action is necessary.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
Message 6
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| Subject: | Re: xponder ant mounting |
At 01:55 PM 6/6/2009, you wrote:
>
>Since I already installed a reinforcement plate into the floor of my
>tailcone to mount my ground power receptacle, is there any reason
>that I can't mount my transponder antenna into the same plate an
>inch or two from the piper plug receptacle?
>
>Thanks in advance. I am loathe to drill holes into my skin without
>checking first.
I see no structural problems with it . . .
One usually mounts the xponder antenna
on the belly centerline. I presume your
ground power jack structure is off to one
side. I doubt that an off-center antenna
location is any big deal.
Bob . . .
---------------------------------------
( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
---------------------------------------
Message 7
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| Subject: | RF interference fix |
Thanks for the response Bob. I have called GRT - they did not seem to
be
familiar with this problem. I am also a follower of the Vans Air Force
list and multiple posts there have turned up only one other individual
with
similar fuel pressure symptoms, and I wasnt ever able to confirm that h
is
problem was RF related. This all seems strange to me as there are many
RVs
with similar equipment (SL-40 comm, GRT EFIS/EIS) and wiring (Z-13/8) t
hat
have my problem. Anyway, GRT suggested a ferrite filter , which I trie
d
without success. Looks like I may just have to live with this issue.
Good to know that the noise I hear on my handheld will likely disappear
by
connecting it to ships antenna. I have a small device that allows
switching between two radios on a single antenna, but have not yet hook
ed
it up - on my to do list.
thanks again
Erich
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