Lowering cockpit RF interference

Been a long time since EE school, but, IIRC, "switching on and off" creates
a LOT of RF noise.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 11:02:57 -0800, bumper wrote:


When you say you soldered it "across" the power switch, that might lead
one to assume you soldered it to the two terminals of your switch, i.e.
in parallel with the switch contacts). Normally a filter capacitor would
be connected across the power leads, i.e. from + to -. While a filter
inductor (such as a ferrite) is connected in series.

The switch is a double pole push-on push-off type, so one pole each for
ground and 12v I was able to use it as a two position tag strip, which
made wiring easier. The capacitor is across the +12 and ground connectors
on the T&B side of the switch.

When trouble shooting RFI, it helps to be able to quantify results. If
it's interferance on comm freqencies, a hand held radio can be useful -
squelch off, and distance away from suspected RFI source as appropriate
one can use the handheld as a field strength meter for radiated
interferance.

It was purely electrical noise that cut in when the T&B was nearly up to
speed. I don't think it was RF because:

(a)ferrites didn't have any effect

(b) if it was coming from the DC-DC 12-28v solid state adapter I'd have
expected to hear it loudest when the T&B started because thats when you
see the biggest current drain from an electric motor.

(c) an old, mechanical T&B shouldn't be an RF source unless its sparking
like hell. This is a Mil-spec R C Allen type MD-4A, so possibly ex-USAF,
that has been modified to suit a glider's rate of turn rather than a
power plane. I'm told that these devices have a centrifugal switch that
sets the RPM and has the side effect of switching in and out at several
tens of cycles a second when its up to speed. That is what the racket
sounded like too.

This is not so much use when dealing with closely spaced instruments on
the panel where one is causing problems with another (LNAV did that to
Comm on one of previous glider). There, just slapping in clamp-ferrites
did the job.

This was very easily localized: T&B off: no problem. T&B turned on: nasty
noise on the radio once it had spun up. No effect from turning any other
stuff on or off.

...other good stuff snipped.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Sun, 16 Dec 2012 17:12:24 -0700, Dan Marotta wrote:

Been a long time since EE school, but, IIRC, "switching on and off"
creates a LOT of RF noise.

Even at those low voltages and currents?

But what does solving the problem by fitting a simple capacitor across
the supply say about the relative amounts of RV vs. electrical noise?
Bear in mind that there's around 50cm of unshielded cable between the
capacitor on the switch and the T&B - cable that was supplied with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

You're making my brain hurt thinking about these things! Switching DC on
and off amounts to a square wave which, as we all know, is made up of an
infinite series of sine waves, i.e., DC to light, and somewhere in there is
the RF spectrum.

From my days as an Air Force radio tecnhician, before becoming an edumacated
AF ossifer and pilot, my experience with receivers indicated that just a
couple of micro volts at the antenna input would break squelch. Now that
was from a 5 watt transmitter many, many miles away (line of sight); and
your source is merely inches away. A capacitor of the correct value acts as
a short circuit to RF at the tuned frequency whereas an inductor (RF choke)
acts like a high resistance (at the tuned frequency). Either or both of
these of the proper values and connected in the proper way will greatly
reduce or eliminate RF interference.

In my case, with the turn indicator causing the interference, I rummaged
through an electronics parts bin (actually a jelly jar) and found a
ready-made RF choke which consisted of a torroid with two wraps of wire
giving 4 loose ends. I connected one wrap (about 4 or 5 turns) in series
with +12v and the other in series with the ground to the instrument.
Problem solved.


"Martin Gregorie" wrote in message
...
On Sun, 16 Dec 2012 17:12:24 -0700, Dan Marotta wrote:

Been a long time since EE school, but, IIRC, "switching on and off"
creates a LOT of RF noise.

Even at those low voltages and currents?

But what does solving the problem by fitting a simple capacitor across
the supply say about the relative amounts of RV vs. electrical noise?
Bear in mind that there's around 50cm of unshielded cable between the
capacitor on the switch and the T&B - cable that was supplied with it.


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Mon, 17 Dec 2012 09:42:00 -0700, Dan Marotta wrote:

You're making my brain hurt thinking about these things! Switching DC
on and off amounts to a square wave which, as we all know, is made up of
an infinite series of sine waves, i.e., DC to light, and somewhere in
there is the RF spectrum.

From my days as an Air Force radio tecnhician, before becoming an
edumacated AF ossifer and pilot, my experience with receivers indicated
that just a couple of micro volts at the antenna input would break
squelch. Now that was from a 5 watt transmitter many, many miles away
(line of sight); and your source is merely inches away. A capacitor of
the correct value acts as a short circuit to RF at the tuned frequency
whereas an inductor (RF choke) acts like a high resistance (at the tuned
frequency). Either or both of these of the proper values and connected
in the proper way will greatly reduce or eliminate RF interference.

Thanks for the clear explanation. Much appreciated.

In my case, with the turn indicator causing the interference, I rummaged
through an electronics parts bin (actually a jelly jar) and found a
ready-made RF choke which consisted of a torroid with two wraps of wire
giving 4 loose ends. I connected one wrap (about 4 or 5 turns) in
series with +12v and the other in series with the ground to the
instrument. Problem solved.

Good one, and another good explanation. I've not previously understood
exactly what an RF choke did.

BTW, about 10 years ago I remember meeting a fellow glider pilot, name of
Dan, at the Sierra Cup free flight bash, but didn't catch his last name.
Was that you by any chance?


--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |

On Monday, December 17, 2012 11:42:00 AM UTC-5, Dan Marotta wrote:

Either or both of these of the proper values and connected in the proper way will greatly reduce or eliminate RF interference.


Thanks Dan for for your input. Since exercise is good for the brain, inquiry minds want to know how to determine proper capacitor and RF choke values and how to connect correctly.

BTW, about 10 years ago I remember meeting a fellow glider pilot, name of
Dan, at the Sierra Cup free flight bash, but didn't catch his last name.
Was that you by any chance?

martin@ | Martin Gregorie
gregorie. | Essex, UK
org |


Nope! Wasn't me...

Frankly, I've been out of school far too long to do a decent job of
designing a filter. I've included a link on filter design below if you want
to slap a few components together. Or you could simply go to your local
electronics supplier and tell them that you've got a buzzing in your radio
caused by something switching DC and you want to insert a low pass filter or
an RF choke in the DC line.

This will help you design a lowpass filter:

http://www.radio-electronics.com/inf...ter-design.php

However, simply wrapping your +12v line through a ferrit ring will serve the
purpose.
wrote in message
...
On Monday, December 17, 2012 11:42:00 AM UTC-5, Dan Marotta wrote:

Either or both of these of the proper values and connected in the proper
way will greatly reduce or eliminate RF interference.


Thanks Dan for for your input. Since exercise is good for the brain,
inquiry minds want to know how to determine proper capacitor and RF choke
values and how to connect correctly.

Your question is less about "I have a problem and want to fix it." versus, "I don't want any problems in the first place. What are the best practices to follow?"

Literally the only interference problem I have every had was a vario that deflected when I keyed the transceiver. Solution: replace the vario. I must live a blessed life. This has also been in pure gliders only so I don't have to worry about ignition noise which is by far the worse culprit for interference. However, new electronics run at some high frequencies (1Mhz processors are sloooow these days), so interference can happen.

So back to what I do to prevent problems in the first place.

1) Start with QUALITY - Components, wiring, terminals, switches, etc. Buy from an aviation source like Wicks or Aircraft Spruce. Radio Shack and ACE Hardware DO NOT have aviation aisles. Modern avionics seldom seem to create issues as they have to pass some pretty stringent testing to be certified. Step away from anything still using crystals (are they still legal?).
2) Coax for microphones and antennas. But every microphone and antenna already comes with coax. The more important bit is making sure that the shield has a good ground at the avionics end. A good BNC connection for the antenna (have someone knowledgeable help). Many transceivers have a separate ground pin for the microphone - use it!
3) Ferrite beads - While my EE specialty is not in the RF spectrum (get it?), putting these on doesn't hurt and might help ... so I use them. As someone in this thread said, run both the +/- wires through the bead twice if you have the slack. Non-split beads are generally better than split beads. The trick is to prevent high frequency noise from one device getting into another device through your power bus.
4) Air gaps - Wires that might have data flowing through them, like a GPS to a PDA, can induce noise into another wire running right along side of it in a bundle. So separate any suspect wires from one another.
5) Grounding - Ground all your devices to a single common location as close to the electronics as possible, including the negative side of the battery.. Even ground the metal stick, push tubes, etc, if possible.

Good luck, John

Thank you Dan for your article link

John DeRosa wrote:
Your question is less about "I have a problem and want to fix it." versus, "I don't want any problems in the first place.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Am I correct to assume the best grounding point would be the heaviest metal frame work closest to the instrument panel such as the control yoke or the adjustable rudder pedal track?

So in the perfect world all things metal (especially long lengths like push rods) and your negative instrument buss should be grounded to a common ground point?

On 12/20/2012 7:58 AM, wrote:
Thank you Dan for your article link

John DeRosa wrote:
Your question is less about "I have a problem and want to fix it."
versus, "I don't want any problems in the first place.

Exactly.

Do late model sailplanes actually have a "factory" grounding point?

Am I correct to assume the best grounding point would be the heaviest
metal frame work closest to the instrument panel such as the control
yoke or the adjustable rudder pedal track?

So in the perfect world all things metal (especially long lengths
like push rods) and your negative instrument buss should be grounded
to a common ground point?

I don't know how many adhere to the "single grounding point"; however,
note that the point is for the instruments, not necessarily everything.
The other metal bits, which are not carrying signals, can be grounded
elsewhere, such as the near the battery. A powered sailplane will likely
also have an "engine" ground, so the starter currents, ignition system,
generator, etc. are kept from the instrument ground.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)